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Full text of "Plant morphology"

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McGRAW-HILL PUBLICATIONS IX THE 
BOTANICAL SCIENCES 

Edmund W. Sinnott. Consulting Editor 



PLANT MORPHOLOGY 



McGRAW-HILL PUBLICATIONS IN THE 
BOTANICAL SCIENCES 

Edmund W. Sinnott, Consulting Editor 



Arnold — An Introduction to Paleobotany 

Avery et al. — Hormones and Horticulture 

Babcock and Clausen — Genetics 

Curtis and Clark — An Introduction to Plant Physiology 

Eames — Morphology of Vascular Plants 

Eames and MacDaniels — An Introduction to Plant Anatomy 

FiTz PATRICK — The Lower Fungi 

Gates — Field Manual of Plant Ecology 

Gaumann and Dodge — Comparative Morphology of Fungi 

Haupt — An Introduction to Botany 

Haupt — Laboratory Manual of Elementary Botany 

Haupt — Plant Morphology 

Hill — Economic Botany • 

Hill, Overholts, and Popp — Botany 

Johansen — Plant Microtechnique 

Kramer — Plant and Soil Water Relationships 

Lilly and Barnett — Physiology of the Fungi 

Maheshwari — An Introduction to the Embryology of Angiosperms 

Miller — Plant Physiology 

Pool — Flowers and Flowering Plants 

Sharp — Fundamentals of Cytology 

Sharp — Introduction to Cytology 

Sinnott — Botany: Principles and Problems 

Sinnott — Laboratory Manual for Elementary Botany 

Sinnott, Dunn, and Dobzhansky — Principles of Genetics 

Smith — Cryptogamic Botany 

Vol. I. Algae and Fungi 

Vol. II. Bryophytes and Pteridophytes 
Smith — The Fresh-water Algae of the ITnited States 
Swingle — Textbook of Systematic Botany 
Weaver — Root Development of Field Crops 
Weaver and Clements — Plant Ecology 

There are also the related series of McGraw-Hill Publications in the Zoo- 
logical Sciences, of which E. J. Boell is Consulting Editor, and in the Agri- 
cultural Sciences, of which R. A. Brink is Consulting Editor. 



H ^'1 



PLANT MORPHOLOGY 



ARTHUR W. HAUPT 

Professor of Botany 
University of California, Los Angeles 



"Morphology .. .is one of the most interesting departments of 
natural history, and may almost be said to be its very soul." 
— Charles Darwin, Origin of Species. 



New York Toronto London 
McGRAW-HILL BOOK COMPANY, INC. 

1953 



PLANT MORPHOLOGY 

Copyright, 1953, by the McGraw-Hill Book Company, Inc. Printed in the 
United States of America. All rights reserved. This book, or parts thereof, 
may not be reproduced in any form without permission of the publishers. 

Library of Congress Catalog Card Number: 52-13810 



THE MAPLE PRESS COMPANY, YORK, PA. 



OGlC/17 









,3: LIBRakY ^ 

MASS. Qv 




PREFACE 

This book deals A\'ith the principal groups of plants from the standpoint 
of their structure, reproduction, and development. It presents a survey 
of the plant kingdom Avith emphasis upon relationships as revealed by 
basic similarity in bodil}' organization and life histories. It gives an 
account of the general course of evolution that existing groups appear to 
have followed. It endeavors to interpret, as far as possible, the struc- 
tural and developmental complexities of the higher plants in terms of 
the simpler conditions prevailing among the lower plants. 

The principal groups of plants are taken up in an ascending sequence 
based on ever-increasing structural complexity. This order of pre- 
sentation does not impl}^ direct phylogenetic relationship between 
successive groups, even though in some cases such relationship may exist. 
It merely denotes different degrees of progress from what is assumed to 
have been a more primitive condition. Conclusions as to the derivation 
of one group from another are based on substantial morphological 
evidence, but are always tentative and subject to confirmation by 
paleontological evidence. A true understanding of phylogeny can rest 
onl\^ on the fossil record and, with a few notable exceptions, this is very 
incomplete. 

The system of classification used as a basis for the presentation of the 
principal plant groups is at once simple and conservative. The older 
classification, which continues to be the one most widely used, has 
certain limitations, but these arise mainly from uncertainties regarding 
the affinities of many groups, particularly the lower ones. As long as 
these uncertainties remain, there is little justification for abandoning an 
established system of classification for a newer one. A somewhat simpli- 
fied classification is adopted because of its greater convenience and 
because more detailed schemes may be found in advanced works dealing 
^vith special plant groups. It is comprehensive enough to embrace, under 
almost every class, the most important orders; but it generally does not 
include families. 

Usually the outstanding features of each order are developed through 
a detailed discussion of one or, more frequently, of several of its repre- 
sentative genera. The distinguishing characters of the order are then 
given in the form of a summar3^ Likewise the characters of each class 
are summarized after all its members have been considered. These are 
usually presented with the characters of related classes, so that a com- 



vi PREFACE 

parison can be made. General conclusions are given at the end of the 
account of each of the major divisions of the plant kingdom, viz., algae, 
fungi, bryophytes, pteridophytes, and spermatophytes. Here are 
emphasized the evolutionary tendencies within the group, its contribu- 
tions to the evolution of the plant kingdom, and the interrelationships 
of its classes. 

This book is designed for use in a two-semester course with adequate 
laboratory work. It is intended to follow a course in general botany, 
where the student has gained a knowledge of such material as the many 
available elementary textbooks present. In particular, the student 
should understand the cytological relations involved in alternation of 
generations, including the behavior of the chromosomes in vegetative 
mitosis, fertilization, and meiosis. Much material properly belonging 
to the special fields of plant anatomy, cytology, and taxonomy has been 
omitted from the present work, especially in the treatment of the angio- 
sperms. Emphasis is placed throughout on the evolution of the plant 
kingdom as revealed by a comparative study of the morphology of the 
main groups. At the end of the book a list of supplementary readings 
has been added. These will serve to introduce the student to the current 
literature dealing with special groups and topics. 

More than two-thirds of the illustrations are original, and most of 
these have not hitherto been pubhshed elsewhere. Some have been 
taken from the author's earlier writings. Of the figures borrowed from 
the works of others, for which credit is given in every case, almost all 
have been redrawn and are designated in the legends by the word "after." 

The author is indebted for many valuable suggestions to his colleague, 
Prof. Orda A. Plunkett, and to H. R. Bennett of Chicago, who read 
Chap. IV; to Prof. P. Maheshwari, University of Delhi, India, who read 
Chaps. VIII and IX, and to Prof. Paul D. Voth, The University of 
Chicago, who read the entire manuscript. The author is also grateful 
to his wife for making some of the slides from which illustrations have 
been made and for much assistance in proofreading. 

Arthur W. Haupt 
Los Angeles, Calif. 
April, 1953 



CONTENTS 

Preface v 

I. INTRODUCTION 1 

Classification of Plants 1 

Plant Life of the Past 4 

II. thallophyta: algae 7 

1. Cyanophyceae 8 

2. Euglenophyceae 14 

3. Chrysophyceae 16 

4. Dinophyceae 18 

5. Xanthophyceae 18 

6. Bacillariophyceae 21 

7. Chlorophyceae 25 

1. Volvocales 26 

2. Chlorococcales 32 

3. Ulotrichales 38 

4. Oedogoniales 45 

. 5. Conjugales 48 

6. Siphonocladiales 54 

7. Siphonales 57 

Summary of Chlorophyceae 61 

III. thallophyta: algae (continued) 63 

8. Charophyceae 63 

9. Phaeophyceae 66 

1. Ectocarpales 67 

2. Sphacelariales 69 

3. Cutleriales 70 

4. Dictyotales 72 

5. Laminariales 76 

6. Fucales 80 

Summary of Phaeophyceae 85 

10. Rhodophyceae 85 

Comparison of the Classes of Algae 93 

General Conclusions 94 

IV. thallophyta: fungi 100 

1. Schizomycetes 100 

2. Myxomycetes 104 

3. Phycomycetes 103 

vii 



67807 



viii CONTENTS 

1. Chytridiales 108 

2. Monohlepharidales Ill 

3. Plasmo(iiophoralps 112 

4. Saprolegnialos 113 

5. Peronosporales 114 

6. Mucorales 117 

7. Entomophthorales 120 

4. Ascomycetes 121 

1. Protoascales 122 

2. Protodiscales 123 

3. Plectascales 124 

4. Perisporiales 127 

5. Pezizales 129 

6. Helvellales 133 

7. Tuberales 134 

8. Pyrenomycetales 134 

9. Laboulbeniales 140 

5. Basidiomycetes 140 

1. Ustilaginales 140 

2. Uredinales 142 

3. Auriculariales 146 

4. Tremellales 146 

5. Exobasidiales 146 

6. Hymenomycetales 147 

7. Gasteromycetales 151 

Fungi Imperfecti 153 

Lichenes 153 

Comparison of the Classes of Fungi 156 

General Conclusions 157 

V. BRYOPHYTA 160 

1. Hepaticae 161 

1. Marchantiales 161 

2. Sphaerocarpales 173 

3. Jungermanniales 177 

4. Anthocerotales 186 

2. Musci 192 

1. Sphagnales 192 

2. Andreaeales 197 

3. Bryales 198 

Comparison of Liverworts and Mosses 205 

General Conclusions 205 

VI. PTERIDOPHTTA 209 

The Vascular System 210 

1. Psilophytinae 212 

1. Psilophytales . 212 

2. Psilotales 214 

2. Lycopodiinae 218 

1. Lycopodiales 218 



CONTENTS ix 

2. Selaginellales 230 

3. Lepidodcndrales 238 

4. Isoetales 241 

3. Equisetinae 247 

1. Hyeniales 247 

2. Sphenophyllales 248 

3. Equisotales 250 

4. Calamitales 258 

VII. PTERIDOPHYTA (CONTINUED) 260 

4. Filicinae 260 

1. Coenopteridales 260 

2. Ophioglossales 261 

3. Marattiales 270 

4. Filicales 276 

5. Hydropteridales 292 

Comparison of the Classes of Pteridophytes 303 

General Conclusions 304 

VIII. SPERMATOPHYTA 309 

1. Gymnospermae 310 

1. Cycadofilicales 311 

2. Bennettitales 315 

3. Cycadales 319 

4. Cordaitales 330 

5. Ginkgoales 333 

6. Coniferales 340 

7. Gnetales 354 

IX. SPERMATOPHYTA (CONTINUED) 362 

2. Angiospermae 362 

Vegetative Organs 363 

The Flower 372 

Chief Orders of Angiosperms 399 

Comparison of Gymnosperms and Angiosperms 408 

General Conclusions 409 

X. EVOLUTION OF THE PLANT KINGDOM 412 

Prominent Evolutionary Tendencies 415 

Evolution of Sex 417 

Alternation of Generations 422 

Selected References 427 

Glossary 431 

Index 445 



CHAPTER I 
INTRODUCTION 

Morphology deals with the form and structure of plants. It is con- 
cerned with both gross, external features and minute, internal details. 
It includes a study of the development of plants throughout all their 
growth stages, called ontogeny, as well as their evolutionary development, 
or phyJogeny, by means of which all existing plants have been derived from 
those of past ages. Morphology considers the interrelationships of the 
groups forming the larger units of classification, but does not deal with 
species, the study of which belongs to taxonomy. One of the main objec- 
tives of morphology is the determination, so far as possible, of lines of 
descent. 

A sound knowledge of the structure and development of plants is a 
necessary foundation for successful specialization in any phase of botany, 
whether it be taxonomy, physiology, ecology, pathology, or genetics. 
A study of the lower plants is often neglected, since the higher ones are 
more famihar and, in general, more important. Many of the problems 
encountered in the higher plants, however, are more easily studied in 
the lower plants, whose structure and functions are much simpler. The 
logical procedure is to study simple plants before attempting to under- 
stand complex plants. 

CLASSIFICATION OF PLANTS 

For many years the system of classification most widely adopted by 
botanists has been one in which the plant kingdom is separated into four 
major divisions: Thallophyta, Bryophyta, Pteridophyta, and Spermato- 
phyta. Each division comprises a number of classes. A class is made up 
of orders, an order of Jamilies, a family of genera, and a genus of species. 
Categories of intermediate rank are designated by the prefix suh. 

At one time the two subkingdoms Cryptogamia and Phanerogamia 
were recognized, the former including the three lower divisions and the 
latter the fourth division. These names have fallen into disuse because 
they are inappropriate. Cryptogam means "fertilization concealed" and 
phanerogam means "fertiUzation evident." The names were given 
because stamens and pistils, the organs once thought to produce directly 
the cells which unite in fertilization, are present in seed plants but not in 
plants without seeds. After the true nature of fertiUzation was dis- 

1 



2 PLANT MORPHOLOGY 

covered, it was found to be actually more evident in the so-called crypto- 
gams than in the phanerogams. 

Often all plants above the level of the Thallophyta are grouped 
together as the Embryophyta, plants in which the zygote gives rise to 
an embryo that undergoes its early development within either an archego- 
nium or an embryo sac. A less suitable name for these plants, but one 
sometimes used, is Cormophyta, meaning "plants with a stem." Many 
bryophytes have a stem, but it is not homologous with the stem of 
pteridophytes and spermatophytes. Sometimes the bryophytes and 
pteridophytes are combined into a single group, the Archegoniatae, a 
name that is not distinctive because archegonia are present in nearly all 
gymnosperms, which form the lower class of spermatophytes. A recent 
tendency is to place the pteridophytes and spermatophytes together 
under the name of Tracheophyta, which signifies that they are vascular 
plants. 

Classification of Thallophyta. Some botanists disapprove of the term 
Thallophyta on the ground that it includes a heterogeneous assemblage of 
plants which are not closely related. This objection is more valid v/hen 
the term is applied to one of the four divisions of the plant kingdom 
rather than to one of two subkingdoms; for the same objection could be 
raised against the term Embryophyta. A partial solution of the difficulty 
is to consider the Thallophyta as a subkingdom and to raise the algae and 
fungi to the rank of divisions, as follows: 

A. Thallophyta 

I. Phycophyta (Algae) 

II. Mycophyta (Fungi) 

B. Embryophyta (Cormophyta) 

I. Bryophyta 

II. Pteridophyta 

III. Spermatophyta 

The thallophytes comprise a number of subordinate groups. These 
may either be considered as classes and assigned to the algae or the fungi, 
or may be distributed among a larger number of divisions. The first 
arrangement is a convenient one, but some of the groups classified as algae 
or fungi have little in common with the others. Furthermore, it makes 
the presence or absence of chlorophyll the basis for establishing the two 
divisions Phycophyta and Mycophyta, a distinction which cannot be 
maintained among the flagellates, where both green and colorless forms 
occur. The flagellates were formerly regarded as constituting a distinct 
class of thallophytes, but are now broken up into a number of separate 
groups. 

Some of the groups commonly included among the algae and fungi 
are so distinctive that their separation seems justified. The.se include the 



INTRODUCTION 3 

Cyanophj^ceae, Schizomyoetes, M^^\omycetes, Bacillariophyceae, and 
possibly some of the flagellate groups. The remaining classes of algae 
might then be retained in one division and the remaining classes of fungi 
in another, or some or all of these classes might be raised to the rank of 
divisions. Much difference of opinion exists as to which classes should be 
placed together. 

Two different arrangements for classifying the thallophytes are as 
follows : 



A. 


Thallophyta 


I. Phycophyta' 




I. 


Schizophyta 


1. Cyanophyceae 






1. Cyanophyceae 


2. Xanthophyceae 






2. Schizomycetes 


3. Bacillariophyceae 




II. 


Myxomycophyta (Myxomycetes) 


4. Chlorophyceae 




III. 


Bacillariophyta (Diatomeae) 


5. Charophyceae 




IV. 


Euphycophyta (Euphyceae)' 


6. Phaeophyceae 






1. Xanthophyceae 


7. Rhodophyceae 






2. Chlorophyceae 


II. Mycophyta 






3. Charophyceae 


1. Schizomycetes 






4. Phaeophyceae 


2. Myxomycetes 






5. Rhodophyceae 


3. Phycomycetes 




V. 


Eumycophyta (Eumycetes) 


4. Ascomycetes 






1. Phycomycetes 


5. Basidiomycetes 






2. Ascomycetes 



3. Basidiomycetes 

' Several other classes, consisting almost entirely of flagellates, are generally recog- 
nized. These are the Euglenophyceae, Chrysophyceae, Cryptophyceae, and 
Dinophyceae. 

The elevation of a great number of classes to the rank of divisions, thus 
making each coordinate with the bryophytes, pteridophytes, and sperma- 
tophytes (or even with the tracheophytes, if the last two are combined), 
tends to conceal relationships and gives a prominent place to small, 
obscure groups. Moreover, when the names of all the divisions are 
given the termination phyta, in order to make them consistent through- 
out the plant kingdom, many lose their distinctive meanings, and we find 
the various groups of algae called Chlorophyta (green plants), Phaeophyta 
(brown plants), Rhodophyta (red plants), etc. 

Classification of Embryophyta. The position of the Bryophyta 
as a division of the plant kingdom seems secure. Those botanists 
who classify the higher plants on the basis of vascular anatomy discard 
the names Pteridophyta and Spermatophyta and designate all vascular 
plants as Tracheophyta. They point out that a marked tendency 
toward seed formation was present in several extinct groups of pterido- 
phytes, and that the most primitive group of seed plants, the extinct 
Cycadofilicales, were very fern-like. The Tracheophyta, constituting a 
division, are separated into four classes, the Psilopsida, Lycopsida, 



PLANT MORPHOLOGY 



Sphenopsida, and Pteropsida. The first three correspond to established 
classes of pteridophytes under the older classification, while the Pterop- 
sida include the ferns (Filicinae), gymnosperms, and angiosperms. 

The presence of leaf gaps in the vascular cylinder is thought to indicate 
a closer relationship between the ferns and seed plants than exists 
between the ferns and other pteridophytes. However, the basis used in 
distinguishing the ferns from the gymnosperms and angiosperms, when 
the three are grouped together as Pteropsida, is the same as when the 
ferns are placed in the Pteridophyta and the other two groups in the 
Spermatophyta. Furthermore, if the existing classes of pteridophytes 
represent collateral lines of descent from the psilophytes of the Devonian, 
a view widely accepted, their relationship to one another can better be 
expressed by including them in a division of their own. Certainly no 
greater degree of relationship is expressed by placing them in a division 
that also includes the gymnosperms and angiosperms. 

The t"svo different schemes of classifying the embryophytes are as 
follows: 

B. Embryophyta 

I. 



I. Bryophyta 

1. Hepaticae 

2. Musci 

II. Pteridophyta 

1. Psilophytinae 

2. Lycopodiinae 

3. Equisetinae 

4. Filicinae 
III. Spermatophyta 

1. Gymnospermae 

2. Angiospermae 



Bryophyta 

1. Hepaticae 

2. Musci 

II. Tracheophyta 

1 . Psilopsida 

2. Lycopsida 

3. Sphenopsida 

4. Pteropsida 

a. Filicinae 

b. Gymnospermae 

c. Angiospermae 



PLANT LIFE OF THE PAST' 

The plants of today are the modified descendants of other plants that 
have lived on the earth throughout the course of geologic history. They 
are the products of a process of evolution that has been in operation since 
life first began. Our knowledge of the plants of the past has come from a 
study of fossil remains found embedded in the layers of rock that form the 
earth's crust. These remains constitute a direct record of the changes 
undergone by plants down through the ages. This record, incomplete 
as it is, helps us to follow the course of evolution and to understand the 
relationships that occur among the various existing plant groups. 

It is not known how or when life arose on the earth. It is not even 

known in what form it arose, although much evidence indicates that the 

first living things were extremely simple and from them forms more and 

' This subject is presented in much greater detail in Arthur W. Haupt, An Introduc- 
tion to Botany, 2d ed., Chap. XX, New York, 1946. 



INTRODUCTION 5 

more complex have been evolved. Some groups have made more 
progress than others. That is why existing groups are at different levels 
of development. Along with the tendency toward ever-increasing 
complexity, much retrogression has occurred and, as a result, some 
modern groups are more or less degenerate. 

Paleobotany, the study of fossil plants, has made great progress because 
methods have been developed making it possible to study thin sections 
of petrified material under the microscope. Many of these sections 
show such an amazing wealth of structural detail that almost as much can 
be learned from them as from sections of living plants. Unlike petri- 
factions, fossils in the form of casts or impressions, made when some part 
of a plant falls into soft earth that later hardens into stone, show no 
internal structvu-e but preserve many external features. Most fossils 
are of this kind. 

Geologic time, whose total duration is about 2 billion years, is divided 
into five great eras. The Archeozoic era came first. Then followed, in 
order, the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras. Each 
era is divided into periods. The Archeozoic and Proterozoic, with an 
estimated duration of 800 million and 650 million years, respectively, 
comprise nearly three-fourths of all geologic time. Most of the evidence 
for the existence of Hfe during these two great eras is indirect, consisting 
of extensive deposits of graphite, limestone, and iron ores, substances 
that are formed, at least to some extent, by organisms. The earliest 
plants may have been similar to certain existing bacteria and blue-green 
algae. 

The fossil record of nonwoody plants is very fragmentary. Because 
of their soft and perishable nature, few have left any direct evidence of 
their existence. Remains are more numerous of such algae as diatoms, 
which have siliceous shells, and of lime-secreting seaweeds. Bryophytes 
have been poorly preserved and their remains are scanty. Vascular 
plants, on the other hand, are represented by an abundance of well- 
preserved fossil material, and much is known of the geologic history 
of many groups. 

The fossil record really begins with the Paleozoic era, since so little 
is known of the life of the Archeozoic and Proterozoic. The periods into 
which the Paleozoic, Mesozoic, and Cenozoic eras are divided are given 
in the table on page 6. The figures in the time scale denote millions of 
years. 

Fossil algae furnish the only record of plant life during the Cambrian 
and Ordovician, and the diversity of types which have been found indi- 
cates that all four of the great algal groups were represented in both 
periods. Silurian deposits have yielded remains of the oldest known land 
plants, the psilophytes, but these are scanty. During the Devonian so 



6 



PLANT MORPHOLOGY 



much progress was made, that not only were many kinds of primitive 
land plants in existence, but even such highly developed forms as large 
lycopods, ferns, and primitive gymnosperms were abundant. 

The Carboniferous was characterized by a wonderful display of plant 
life. Tree lycopods and horsetails, as well as fern-like and other primi- 
tive gymnosperms, formed a most luxuriant growth surpassing even the 



Time 
scale 


Periods 


Eras 





Recent 




1 


Quaternary 


Cenozoic 


60 


Tertiary- 




100 


Upper Cretaceous \ 




125 
160 


Lower Cretaceous 

■ 

Jurassic 


Mesozoic 


195 


Triassic 




220 


Permian \ 




255 


Upper Carboniferous 1 




305 


Lower Carboniferous/ 




355 


Devonian \ 


Paleozoic 


395 


Silurian I 




480 


Ordovician j 




550 


Cambrian / 





densest tropical jungles of today. The accumulated remains of the plants 
that lived in the vast Carboniferous swamp forests have formed our most 
extensive coal deposits. 

The plant life of the Mesozoic, except during the Upper Cretaceous, 
was dominated by the gymnosperms, these being of much more advanced 
types than had lived during the Paleozoic. Nearly all the large pterido- 
phytes of the late Paleozoic, as well as the primitive gymnosperms, 
became extinct early in the Mesozoic. A striking feature of the Creta- 
ceous was the rise of the angiosperms, as a result of which they came to 
dominate the vegetation of the entire earth, a position they have main- 
tained ever since. With the rise of the angiosperms, the gymnosperms 
have become a subordinate group. 




CHAPTER II 
THALLOPHYTA: ALGAE 

The thallophytes comprise a large and diverse assemblage of simple 
plants forming the lowest division of the plant kingdom. They number 
about 88,000 species. The plant body may be unicellular but, where 
multicellular, as is generally the case, it is a thallus — a body without 
differentiation into true vegetative organs, such as characterize the higher 
plants. This distinction is not absolute, however, as some of the marine 
algae have parts that superficially resemble true vegetative organs, while 
some of the bryophytes have thallus bodies. A more tenable distinction 
is based on the structure of the reproductive organs. The sporangia of 
thallophytes, with only a few exceptions, are unicellular; those of the 
higher plants are always multicellular. The gametangia of thallophytes 
are prevailingly unicellular but, where multicellular, have no outer layer 
of sterile cells (except in the Charophyceae). In the thallophytes the 
zygote does not produce an embryo within the female sex organ, as it 
does in all the higher groups. 

The Thallophyta include two main series, the algae (Phycophyta) and 
the fungi (Mycophyta), the former with 18,000 species and the Latter with 
70,000. The algae, having chlorophyll, are able to make food by photo- 
synthesis and so are independent {autotrophic) plants. The fungi, lack- 
ing chlorophyll, must obtain their food from an external source and so 
are dependent (heterotrophic) plants. This distinction, being physiologi- 
cal, is a convenient one but does not necessarily express relationship; 
thus it may be without phylogenetic significance. For this reason the 
various classes of algae and fungi are often regarded as separate and more 
or less coordinate groups of thallophytes rather than as members of two 
different series. 

Algae live in both fresh and salt water, while a few grow on moist soil, 
wet rocks, tree trunks, or in other terrestrial habitats. They include the 
pond scums, kelps and other seaweeds, and a host of less famihar forms. 
Many are microscopic, but some kelps reach a large size. Because of 
their perishable nature, algae have left few reliable records of their exist- 
ence during geologic times. Most of those preserved as fossils are lime- 
secreting seaweeds and forms with sihceous shells (diatoms). As here 
presented, the algae are distributed among 10 main classes, the Cyano- 
phyceae, Euglenophyceae, Chrysophyceae, Dinophyceae, Xanthophy- 

7 



8 PLANT MORPHOLOGY 

ceae, Bacillariophyceae, Chlorophyceae, Charophyceae, Phaeophyceae, 

and Rhodophyceae. 

1. CYANOPHYCEAE 

The Cyanophyceae,^ or blue-green algae, are the simplest and lowest 
group of green plants. They are characterized by having, in addition 
to chlorophyll and carotinoids, a blue pigment, phycocyanin, the combina- 
tion resulting in a blue-green color. Some of the Cyanophyceae, how- 
ever, also possess a red pigment, phycoerythrin, the presence of which, in 
varying amounts, produces shades of red, brown, or purple. The Red 
Sea is said to have received its name from a floating species, Trichodes- 
mium erythraeum, which is red and sometimes occurs in such abundance 
as to color the water. The Cyanophyceae are unicellular plants, the 
cells being nearly always grouped to form colonies of various kinds. 
About 1,500 species are known. 

The Cyanophyceae comprise two orders: (1) the Coccogonales, whose 
cells are either solitary or arranged in nonfilamentous colonies; and (2) 
the Hormogonales, whose cells are in filamentous colonies. Some of the 
genera belonging to the Coccogonales are Chroococcus, Gloeocapsa, Meris- 
mopedia, Coelosphaerium, and Chamae siphon. The main genera of the 
Hormogonales include Oscillatoria, Lynghya, Nostoc, Anahaena, Rivula- 
ria, Gloeotrichia, Tolypothrix, Scytonema, and Stigonema. 

Distribution and Habitat. Blue-green algae are found in all parts of 
the world where plants can grow. Most of them live in fresh water, 
some occur on moist earth, rocks, and trees, while others live in the ocean. 
They commonly form scums, slimy mats, or gelatinous lumps. They 
are especially prevalent in stagnant water, where large quantities of 
organic matter accumulate. Some live in hot springs at temperatures 
as high as 75°C. Many forms extract calcium and magnesium from the 
water and cause minerals, which are often brightly colored, to be depos- 
ited on rocks in the vicinity. Some species of Nostoc and Anahaena live 
as endophytes in the intercellular cavities of other plants, as in the 
thallus of Anthoceros, the leaves of Azolla, and the roots of cycads. Some 
blue-green algae enter into the formation of lichens. 

The Cyanophyceae living in hot springs thrive under conditions that 

1 Sometimes called Schizophyceae or Myxophyceae. Schizophyceae means "split- 
ting algae"; Myxophyceae means "slime algae." These names are used by some 
botanists in preference to Cyanophyceae, which means "blue algae," because not all 
the members are blue-green. But it is also true that some of them lack the slippery 
feel. As long as we retain the names Chlorophyceae, Phaeophyceae, and Rhodo- 
phyceae for other algal groups, we might as well retain the name Cyanophyceae for 
the sake of uniformity, especially since some of the Chlorophyceae are not green, 
some of the Phaeophyceae are not brown, and some of the Rhodophyceae are not red. 
The things most desired in a name are that it shall express the most prominent feature 
of the group and that it shall be consistent with the names of coordinate groups. 



THALLOPHYTA: ALGAE 9 

would be fatal to almost all other forms of life. For this reason, because 
of their simplicity in cellular organization, and because they are auto- 
trophic, members of this group may have lived on the earth before con- 
ditions were favorable for the existence of other organisms, with the 
possible exception of bacteria. Their great antiquity is indicated by the 
presence, in rocks of Proterozoic age, of what seem to be fossil Cyano- 
phyceae, as well as numerous calcareous deposits resembling those made 
by blue-green algae now living in hot springs. There is more certain 




V-/ Vi/ J t^ 



'o'iG 0,000 OqX/OJ 



c-N /^ /'^ /~^ f^ /^ rrs 



B 








>■ 



'ii- 



i. 



■I. 






'1:! 



}': 



'• 'V. 






D 

Fig. 1. Some simple colonial blue-green algae. A, Gloeocapsa, X750; B, Merismopedia, 
X750; C, Nostoc, X 1,000; D, Oscillatoria, X600. Except in Oscillatoria, the cells are 
embedded in a mucilaginous matrix. 



evidence of their existence in the Paleozoic era, particularly in the 
Cambrian, Silurian, and Devonian periods. 

Plant Body. All the Cyanophyceae are unicellular and in nearly all 
of them the cells are organized to form colonies (Fig. 1). None has a 
truly multicellular body, although this condition is approached by the 
higher members of the group. In some species of Chroococcus the cells 
are solitary, while in Gloeocapsa they form small irregular colonies loosely 
held together in a gelatinous matrix. In Merismopedia the colonies are 
plate-hke, the cells being arranged in regular rows. In Coelosphaerium 
the colonies are globular and hollow, in Nostoc they resemble a string of 
beads, while in Oscillatoria they form a compact filament. The fila- 
mentous type of colony is most common. 



10 



PLANT MORPHOLOGY 



Although ciUa are never present, many of the filamentous blue-green 
algae have the power of movement. If a mass of Oscillatoria growing 
on mud is placed in a flat dish, the filaments soon creep out in all direc- 
tions. Under the microscope the filaments are seen to shift frequently 
their position laterally in the water. In performing these movements, 
the cause of which is unknown, the cells of the colony function as a unit, 
thus approaching a condition characteristic of multicellular plants — a 
cooperation of cells in the performance of their functions. 





Fig. 2. Longitudinal sections through cells of Anabaena circinalis, some of which are 
dividing, X 2,750. The nuclear material is in the form of irregular masses. The spherical 
bodies are cyanophycin granules and represent reserve food. {After Haupt.) 



Cell Structure. The Cyanophyceae are characterized by a very primi- 
tive cell structure. A thin cell wall, composed of cellulose and pectic 
compounds, seems always to be present. Generally it becomes mucilag- 
inous and forms a matrix around the cell. The protoplast lacks the 
degree of organization seen in the higher plants. It consists of an outer 
colored portion, containing the blue and green pigments, and a central 
colorless portion. The latter, representing an incipient nucleus, con- 
tains a mass of scattered chromatin granules not surrounded by a mem- 
brane and without a nucleolus (Fig. 2). Plastids are not organized, the 
pigments being merely diffused throughout the peripheral region of the 
cell. Carbohydrate food is stored as glycogen, starch being absent. 
Reserve food often occurs also as minute oil droplets and as spherical 
bodies (cyanophycin granules) that are probably protein in nature. 



THALLOPHYTA: ALGAE 11 

These granules usually lie in the outer part of the cell, in many fila- 
mentous forms being commonly grouped along the cross walls. 

Cell division is accomplished by a ring-like wall that develops from 
the outside toward the center, finally cutting the cell in half (Fig. 2). 
At the same time the chromatin separates into two approximately equal 
masses without the formation of chromosomes or other features of 
mitosis. 

Reproduction. Because the Cyanophyceae are unicellular plants, cell 
division results in reproduction, a method called fission. The division 
of a cell to form two new individuals directly is the simplest method of 
reproduction in the plant kingdom. In most of the Cyanophyceae the 
cell walls break down to form abundant mucilage. Generally, as in 
Gloeocapsa, this holds together a group of cells derived from a single cell 
by repeated division, thus forming a colony (Fig. L4). Here the mucilage 
surrounding the cells is in concentric layers; but in many other genera it 
is in a continuous mass made up of the confluent sheaths of the individual 
cells. 

In some of the filamentous types, such as Lynghya, a firm mucilaginous 
sheath is present around the whole colony, but in Oscillatoria, a related 
genus, the cell walls are more resistant and no sheath is formed (Fig. \D). 
In both genera the cells are compactly arranged in the colony, each cell, 
except the terminal one, being shortly cylindrical. That their shape 
results from mutual pressure is shown by the fact that the free surface of 
the end cell is convex. This is also true of cells adjacent to a dead cell 
in the filament. 

The type of colony produced depends on the way in which the cells 
divide. In a filament all the divisions occur in one plane. Where the 
cells divide in two planes, a plate or a hollow sphere one layer thick is 
produced. Divisions in three planes usually result in a somewhat 
massive type of colony. 

In most of the filamentous forms, with the exception of Oscillatoria 
and its relatives, differentiated cells, called heterocysts, appear in the 
colony. They may be seen in such common genera as Nostoc and 
Anabaena (Fig. IC). A heterocyst is an enlarged vegetative cell that 
becomes thick- walled and transparent. Heterocysts usually occur singly 
but at rather frequent intervals, thus dividing the filament into segments 
called hormogonia. These become detached and move away from one 
another to form new colonies. A hormogonium is merely an isolated 
portion of the original filament. In Oscillatoria and related genera 
hormogonia are formed by the death of unmodified cells here and there 
in the colony (Fig. ID). 

Although none of the Cyanophyceae produces zoospores or gametes, 
most of the filamentous members form nonmotile resting spores. One 



12 



PLANT MORPHOLOGY 



kind, called an akinete, arises from a v^egetative cell that enlarges by the 
accumulation of food and develops a thick cell wall (Fig. 3A, B). These 
cells are very resistant to unfavorable conditions. Akinetes may be 
separated in the filament or several may occur together. In some forms 
they always appear next to a heterocyst, either at the end or in the 
middle of the filament. Another kind of resting cell, called an endospore, 
is developed in some genera, as in Chamae siphon and in the marine genus, 
Dermocarpa (Fig. 3C). Endospores are small thick- walled spores that 




B C 

Fig. 3. Formation of resting spores in the blue-green algae. A, portion of filament of 
Anabaena with a heterocyst and an akinete containing many food granules, X750; B, 
Gloeotrichia, showing a young filament and two stages in the development of an akinete, 
X500; C, Dermocarpa, an epiphytic form, with two empty cells and others containing 
endospores. (C, aftei- Bornet and Thuret.) 



arise from a protoplast by divisions within the cell cavity and from which 
they later are liberated. 

Rivularia is a filamentous form in which the basal cell of a filament is 
always a heterocyst, while the other cells become gradually smaller 
toward the very slender apex. A thick mucilaginous sheath, confined 
to the base of the filament, begins next to the heterocyst. Gloeotrichia 
is similar to Rivularia except that the first basal vegetative cell becomes 
transformed into an elongated akinete (Fig. 35). 

Branching. In some of the filamentous members branching occurs 
(Fig. 4). In Tolypothrix the filaments exhibit "false branching." 
Here the cells on one side of a heterocyst grow out beyond it to form 
a branch. In Scytonema the false branches arise laterally in pairs but 



THALLOPHYTA: ALGAE 



13 



usually not in connection with a heterocyst. In Stigonema a true branch 
arises as a lateral outgrowth from a single cell of the filament. This type 
of branching is rare in the Cyanophyceae, occurring in only a few genera. 
Summary. The Cyanophyceae are an ancient group of plants showing 
an extremely primitive condition of structural organization. In addition 
to chlorophyll and carotinoids, a blue pigment (phycocyanin) is present 
and often a red pigment (phycoerythrin) as well. The plant body is 
unicellular, the cells nearly always forming colonies. The cell wall is 





A B C 

Fig. 4. False branching in Tolypothrix (A) and Scytonema (B), and true branching in 
Stigonema (C) ; A, X750; B, X200; C, X300. 



more or less unstable, usually producing abundant mucilage. The 
protoplast shows little organization. The pigments forming the charac- 
teristic blue-green color are diffused throughout the peripheral part of the 
protoplast, no plastids being present. Reserve carbohydrate food is 
stored as glycogen. A nucleus is represented only by scattered chromatin 
granules, there being no nuclear membrane or nucleolus. Reproduction 
occurs by fission and by nonmotile spores. It is entirely asexual. 
Ciliated cells are never produced. The resting ceUs (akinetes) are 
merely enlarged protoplasts with a thick wall. There is a tendency 
toward cellular differentiation, in some forms resulting in the establish- 
ment of a distinct apex and base. The relationships of the Cyanophyceae 
to the other algae are obscure. They appear to be closely related to the 
bacteria. In fact, the blue-green algae and bacteria are sometimes placed 
in the same group, the Schizophyta, and made an independent class of 
thallophytes. 



14 



PLANT MORPHOLOGY 



2. EUGLENOPHYCEAE 

Flagellates are unicellular organisms combining characters of both 
plants and animals. Zoologists regard them as one-celled animals, 
while botanists consider at least those with chlorophyll as plants, as well 
as certain colorless ones evidently derived from them. Nearly all 
flagellates are solitary and free-swimming, but some form loose gelatinous 

colonies and some are attached. Most of them live in 
fresh or salt water, some occur on damp earth, and 
some are parasitic. Formerly they were placed in a 
separate group, the Flagellatae, but they show so 
many differences among themselves that they are 
now distributed, so far as possible, into other groups. 
One of these, the Euglenophyceae, includes about 350 
species of green or colorless, mainly fresh-water 
flagellates. The best-known genus, Euglena, is widely 
distributed and common in stagnant pools and ditches, 
often occurring in such abundance as to color the 
water a deep green. 

Cell Structure. Euglena is somewhat pear-shaped, 
being blunt at its anterior end and gradually tapering 
behind (Fig. 5). As in other flagellates, there is no 
cell wall, each cell consisting of a naked protoplast. 
In Euglena the outer part of the protoplast is differ- 
entiated into a thin pellicle that is somewhat firm but 
flexible enough to permit the cefl to undergo frequent 
changes in shape. In some flagellates the pellicle is 
more rigid, giving the cell a constant form, while in 
others it is wanting. Some flagellates that lack a 
pellicle are amoeboid, putting out slender pseudopodia. 
Flagellates are characterized by having, in the vege- 
tative condition, one or two (rarely more) cilia — slender 
protoplasmic threads that lash back and forth in the water. Long whip- 
like ciHa are called flageUa,! the possession of which gives the flagellates 
their name. The flagella are generally borne at the anterior end of the cell 
and, where two are present, they are either equal or unequal in length. 

Euglena has a single flagellum attached anteriorly. Near its base 
is a conspicuous red eyespot, which is thought to be sensitive to fight. 
Although avoiding direct sunlight, the organism tends to swim toward 
the best-illuminated part of the water. At the anterior end of the cell 

1 If a distinction is to be made between cilia and flagella, the latter are not only 
longer than the cell that bears them but coarser and fewer in number. Cilium means 
"eyelash"; flagellum means "whip." 




Fig. 5. Euglena 
viridis, X750. The 
cell contains a large 
nucleus and a num- 
ber of chloroplasts. 
At the anterior end 
is a long flagellum, 
a narrow gullet lead- 
ing to the reservoir, 
a contractile vacu- 
ole, and an eyespot. 



THALLOPHYTA: ALGAE 15 

is a short narrow tube, called the gullet, that leads to a spherical cavity, 
the reservoir. The flagelliim is inserted inside the reservoir and projects 
through the gullet. Near the reservoir is a contractile vacuole (more than 
one in some species), which alternately contracts and expands. The 
contractile vacuole discharges its contents into the reservoir. Con- 
tractile vacuoles, usually regarded as organs of excretion, are found in 
many one-celled animals, as well as in the motile cells of many algae. 
Flagellates show an advance over the blue-green algae in having 
a definite nucleus. Moreover, their photosynthetic pigments, where 
present, are always confined to definite plastids. Euglena has many small 
green plastids (chloroplasts) , but some flagellates, not belonging to the 
Euglenophyceae, have yellow or brown plastids. All these colored forms 
carry on photosynthesis. Other flagellates, some belonging to the 
Euglenophyceae, are colorless and live either as saprophytes, absorbing 
organic matter in solution through the plasma membrane, or as animals, 
ingesting solid particles of food either through the gullet or by means of 
pseudopodia. A few flagellates are parasitic on animals, one of these, 
Trypanosoma, causing a disease of man known as African sleeping 
sickness. Some forms with chlorophyll ingest solid food particles through 
the gullet. Some species of Euglena carry on photosynthesis in the 
light, but, if kept in darkness and supplied with organic matter in solution, 
become colorless and saprophytic. In all the Euglenophyceae food is 
stored as paramylon, a starch-like carbohydrate, and often as oil. The 
presence of paramylon granules is very characteristic, even in colorless 
cells. True starch is not formed. Flagellates belonging to other groups 
of algae differ with respect to the type of food stored. 

Reproduction. As in all flagellates, reproduction in Euglena occurs 
by fission, the cell dividing longitudinally. In the presence of unfavor- 
able conditions, encystment often occurs. The protoplasm retracts the 
flagellum, rounds up, secretes a thick gelatinous covering about itself, 
and goes into a resting stage. Although later the cyst usually produces 
a single motile protoplast, sometimes it divides internally into a number 
of smaller protoplasts that escape, develop flagella, and grow to mature 
size. Sexual reproduction in the Euglenophyceae is of doubtful 
occurrence. 

Colacium, a member of the Euglenophyceae, is interesting in being an 
attached form lacking flagella in the vegetative condition. Its cells are 
surrounded by a gelatinous wall and are united into small irregular 
colonies. When reproduction occurs, the cell contents escape as a 
naked euglenoid protoplast with a flagellum. 

Relationships. Flagellates are related on the one hand to various 
groups of algae and on the other hand to the Protozoa. The fact that 
they are intermediate between plants and animals strongly suggests 



16 PLANT MORPHOLOGY 

that the earliest forms of life may have been similarly undifferentiated 
and that from such a common ancestry both plants and animals may have 
arisen. Although the relationship of the Cyanophyceae to the flagellates, 
if any, is very obscure, most of the higher algal groups are thought to have 
been derived from flagellate ancestors. Where intermediate forms occur, 
such a derivation seems almost certain. 

Summary. The Eugienophyceae are a group of flagellates with 
bright green chloroplasts containing only chlorophyll and its associated 
carotinoids, the chlorophyll predominating, as in the green algae. Some 
members are colorless, these being either saprophytic or animal-like 
in their nutrition. All are unicellular and uninucleate, the cells being 
solitary or rarely in colonies. The cells have one or two cilia (flagella) 
that may be equal or unequal but are always attached anteriorly. Except 
in colonial forms, the cells are motile and lack a cell wall. Reserve food is 
stored as paramylon and often also as oil. Reproduction occurs by 
longitudinal fission. Resting cells (cysts) are commonly formed. Sexual 
reproduction is doubtful. 

3. CHRYSOPHYCEAE 

The Chrysophyceae, or golden-brown algae, are a small group number- 
ing only about 200 species and occurring mainly in fresh water. Their 
plastids contain chlorophyll and an excess of yellow and brown carotinoid 
pigments, giving them a golden-brown color. Most members are flagel- 
lates, being unicellular motile forms without a cell wall. The cells are 
solitary or in colonies and may be either free-swimming or attached. 
Motile cells have one or two, rarely three, cilia (flagella) attached anteri- 
orly. The two cilia may be equal or unequal in length. A few forms 
have a cell wall and are either filamentous or palmelloid, the latter with 
cells loosely held together in a gelatinous matrix. All members are 
uninucleate. Food is stored as oil or as leucosin, which is a protein-like 
substance of unknown composition. Some forms are colorless, while 
a few, with chlorophyll, may ingest soUd food. Reproduction occurs by 
fission, mainly longitudinal. A characteristic feature is the occurrence of 
cysts with a silicified cell wall having a small plug at one end. Zoospores 
may be produced in members that are not flagellates. Sexual repro- 
duction is of doubtful occurrence. 

Chromulina is a motile unicellular form (Fig. 6). Chrysamoeba is 
amoeboid. Symira and Uroglena are globular free-swimming colonies. 
Dinobryon has species in which the cells form a dendroid colony. Hydru- 
rus and Phaeocystis are palmelloid forms. Phaeothamnion is a branched 
filament and represents the highest type of organization attained by the 
group. 



THALLOFHYTA: ALGAE 



17 




Fig. 6. Group of golden-brown algae. A, Chromulina ovalis, X 1,450; B, Chrysamoeba 
radians, X960; C, Synura uvella, X600; D, Dinohryon sertularia, X900; E, Hydrurus 
foetidus, X480; F, Phaeothamnion confervicolum, X-440. (A and B, after Klebs; C, after 
Stein; D, after Senn; E, after Berthold; F, after G. M. Smith.) 

Another group composed almost entirely of flagellates are the Crypto- 
phyceae, yellow-green and brown forms that store food as starch or a 
related substance. Motile cells have two unecjual cilia (flagella). 
Reproduction occurs by fission. Sexual reproduction has been reported 
in only one species. The Cryptophyceae comprise only 30 species, 
mostly occurring in fresh water, and are rarely seen. In some respects 
they resemble the next class. 



18 



PLANT MORPHOLOGY 



4. DINOPHYCEAE 

The Dinophyceae comprise a group of nearly 1,000 species of organisms, 
most of which are known as dinofiagellates. Although some occur in 
fresh water, most of them are free-swimming marine forms. A few have 
naked protoplasts, but nearly all have sculptured walls of cellulose usually 

composed of a definite number of jointed 
plates (Fig. 7). All the dinofiagellates are 
unicellular and most of them are solitary ; 
some occur in chain-like colonies. The 
cells are small and generally have a pair 
of laterally attached cilia (flagella). A 
characteristic feature is the occurrence of 
two grooves, one encircling the cell trans- 
versely and the other extending longi- 
tudinally along one side. The cilia arise 
at the point of intersection of the grooves. 
One lies in the transverse groove and the 
other is directed backward. 

The dinofiagellates have a definite nu- 
cleus and usually a number of brownish 
yellow plastids, in which there is a predom- 
inance of carotinoids over the chlorophyll. 
Some are colorless. The colorless forms 
live either as saprophytes or as animals, 
the latter ingesting solid food particles. 
Some are parasitic. Reserve food occurs 
either as starch or as oil. Many of the 
dinofiagellates are phosphorescent. The 
prevailing method of reproduction is by 
fission, but some members produce zoo- 
spores. As in the other flagellates, cysts are often formed. Sexual repro- 
duction has been reported in only one member of the class. 

In addition to the dinofiagellates, the Dinophyceae include a few forms 
with a higher type of cellular organization, such as Gloeodinium, a palmel- 
loid form, and Dinothrix and Dinodadium, both of which are filamentous. 




Fig. 7. Ceratium hirudinella, a 
fresh-water dinoflagellate, X400. 



5. XANTHOPHYCEAE 

The Xanthophyceae,! or yellow-green algae, are a small but distinct 
group of only about 200 species characterized by having an excess of 



1 Also called Heterocontae. 



THALLOPHYTA: ALGAE 19 

yellow pigments, especially carotin, in their plastids. Xanthophijll and 
carotin are the two carotinoid pigments associated with chlorophyll in 
other green plants, but here their proportions are different. Although 
a few are marine, most yellow-green algae are found in fresh water. 
They are either unicellular or multicellular. Many are flagellates. The 
group was formerly classified with the Chlorophyceae, but seems to have 
had an independent origin from a flagellate ancestry and to have fol- 
lowed a line of evolution parallel to that of the green algae. Three rep- 
resentative genera are Chlorochromonas, Tribonema, and Botrydium. 

Chlorochromonas. This is a naked unicellular flagellate with two yel- 
low-green plastids (Fig. 8). It has two cilia (flagella) of unequal length 
attached anteriorly, a contractile vacuole, and a cen- . 

tral nucleus. It stores food as leucosin and probably 
also as oil. A leucosin granule, contained in a vacuole, 
lies at the posterior end of the cell. Reproduction 
takes place by fission. From such a form as Chloro- 
ckromonas, the other Xanthophyceae appear to have 
evolved. 

Tribonema. This is a filamentous alga, widely 
distributed in fresh-water pools (Fig. 9). The fila- 
ments are unbranched and composed of elongated 
cylindrical cells. The walls are made up of two over- 
lapping pieces that appear H-shaped in a longitudinal 
section. The cells contain a nucleus and a number of chromonas minuta, 
yellow-green plastids. Asexual reproduction occurs x 2,000. {After 
by the formation of aplanospores, akinetes, or zoo- 
spores. Aplanospores are nonmotile spores with a wall distinct from the 
wall of the parent cell. Akinetes are also nonmotile but are derived from 
an entire vegetative cell whose wall becomes the wall of the spore. Zoo- 
spores are ciliated and naked. In Tribonema one or more aplanospores 
may be produced within a cell, while the zoospores are usually formed 
singly. Sexual reproduction, which is rare, takes place by the fusion of 
isogametes formed in ordinary cells. Usually one gamete settles down 
before the other unites with it. The motile cells have two cilia of unequal 
length, attached anteriorly, and the reserve food is stored as oil or leu- 
cosin, never as starch. 

Botrydium. Botrydium is a terrestrial alga often found on wet muddy 
flats. The vegetative body is unicellular and multinucleate, consisting 
of a balloon-shaped bladder about 1 to 2 mm. in diameter (Fig. 10). It 
is fastened to the soil by means of branched colorless rhizoids. The 
cytoplasm, containing many nuclei and, in the aerial portion, numerous 
yellow-green plastids, forms a thin layer lining the cell wall and enclosing 




20 



PLANT MORPHOLOGY 



a large central vacuole. Such a multinucleate body, without any cross 
walls, is called a coenocyte. Food is stored as oil or leucosin. 

Asexual reproduction may occur either by zoospores or aplanospores. 
When covered with water, the entire aerial portion may release numerous 
uninucleate zoospores through a terminal pore. The zoospores have two 




D 




Fig. 9. Tribonema homhycinum, a yellow-green 
alga. A, portion of vegetative filament; B, aplan- 
ospores; C, zoospores; D, structure of cell wall, as 
revealed by special treatment. {A, B, C, after Gay; 
D, after Bohlin.) 



Fig. 10. Botrydium 
granulatum, showing 
balloon-shaped aerial 
portion and branched 
subterranean portion, 
X20. 



cilia of unequal length, attached anteriorly. They may either germinate 
immediately or form a wall and go into a resting stage. In the absence 
of sufficient moisture, the aerial portion may give rise to aplanospores or 
all the cytoplasm may move into the rhizoidal portion and there produce 
aplanospores. The aplanospores of Botrydium may be either uninucleate 
or multinucleate and, after a dormant period, may give rise either to zoo- 
spores or to new plants directly. Sexual reproduction is accomplished 
by small biciliate isogametes, each with a single nucleus, that fuse to 



THAU/)PHVTA: ALGAE 



21 



form thick-walled zygotes. These germinate immediately, giving rise 
directly to a new vegetative body. Sometimes the gametes conjugate 
before being liberated. 

6. BACILLARIOPHYCEAE 

The Bacillariophyceae,* or diatoms, constitute an isolated group whose 
relationships to the other algae are very uncertain. They include over 
5,000 species of unicellular plants occurring almost universally in fresh 
and salt water, as well as on damp soil. Some of the more common 
genera are Melosira, Coscinodiscus, Bid- 
dulphia, Pinnularia, Surirella, Cocconeis, 
Navicula, and Pleurosigma. Diatoms 
may be either free-floating or attached. 
Frequently they form slimy brown coat- 
ings on mud at the bottom of shallow 
bodies of water, as well as on sticks, 
stones, shells, other aquatic plants, etc. 
That they were more numerous in geologic 
times is shown by the great accumula- 
tions of diatomaceous earth found in var- 
ious parts of the world. This consists of 
the shells (cell walls) of dead diatoms. 
Deposits of diatomaceous earth were 
formed mainly during the Tertiary, but 
the fossil record of diatoms extends as 
far back as the Jurassic. 

Although most diatoms are solitary, 
some form colonies of diverse types, the 
individuals being held together by a 
sheath of mucilage. Their color, usually 
a golden brown, is due to the presence of 
chlorophyll in association with an excess 
of carotinoids, particularly carotin and 
several brown xanthophyll pigments. 
Diatoms are distinguished from other algae 
by their silicified cell wall. This consists 
of two valves, one overlapping the other like the lid and bottom of a 
pillbox (Fig. 11). The place where the valves overlap is called the girdle. 
The cell wall is composed mainly of pectin impregnated with a large 
amount of silica. It is variously marked with numerous fine transverse 
lines that form regular and elaborate patterns. These make diatoms 
among the most striking and beautiful objects to be seen under the 

1 Also called Diatomeae. 




A B 

Fig. 11. Two views of the shell of 
Pinnularia viridis. A, girdle view; 
B, valve view; g, girdle; pn, polar 
nodule, en, central nodule; r, raphe. 
{After Pfitzer.) 



22 



PLANT MORPHOLOGY 




Fig. 12. Group of common diatoms. A, Triceratium; B, Aitlacodiscits; C, Isthmia; D, 
Sitrirella, E, Navicula; F, Amphipleura; G, Pleurosigma. {Adapted from a Turtox classroom 
chart.) 



THALLOPHYTA: ALGAE 



23 



microscope. A good microscope will show that the striations on the silici- 
fied cell wall generally consist of rows of very minute pores. They appear 
as lines because the pores are very close together.^ 

Two views of a diatom are possible — girdle (side) view and valve 
(top) view (Fig. 11). Many diatoms possess a raphe, which is a longitu- 
dinal slit extending down the center of the valve. Such forms have the 
power of locomotion, movement apparently being accomplished by a 
streaming of protoplasm along the raphe. 

The Bacillariophyceae comprise two orders: (1) the Centrales, which 
are radially symmetrical in valve view, often circular, and have no raphe 




Fig. 13. Triceratium, a large marine diatom, as seen in optical section, X400. The 
nucleus lies in the center of the cell, while numerous small plastids lie just inside the cell wall. 

(Fig. 12A-C) and (2) the Pennales, which are usually bilaterally sym- 
metrical, not circular, and generally have a raphe (Fig. \2D-G). The 
difference in symmetry is clearly shown by the pattern of markings on 
the valves, being radial in the Centrales and bilateral in the Pennales. 

In most diatoms the nucleus is suspended in the center of the cell by 
slender strands or by a broad transverse band of cytoplasm connected 
with a thin laj^er lying next to the cell wall (Figs. 13 and 14/1). Embed- 
ded in the peripheral layer are one or more plastids that are usually 
brown, frequently yellow, or rarely green. In the Centrales the plastids 
are small and numerous. In the Pennales they are large and few in 
number; commonly there are two. The plastids of diatoms vary greatly 
in shape, being often irregular and sometimes elaborately lobed. The 
cell contains no starch, food being stored mainly as oil. 

1 Some diatoms have striae so fine that they are used as test objects in determining 
the efficiency of microscope lenses. A good oil immersion objective will resolve mark- 
ings that are as fine as five striae to the micron. 



24 



PLANT MORPHOLOGY 



Reproduction occurs chiefly by fission, the cell always dividing in the 
plane of the valves (Fig. 14). The two valves separate and each daugh- 
ter protoplast forms a new wall on its naked side, the new wall fitting 
inside the old one. One of the cells is always as large as the parent cell, 





B 




Fig. 14. Cell division in Surirella calcarata, X275. 
Laiderborn.) 



A to D, successive stages. {After 



but the other is smaller. Thus, as cell divisions continue, some of the 
individuals become constantly smaller. After a minimum size for the 
species has been reached, the original size is regained through the forma- 
tion of auxospores. 

In the Centrales an auxospore is formed by the escape from its cell 
wall of a protoplast that soon grows to the original size and develops a 
new cell wall. An auxospore may directly become a new individual or 
may form two new individuals by dividing in half. In most of the 



THALLOPHYTA: AWAE 



25 



Pennales auxospore formation is due to a fusion of cells. In some forms 
two vegetative protoplasts escape and conjugate to produce a single 
auxospore (Fig. 15). In other forms two diatoms unite to produce two 
auxospores. Here the two fusing cells lie within a gelatinous matrix and 
each produces two gametes. Then each of the gametes derived from 
one cell conjugates with one of those derived from the other cell. It is 
apparent that an "auxospore" formed by sexual fusion is really a zygote. 





Fig. 15. Conjugation in Cocconeis placentula, X 1,500. A and B, meiosis in conjugating 
cells; C, fusion of protoplasts; D, zygote with sexual nuclei not yet fused. (After Geitler.) 

Just previous to conjugation the nucleus of each of the pairing proto- 
plasts undergoes a reduction of chromosomes, giving rise to four haploid 
nuclei. Some of these degenerate. 

In some of the Centrales many small biciliate protoplasts arise within 
a vegetative cell and later escape into the water. These have been 
called "microspores." Some observers think that they function as zoo- 
spores, while others regard them as gametes, claiming that they fuse in 
pairs. The occurrence of these ciliated cells in the Bacillariophyceae 
suggests that the group may have been derived from flagellates with 
brown plastids. The connection, however, is a remote one. 

7. CHLOROPHYCEAE 

The Chlorophyceae, or green algae, are predominantly fresh-water 
forms whose plastids contain a preponderance of chlorophyll over its 
associated carotinoids, the green and yellow pigments occurring in 
approximately the same proportions as in the groups above the thal- 
lophyte level. ^ Onlj^ a comparatively few members are marine, but 

' In a few members accessory pigments in the cell sap may mask the green color of 
the chloroplasts. 



26 PLANT MORPHOLOGY 

some of these are widely distributed and often abundant. Some green 
algae grow as scums on the surface of quiet water, while others are 
attached to various objects beneath the surface. A few forms grow on 
moist soil, rocks, or tree trunks. Most of the green algae are multicellular 
but some are unicellular, these occurring either as isolated cells or as 
colonies. The Chlorophyceae are generally regarded as the group of 
algae from which the bryophytes and other higher groups of green plants 
have been derived. Lime-secreting forms are known as fossils as far 
back as the Ordovician. The Chlorophyceae number over 5,000 species, 
nearly all of which are included in seven principal orders: Volvocales, 
Chlorococcales, Ulotrichales, Oedogoniales, Conjugales, Siphonocla- 
diales, and Siphonales. 

1. Volvocales 

The Volvocales are a distinct group of primitive green algae that are 
widely distributed in fresh water. Only a few members are marine. 
They appear to have been derived from green flagellates, which they 
resemble in many ways, and to have given rise, in turn, to the other 
groups of Chlorophyceae. The Volvocales include about 50 genera and 
300 species. The main genera are Chlamydomonas, Sphaerella, Gonium, 
Pandorina, Eudorina, and Volvox. 

Chlamydomonas. This is a unicellular alga that does not form per- 
manent colonies. It is widely distributed in pools and ditches and on 
damp ground. The vegetative cell, which is free-swimming, is generally 
spherical or egg-shaped (Fig. 16A). A cell wall is always present. At 
the anterior end are a pair of cilia, equal in length, a red eyespot, and 
two (rarely more) small contractile vacuoles. Surrounding the nucleus 
is a small m.ass of colorless cytoplasm lying in the depression of a large 
cup-shaped chloroplast. Embedded in the chloroplast is a conspicuous 
spherical pyrenoid. Pyrenoids are protein bodies that function as centers 
of starch formation. Although occurring in some members of certain 
other algal groups, they are especially characteristic of the Chlorophyceae. 

Chlamydomonas reproduces asexually by means of zoospores. The 
vegetative cell becomes quiescent by retraction of the ciha and then its 
protoplast divides internally to form two, four, or eight daughter proto- 
plasts, each of which, after enlarging slightly, forms a new cell wall and a 
pair of cilia while within the parent cell (Fig. 165, C). By the breaking 
down of the original cell wall, the small cells are set free as zoospores, 
each soon undergoing further enlargement to become an adult vegeta- 
tive cell (Fig. 16-D). Under conditions unfavorable for vegetative activ- 
ity, Chlamydomonas may pass into a "palmella" stage. The daughter 
cells, produced by the internal division of a vegetative cell, increase in 
number but, instead of escaping, become surrounded by abundant muci- 



THALLOPHYTA: ALGAE 



27 



lage derived from the cell walls (Fig. XQE). Later, when favorable con- 
ditions return, the cells develop cilia and swim out of the mucilaginous 
matrix. 

Sexual reproduction in Chlamydomonas occurs by the union of similar 
gametes. These arise from a (luiescent vegetative cell by division of its 
protoplast into 16 or 32 daughter protoplasts (Fig. 16F). The gametes 
are smaller than the zoospores and are usually without a cell wall, but 




Fig. 16. Chlamydomonas, a free-swimming, unicellular green alga, X 1,000. A, vegetative 
cell, showing large cup-like chloroplast with embedded pyrenoid, nucleus, eyespot, two 
contractile vacuoles, and two cilia; B and C, formation of zoospores within parent cell 
wall; D, two escaped zoospores; E, "palmella" stage; F, formation of gametes; G, two 
escaped gametes; H, gametes fusing; /, zygote; /, four zoospores escaping from zygote. 

otherwise have the same structural features. They escape and swim 
about in the water. Finally, they come together in pairs and fuse, each 
pair forming a zygote (Fig. 16G, H). The zygote soon loses its cilia, 
secretes a heavy wall about itself, and goes into a resting stage (Fig. 16/). 
While the wall is forming, the two nuclei inside the zygote unite. Upon 
germination, the protoplast of the zygote divides internally to form four 
zoospores that escape and enlarge to become new vegetative cells (Fig. 
16J). The reduction in chromosome number from the diploid to the 
haploid state occurs in connection with the formation of the four zoo- 
spores from the zygote. Because, in most species, the pairing gametes 
are alike in size, Chlamydomonas is said to be isogamous. The fusing of 
similar gametes (isogametes) is known as conjugation. 

In Chlamydomonas eugametos there are two sexually differentiated 



28 



I'LANT MORPHOLOGY 



strains, designated as plus and minus. A zygote may be formed only 
by the union of a plus gamete with a minus gamete. Of the four zoo- 
spores arising from the zygote, two belong to the plus strain and two 
to the minus. These strains may soon undergo another sexual fusion 
or may be perpetuated asexually for an indefinite period. In Chlamy- 
domonas hraunii and a few other species the gametes of the plus strain 
are slightly larger than those of the minus strain, and so here a visible as 
well as a physiological sexual differentiation exists. Such species show 
that Chlamijdomonas displays a slight tendency toward heterogamy. 





A ' B 

Pig. 17. Two species of Gonium, X900. A, side and top views of four-celled colony of 
Gonium sociale; B, top view of sixteen-celled colony of Gonium pectorale. 

In Sphaerella, a close relative of Chlamijdomonas and common in rain- 
w^ater pools, the inner portion of the cell wall is gelatinous and thick and 
is traversed by many delicate cytoplasmic strands. Generally it con- 
tains a bright red pigment, haematochrome , that masks the chlorophyll. 
This is present in the cell sap. 

Gonium. This is a colonial form, each colony consisting of a flat plate 
of cells numbering either four or sixteen, according to the species (Fig. 17). 
The cells are regularly arranged and held together by a mucilaginous 
matrix derived from their cell walls. Each cell is biciliate and otherwise 
similar to an adult Chlamijdomonas. By division of its protoplast, any 
cell may form a new colony that escapes from the parent cell. Sexual 
reproduction occurs by the fusion of similar gametes (isogametes), the 
two coming from separate colonies. The number of gametes formed in 
a cell corresponds to the number of cells in the colony. They escape 
separately. The zygote becomes thick-walled and dormant. Later it 
produces four biciliate zoospores. In the four-celled species these usu- 
ally remain together as a colony; in the sixteen-celled species they separate 
and each forms a new colony. 

Pandorina. This form is similar to Gonium except that the colony is 
spherical or nearly so and consists usually of 16 biciliate cells crowded 



THALLOPHYTA: ALGAE 



29 



together within a mucilaginous matrix and surrounding a small central 
cavity (Fig. 18A). Sometimes the colony consists of only 8 cells or, less 
frequently, of 32 cells. Each cell resembles that of Chlamydomonas. In 
asexual reproduction each cell divides simultaneously to produce a group 
of as many daughter cells as were in the parent colony (Fig. ISB). Each 
group then escapes as a new colony. In sexual reproduction each vegeta- 
tive cell similarly produces a group of daughter cells as numerous as the 
cells in the colony, the groups separate, and the daughter cells escape 
individually as biciliate gametes. Although Pandorina is isogamous, one 




Fig. 18. Pandorina morum, X750. A, free-swimming vegetative colony of 16 cells, 
those lying below not shown; B, colony undergoing ase.xual reproduction; C, a large and a 
small gamete; D, gametic union; E, a zygote. 

of the fusing gametes is slightly larger and less active than the other, thus 
showing a tendency toward heterogamy (Fig. 18C, D). The zygote 
remains motile for a while, finally settling down and secreting a cell wall 
(Fig. 18E). Upon germination the zygote divides internally into four 
protoplasts, but generally only one becomes a zoospore. The zoospore 
produces a new colony. 

Eudorina. Eudorina is a spherical colony usually consisting of 16, 32, 
or 64 biciliate cells, each like a cell of Chlamydo?nonas. The cells are 
loosely arranged in a single layer near the surface of a mucilaginous 
matrix. As in the preceding genera, any cell may give rise to a new 
colony by internal division of its protoplast, but an advance is seen in 
sexual reproduction (Fig. 19). Some of the cells divide to form groups 
of sperms, as many as 64 usually arising from a single vegetative cell. 
The other cells enlarge slightly by the accumulation of food and become 
eggs. Although both male and female gametes are biciliate, only the 
sperms escape from the colony and become free-swimming. At first the 



30 



PLANT MORPHOLOGY 



sperms hang together as a plate, but finally separate and fuse with the 
eggs. The union of a sperm and egg, called fertilization, results in 
the formation of a thick-walled resting zygote. Upon germination, 
the zygote produces four biciliate zoospores but only one functions, the 
other three degenerating inside the zygote. Because the pairing gametes 
are differentiated into sperms and eggs and are therefore unlike, Eudorina 




Fig. 19. Eudorina elcgans. A, colonj' of 32 cells, many of which are dividing to form 
daughter colonies, X 500; B, a female colony surrounded by numerous sperms, two groups of 
which are still intact, while others, having separated, are uniting with the eggs. (After 
Gocbel.) 



is heterogamous. Some species show a further degree of sexual differentia- 
tion in being dioecious. Here all the cells in the male colony give rise 
to sperms, while all those in the female colony become eggs. 

Volvox. This is the most highly developed member of the Volvocales. 
It lives in quiet bodies of fresh water, especially pools, ponds, and lakes. 
It consists of a hollow globular colony composed of hundreds or some- 
times thousands of biciliate cells embedded in mucilage and arranged in 
a single layer (Fig. 20A). Often the colony reaches a diameter of 
nearly 2 mm. It is free-swimming, as in other members of the order. 
Each cell is like an adult Chlamijdomonas, with two cilia, an eyespot, 
contractile vacuoles, a nucleus, and a single chloroplast with a pyrenoid. 
In most species the cells are connected by very fine protoplasmic strands, 
and thus the colony approaches the multicellular condition of organiza- 
tion. This is also shown by the fact that most of the cells function only 
vegetatively during the entire life of the colony, while others become 



THALLOPHYTA: ALGAE 



31 



reproductive cells. Such a "division of labor" is not seen in lower mem- 
bers of the order. 

Volvox reproduces asexually by the formation of new colonies inside 
the old one. A few of the vegetative cells, seldom over 10 or 12, retract 





B 






Fig. 20. Volvox. A, mature colony with young colonies inside; B, j'oung colony in rim 
of mature colony; at the right, a vegetative cell has lost its cilia and is starting to form a new 
colony; C, a group of sperms derived from a single vegetative cell, one of which, to the left, 
has lost its cilia and is enlarging; D, an egg shortly before fertilization and, to the left, an 
egg beginning to develop from a vegetative cell; E, a mature zygote; A, X170; B to E, 
X780. {After Chamberlain.) 

their cilia and increase slightly in size. Each divides to form a small 
group of cells that enter the colony and give rise to a new colony, remain- 
ing inside until the old colony dies (Fig. 20B). 

In sexual reproduction, Volvox is heterogamous. Any cell may retract 
its cilia, enlarge by the accumulation of food, and become an egg (Fig. 
20D). Another cell may enlarge and, at the same time, divide to form 
many small biciliate sperms (Fig. 20C). These arise as a hollow sphere 
or plate of cells that later separate. The sperms and egg? escape into 



32 PLANT MORPHOLOGY 

the colony and there fertilization occurs. The zygote becomes heavy- 
walled and remains dormant for several months (Fig. 2()E). In some 
species it then gives rise to a single biciliate zoospore, while in others it 
forms a new colony directly. In connection with the germination of the 
zygote, the number of chromosomes is reduced one-half. As is Eudorina, 
some species of Volvox are monoecious, others dioecious. 

Summary. The Volvocales are distinguished from the other Chlo- 
rophyceae by the fact that their vegetative cells are cihated and motile. 
They exhibit a range of development from single isolated cells to com- 
plex globular colonies. Each cell has one nucleus and generally one 
chloroplast. Asexual reproduction occurs by zoospores and by the 
formation of a new colony from a single parent cell. The number of 
cells in the colony is definite and is determined during early development. 
It is not subsequently increased by vegetative cell divisions. Sexual 
reproduction shows an advance from isogamy to heterogamy, while 
dioecism is attained by some species of Eudorina and Volvox. 

2. Chlorococcales 

The Chlorococcales constitute a large order of diverse forms that are 
probably not closely related. They are chiefly fresh- water algae, only a 
few occurring in the ocean. Some live in moist places on land. Some 
are endophytic in the intercellular spaces of certain seed plants, while 
others live symbiotically in the lower animals. Others are lichen formers. 
The order contains 90 genera and approximately 700 species. Some 
characteristic genera are Chlorococcum, ChloreUa, Scenedesmus, Pedias- 
trum, Hydrodictyon, and Protosiphon. 

Chlorococcum. This simple alga grows on damp soil or rocks. It is 
unicellular, spherical, and nonmotile. At first it has a single nucleus 
and a large cup-hke chloroplast with one or more pyrenoids (Fig. 21). 
Later the cell becomes multinucleate and the protoplast divides to form 
a variable number of bicihate zoospores that escape. After coming to 
rest, a zoospore loses its ciha, secretes a wall, and becomes a vegetative 
cell. Asexual reproduction may also occur by aplanospores. These 
arise in the same way as zoospores but have no ciha and develop a cell 
wah before being freed. As in Volvox, a "palmella" stage may develop 
by gelatinization of the cell walls in a group of cells. Sexual reproduction 
is accomplished by the production of a large number of bicihate isoga- 
metes by a vegetative cell. These escape and fuse in pairs. In general, 
Chlorococcum is like Chlamydomonas except that the vegetative cells have 
lost their motility. 

ChloreUa. ChloreUa lives on the bark of trees, damp walls, and soil; 
also in various infusoria, the fresh-water sponge, and the green hydra. 
It can be grown easily in water cultures and is much used in experiments 



THALLOPHYTA: ALGAE 



33 



on photosynthesis. The cells are spherical and solitary. They have a 
single nucleus and a cup-shaped peripheral chloroplast usually without 
a pyrenoid. Chlorella resembles Chlorococcum except that it produces 
only aplanospores, no motile cells of any kind. A protoplast divides to 
form as many as 16 daughter protoplasts, each of which, before escaping, 
secretes a cell wall. Gametes are unknown. 




Fig. 21. Chlorococcum infusionum. A, section of vegetative cell with single nucleus and 
pyrenoid; B, multinucleate stage; C, appearance of cleavage furrows, isolating uninucleate 
protoplasts with a pyrenoid fragment; D, section of nearly mature sporangium; E, escape 
of zoospores in a gelatinous vesicle; F, two zoospores; A, X 2,000; others, X 2,700. {After 
Bold.) 

Scenedesmus. This alga is common and widely distributed in fresh 
water. It is a colonial form with generally four or eight cells arranged 
in a short row (Fig. 22). The end cells often bear conspicuous spine- 
like projections. Each cell contains a single nucleus, a large peripheral 
chloroplast, and a pyrenoid. In reproduction, a protoplast divides 
within its own cell wall to form a new colony that escapes as a whole. 
Neither zoospores nor gametes are produced. 

Pediastrum. Pediastrum is a free-floating form widely distributed 
in fresh water. It consists of a colony of cells symmetrically arranged 
in a flat plate (Fig. 23). The number of cells may be 2, 4, 8, etc., up to 
128, but is most commonly 16 or 32. The cells are nearly all alike, 
except that the peripheral ones often bear short spine-like projections. 



34 



PLANT MORPHOLOGY 



In young colonies the cells are uninucleate but later become multi- 
nucleate (coenocytic), as many as eight nuclei being present. Young 
cells have a single peripheral chloroplast with one pyrenoid, while older 
cells have several pyrenoids, the chloroplast becoming diffuse. 

In asexual reproduction a protoplast divides generally into as many 
daughter protoplasts as there are cells in the colony, but often into twice 
as many. These become biciliate zoospores that escape as a group 
enclosed in a common membrane (Fig. 24A). The zoospores then come 
together and form a new colony within the membrane (Fig. 24i?, C). 





Fig. 22. Four-celled colony 
of Scenedesmus, X 750. Each 
cell contains a small nucleus 
and a large peripheral chloro- 
plast with a pyrenoid. 



Fig. 23. Young colony of Pediastrum 
horyanum, its cells forming a plate, X 750. 
Some of the cells have become binucleate. 
Each has a peripheral chloroplast and a 
pyrenoid. 



Sexual reproduction also takes place, Pediastrum being isogamous. 
Division of a vegetative protoplast results in the formation of many 
biciliate gametes. These escape separately and fuse in pairs to form 
zygotes. After increasing in size, the zygote gives rise to a group of 
zoospores. These escape into the water, swim freely and, after coming 
to rest, develop into thick- walled polyhedral cells (Fig. 24Z)). The 
polyhedrons enlarge and divide internally to form a group of zoospores 
that escape in a common membrane, within which they construct a new 
colony by coming together without further division (Fig. 2^E-G). 
Hydrodictyon. This remarkable alga, common in fresh water, is 
a free-floating colony having the form of a large hollow net, the polyg- 
onal meshes of which are made up of elongated cylindrical cells arranged 
end to end (Fig. 2b A). Each mesh consists usually, but by no means 
always, of six cells. A single colony may reach a length of 20 to 30 cm. 
At first each cell contains a single nucleus and a chloroplast with one 
pyrenoid, but later there are many nuclei and a large number of pyre- 



THALLOPHYTA: ALGAE 



35 



noids, the chloroplast becoming reticulate and diffuse (Fig. 255). 
Mature cells have a peripheral layer of cytoplasm surrounding a large 
central vacuole. In asexual reproduction as many as 7,000 to 20,000 
biciliate zoospores may arise from a single vegetative cell by progressive 
cleavage of its protoplast. These do not escape but swim around 
within the parent cell, finally coming together to form a new net (Fig. 
25C). Later the cell walls of the old net dissolve and the young colonies 
are set free. These grow to the adult size without any cell division. 




Fig. 24. Pediastrum horyanum. A, formation of zoospores and escape of one group in a 
common vesicle; B and C, zoospores forming a new colony; D, a thick-walled resting cell 
(polyhedron); E, F, G, zoospores within a polyhedron forming a new colony. {A, B, C, 
after A. Braun; D to G, after Askenasy.) 



In sexual reproduction a single protoplast may give rise to as many as 
30,000 to 100,000 biciliate isogametes. These escape from the parent 
cell through a small pore and fuse in pairs to form thin-walled zygotes 
(Fig. 25D--F). After undergoing a short resting period, the zygote 
turns green and increases in size. It then produces four large zoospores 
and, in connection with their formation, the number of chromosomes 
is reduced one-half (Fig. 25G, H). As in Pediastrum, the zoospores 
escape into the water, settle down, and become large heavy-walled 
polyhedrons (Fig. 25/). These remain dormant until the following 
spring and represent the real resting stage. Upon germination, a poly- 
hedron produces 200 to 300 small zoospores that escape enclosed in a 
membrane, where they arrange themselves to form a new net (Fig. 25 J). 
These nets are much smaller than the ones developed later by the zoo- 
spores arising within the vegetative cells of the colony. 



36 



PLANT MORPHOLOGY 



Protosiphon. Protosiphon is a unicellular coenocytic alga occurring 
on damp earth. It shows a striking resemblance to Botrydium, one of 
the Xanthophyceae, and often grows with it in the same habitat. The 
plant has a green aerial portion that is tubular or bladder-like and a 




Fig. 25. Hydrodictyon reUcnlaium. A, portion of colony, X 150; B, single cell with many 
nuclei and pyrenoids, X350; C, young net formed within a parent cell; D, a gamete; E, 
gametes fusing; F, zygote; G, four zoospores escaping from zygote; H, a zoospore escaped 
from zygote; /, polyhedron formed by a zoospore; /, young net escaping from polyhedron. 
(C to F, after Klebs; G to J, after Pringsheim.) 

colorless underground portion that resembles a rhizoid (Fig. 26A). 
It is entirely without cross walls. The cytoplasm, in a thin layer sur- 
rounding a large central vacuole, contains numerous scattered nuclei 
(Fig. 26B). When young, the cell has a large reticulate chloroplast 



THALLOPHYTA: ALGAE 



37 



with many pyrenoids; later there may be several chloroplasts. Reserve 
food occurs chiefly as starch. The aerial portion may bud off new plants 
that later become detached. When covered with water, the protoplast 
may give rise to a number of biciliate zoospores or isogametes that escape 




E ^-^ F ^<=^ G H 

Fig. 26. Protosiphon hotryoides. A, longitudinal section of vegetative plant; B, upper 
portion, showing scattered nuclei; C, an older stage, the cytoplasm undergoing progressive 
cleavage; D, formation of zoospores; E and F, gametic union; G, a zygote; H, germinating 
zygote with four nuclei. (After Bold.) 

through an apical pore (Fig. 26C-H). Gametes from the same plant may 
pair and fuse. The zj^gote, which becomes thick-walled and dormant, 
produces a new plant directly. If the soil becomes dry, the vegetative 
protoplast may form aplanospores by progressive cleavage of the cyto- 
plasm. These may be either small and uninucleate or larger and multi- 
nucleate. The latter, upon germination, usually give rise to zoospores or 
isogametes, but may develop into a new vegetative plant directly. 



38 



PLANT MORPHOLOGY 



Summary. The Chlorococcales range from simple isolated cells to 
complex colonies. In this and succeeding orders nonmotility is the 
permanent condition of the vegetative cells. Although usually uninu- 
cleate, frequently these are multinucleate and often contain more than 
one chloroplast. Colonies are formed by the coming together of free 
cells (usually zoospores) derived from a single parent cell and there 
is no subsequent division of vegetative cells. Cell division occurs 
only in connection with the formation of reproductive cells. Reproduc- 
tion is accomplished by zoospores, aplanospores, or akinetes, and usually 
also by isogametes. 

3. Ulotrichales 

The Ulotrichales have been called the representative group of the 
Chlorophyceae. Most of them live in fresh water but some are marine. 



.Q.-- 



-<3' 



•.--G>.0;: 



K® 



®-. 



'01 



feiO- 



<^ 



iSti 




ABC 

Fig. 27. Ulothrix zonata, vegetative and reproductive stages, X700. A, basal portion of 
filament, showing holdfast cell and three vegetative cells, each with a single nucleus and a 
peripheral band-like chloroplast with many pyrenoids; B, formation and escape of zoospores; 
C, formation and escape of gametes, some of which are pairing. 

A few live in damp places on land. Trichophilus grows inside the hair of 
the South American sloth. To this order belong 85 genera and approxi- 
mately 500 species, the principal genera being Ulothrix, Chaetophora, 
Draparnaldia, Stigeoclonium, Protococcus, Ulva, and Coleochaete. 

Ulothrix. This alga is of widespread occurrence in streams, lakes, 
and ponds, where it grows attached to objects in the water. A few 
of its species are marine. The plant body is multicellular, consisting 
of a simple unbranched filament (Fig. 27 A). The basal cell is elongated 



THALLOPHYTA: ALGAE 39 

and modified to serve as a holdfast, but all the other cells are alike, 
being shortly cylindrical. Each contains a central nucleus and a periph- 
eral band-like chloroplast usually with many pyrenoids. The chloro- 
plast may form either a complete or a partial band. Any cell in the 
filament, except the basal one, may divide by the formation of a cross 
wall between two daughter protoplasts, thus resulting in growth of the 
filament. 

In asexual reproduction, the contents of any vegetative cell, except 
the holdfast, may divide to form mostly 2, 4, 8, or 16 zoospores (Fig. 21 B). 
These escape through a pore in the cell wall and swim by means of four 
equal cilia attached apically. When discharged, the zoospores are 
enclosed in a common membrane that soon disappears. As in the 
vegetative cells of the Volvocales, each zoospore has a red eyespot and a 
contractile vacuole. After a period of free swimming, a zoospore comes 
to rest, withdraws its cilia, and secretes a cell wall. It then gives rise to 
a new filament by repeated cell divisions. Sometimes aplanospores are 
formed instead of zoospores. They frequently germinate within the 
parent cell. 

Sexual reproduction takes place in Ulothrix by the conjugation of 
isogametes (Fig. 27C). These originate from the vegetative cells 
in the same way as the zoospores do, but are smaller, more numerous 
(usually 32 or 64 in a cell), and have only two cilia. They escape 
through a pore in the cell wall, enclosed in a common membrane that 
soon breaks down. Following pairing and fusing of the gametes, the 
resulting zygotes secrete a heavy wall and generally do not germinate 
until the following spring. Then each produces 4 to 16 zoospores 
(or sometimes aplanospores) that, in turn, give rise to new vegetative 
filaments. The zygote is the only diploid cell in the life history. When 
its nucleus divides, the chromosome number is reduced one-half. 
Although isogamous, Ulothrix shows some degree of sexual differentiation 
in that the gametes of one filament fuse only with those of another. 

Chaetophora. Some of the I lotrichales are branching filaments, 
often with cells showing a differentiation in size. One such member is 
Chaetophora, frequently found in standing water attached to submerged 
objects. The cells of the branches become progressively smaller and 
end in hair-like appendages that taper to a point. In a closely related 
form, Draparnaldia, common in clear, cool streams, the cells of the main 
filament are much larger than those of the branches (Fig. 28). Cell 
structure and reproduction in both genera are much the same as in 
Ulothrix. Stigeoclonium, another relative of Chaetophora, is differentiated 
into a cushion-like basal portion from which arise a number of sparingly 
branched upright filaments. When exposed to dry conditions, the cells 
round off and separate, giving rise to a "palmella" stage. These cells 



40 



PLANT MORPHOLOGY 



are thiek-walled and divide in any plane. They may remain in groups or 
become separate. With the return of favorable conditions, they produce 
a new filamentous body. 

Protococcus. One of the commonest and most widely distributed 
of the green algae is Protococcus,^ a terrestrial form growing on the shaded 
side of damp tree trunks, moist rocks, walls, etc. It is a unicellular alga, 




Fig. 28. Drapamaldia, portion of plant, a branching filament with a marked differentiation 
in size of vegetative cells, X 200. Each cell has a central nucleus obscured by the peripheral 
band-like chloroplast with many pyrenoids. 



consisting of a spherical protoplast enclosed by a cell wall (Fig. 29). 
It has a small nucleus and a large, peripheral, irregularly lobed chloroplast 
usually without pyrenoids. Reproduction occurs entirely by cell 
division, spores and gametes being unknown. Permanent colonies are 
not formed but, instead of separating immediately, the cells usually hang 
together temporarily in small groups. In the presence of excessive mois- 
ture, the number of cells in a group is greatly increased and sometimes 
some of them grow into short filaments. 

In most unicellular algae the division of a cell involves the formation of 
a new cell wall completely around each daughter protoplast and the 
disintegration of the wall of the parent cell. In Protococcus, however, a 

' Often called Pleurococcus. 



THALLOPHYTA: ALGAE 



41 



cross wall is developed across the parent cell, a method characteristic of 
Ulothrix and multicellular algae in general. If a second wall appears in 
one or both of the daughter cells before they separate, it comes in at right 
angles to the first one. Later divisions may be in the third plane. Thus 
there is a slight tendency in Protococcus toward the development of a 
multicellular body. 

Protococcus is now generally regarded, not as a primitive form, but as 
one that has become reduced from more highly developed ancestors, 
probably as a result of its terrestrial mode of life. This is indicated by 










Fig. 29. Protococcus viridis, a unicellular green alga, X 1,000. Some of the cells have 
divided to form small temporarj' groups. Each cell has a central nucleus and a peripheral 
lobed chloroplast. 

its advanced method of cell division combined with a failure to develop an 
extensive multicellular plant body like that of other Ulotrichales and by 
the absence of zoospores and gametes, which even such truly primitive 
forms as Chlamydomonas possess. 

Ulva. This is a widely distributed marine alga commonly known as 
"sea lettuce." It grows along seacoasts between the high- and low-tide 
lines. The vegetative body consists of a plate-like thallus two layers of 
cells in thickness (Fig. 30). It is attached to rocks and other objects in 
the water by means of a basal holdfast consisting of long colorless rhizoids. 
The thallus may reach a length of 30 cm. or more. Each cell is uni- 
nucleate and has a single chloroplast with a pyrenoid. 

Reproduction in Ulva closely resembles that of Ulothrix. Zoospores 
arise from ordinary vegetative cells situated along the thallus margin, four 
or eight zoospores being produced in each cell. They are liberated into 
the water through an opening in the cell wall and swim by means of four 
cilia. Upon germination, a zoospore gives rise to a plant that produces 
only gametes. These are smaller than the zoospores, more numerous 
(16 or 32 in a cell), and biciliate. Two similar gametes^ coming from 

' Although some species of Ulva are strictly isogamous, others produce two kinds of 
gametes that differ slightly in si?e. 



42 



PLANT MORPHOLOGY 



separate plants unite to form a zygote. Instead of becoming a thick- 
walled resting cell, the zygote germinates immediately and produces a 
plant that bears only zoospores. 

Thus Ulva illustrates the phenomenon of alternation of generations. 
Two separate plants, one producing gametes and the other spores, are 
involved in each life cycle and, although they look alike, the gamete- 
producing plants are haploid and the spore-producing plants are diploid. 




Fig. 30. Ulva lactuca, the sea lettuce, about one-half natural size. The bright green 
thallus is only two layers of cells thick. (After Thuret.) 

The doubling of chromosomes, resulting from the conjugation of two 
gametes, is carried over by the zygote to the cells of the spore-producing 
plant. The reduction of chromosomes takes place when the zoospores 
are produced. These haploid spores give rise to the gamete-producing 
plants. The haploid plants are called gametophytes and the diploid 
plants sporophytes. Because the two kinds of plants are alike vegeta- 
tively, Ulva displays an isomorphic alternation of generations. 

Coleochaete. Coleochaete is a small fresh-water alga that usually 
grows attached to leaves and stems of aquatic seed plants, such as water 



THALLOPHYTA: AWAK 



43 



lilies and cattails. Depending on the species, the vegetative body is 
either a branching filament, a cushion with free branches, or a circular 
disk with radiating rows of cells (Fig. 31 A). When disk-like, it rarely 
exceeds 5 mm. in diameter. Some of the cells bear hair-like outgrowths, 




Fig. 31. Coleochaete scutata, a discoid species. A, a small vegetative plant with numerous 
zygotes overgrown by the surrounding cells, X150; B, a small group of vegetative cells, 
one of which is giving rise to a zoospore, and an escaped zoospore, X500; C, vegetative cells 
giving rise to antheridia, and an escaped sperm, X500; D, cross section of portion of thallus, 
showing a zygote, X350. 



each with a sheath at its base. Each cell has a single nucleus and a 
chloroplast with one or sometimes two pyrenoids. Growth is always 
apical, in the discoid species occurring by means of a marginal meristem. 
Biciliate zoospores, formed singly, may arise in any vegetative cell (Fig. 
31JB). They escape through a pore in the cell wall. 

In being heterogamous, Coleochaete makes an advance over the other 
Ulotrichales that have been considered. In the discoid species antheridia 
are formed by the division of a vegetative cell into smaller cells, the 



44 



PLANT MORPHOLOGY 



protoplasts of which escape into the water as biciHate sperms (Fig. 31C). 
An oogonium is formed near the margin of the thallus by the enlargement 
of a vegetative cell, its protoplast becoming a nonmotile egg. In the 
branched species the antheridia and oogonia are borne at the ends of 
separate branches. Here the oogonium has a long, slender extension 
(trichogyne) with a terminal opening. A few species are dioecious. 

A sperm enters the oogonium and fertilizes the egg, the zygote enlarg- 
ing and becoming thick-walled. At the same time adjacent vegetative 
cells grow up around the oogonium and form a case (Figs. 3 ID and 32.4). 




Fig. 32. Coleochaete pulvinata. A, section of oogonium containing a zygote and sur- 
rounded by jacket produced by adjacent vegetative cells; B, section of oogonium containing 
a group of cells derived from the zygote, each of which gives rise to a zoospore. (After 
Oltmanns.) 



After undergoing a period of rest, the zygote germinates inside the 
oogonium and produces a spherical body consisting of 16 or 32 cells, 
each cell in turn producing a biciliate zoospore (Fig. 32J5). This escapes 
and gives rise to a new vegetative plant. In the discoid species the 
zygote produces an eight-celled body. The reduction of chromosomes 
takes place when the zygote germinates. Consequently, the body of 
spore-producing cells that develops from it is haploid and so cannot be 
regarded as a sporophyte. Thus Coleochaete is without a true alternation 
of generations. 

Summary. The plant body of the Ulotrichales is multicellular (except 
in Protococcus) , being either a simple filament, a branched filament, or a 
flat plate-like thallus. The cells contain one nucleus and a single chloro- 
plast. Growth occurs by division of the vegetative cells. Nearly all 
members produce zoospores, these being either biciliate or quadriciliate. 
Asexual reproduction may also occur by aplanospores or by akinetes. 
Sexual reproduction ranges from isogamy to heterogamy. 



THALLOPHYTA: ALGAE 45 

4, Oedogoniales 

The Oedogoniales are related to the Ulotrichales and are often classified 
with them. They are a fresh-water group including onl}^ 3 genera and 
approximately 400 species. The two chief genera are Oedogonium and 
Bulbochaete, both occurring throughout the world. 

Oedogonium. This widely distributed alga, comprising nearly 300 
species, generally lives in ponds, lakes, and quiet streams, often attached 
to sticks, stones, and other aquatic plants. It consists of a simple un- 
branched filament that, when young, has a basal holdfast cell but later is 
usually free-floating. The cells are elongated and uninucleate. Each 
contains a peripheral chloroplast with many pyrenoids. The chloroplast 
is band-like and reticulate. Any vegetative cell except the basal one 
may divide. 

Oedogonium has a peculiar method of cell division seen only in the other 
members of its order (Fig. 33). It results in the formation of distinctive 
"apical caps." The nucleus divides near the upper end of the cell, where 
simultaneously a ring-like thickening of cellulose is developed on the 
inside of the lateral wall above the dividing nucleus. A groove appears in 
this ring and the cell wall splits transversely opposite the groove. A thin 
cross wall now appears between the daughter nuclei and the protoplast is 
divided in half. The ring stretches into a cylinder as each daughter 
protoplast elongates, the new cross wall moving upward to the top of the 
parent cell, where it unites with the lateral wall very close to where the 
transverse split occurred. The upper cell, which has a new cell wall, con- 
tinues to elongate until it reaches the size of the lower cell, which possesses 
the old cell wall. 

Asexual reproduction occurs by the formation of large zoospores, each 
of which arises from the entire contents of an ordinary vegetative cell 
(Fig. 34: A, B). This escapes as a naked protoplast that bears a crown of 
cilia. The liberation of the zoospore is accompanied by a transverse 
splitting of the cell wall at the apical end. After a period of free swim- 
ming, the zoospore comes to rest with its ciliated end downward, retracts 
its cilia, forms a cell wall, and gives rise by repeated divisions to a new 
filament. Oedogonium may also produce akinetes, although these are 
relatively uncommon. The akinetes may occur either singly or in a 
linear series. They germinate directly into new filaments. 

Oedogonium is heterogamous. An antheridium arises as a short cell 
that is cut off at the apex of an ordinary vegetative cell. It may remain 
the only one, but generally more (from 2 to 40) are produced by continued 
division of the lower cell or by division of antheridia already formed (Fig. 
34C). Each antheridium gives rise to one or, more commonly, to two 
sperms, either by a vertical or a transverse division of the protoplast, 



46 



PLANT MORPHOLOGY 



depending on the species. The sperms escape into the water and, like 
the zoospores, swim by means of a crown of cilia. An oogonium also 
commonly arises from the smaller upper cell produced by the division of 
an ordinary vegetative cell, but this cell subsequently enlarges by the 
accumulation of food (Fig. 34D). The oogonia may occur separately or 




wmm^ 

















mmm\ 



Fig. 33. Nuclear and cell division in Oerfogo/iiMm gra?ic?e, X320. A, elongation of nucleus 
and appearance of young ring; B, metaphase; C, anaphase; D, formation of cross wall and 
separation of nuclei; E, broken outer layer of cell wall and stretching of ring; F, straighten- 
ing of ring and migration of cross wall upward to unite with inner layer of cell wall. {After 
Ohashi.) 

several may be cut off in a series. The entire protoplast of the oogonium 
becomes a large nonmotile egg. 

A sperm enters an oogonium through a pore in its wall and unites with 
the egg. The zygote becomes a heavy-walled resting cell that later 
produces four zoospores (Fig. ME, F). When liberated, these are 
enclosed by a common membrane that soon disappears. From each of 
the zoospores a new filament is developed. The reduction of chromo- 
somes occurs in connection with the germination of the zygote, and so the 
four zoospores are haploid. 



THALLOPHYTA: ALGAE 



47 



Some species of Oedogonium are monoecious, the antheridia and oogonia 
occurring in the same filament. Other species are dioecious, the two 
kinds of sex organs being borne on separate filaments. In some dioecious 
species the male and female filaments are approximately equal in size. 





i^^ 




B 



CD E 

Fig. 34. Reproduction in Ocdoj7ont?/m, X500. ^ and B, the entire fontents of a vegetative 
cell escaping as a zoospore; C, portion of filament with two groups of antheridia; also a single 
escaped sperm; D, portion of filament with an oogonium containing a mature egg in which 
are many pyrenoids and starch grains; E, heavy-walled zygote still within the oogonium; 
F, group of four zoospores produced by the zygote. {A and B, after Him; F, after Juranyi.) 

In others the male filaments are very small, consisting of only a few cells. 
These dwarf filaments are produced by special small zoospores, called 
androspores, that originate singly in rows of small cells resembling 
antheridia. The androspores germinate on the female filaments near or 
on an oogonium (Fig. 35). The dwarf filament usually consists of a 
single vegetative cell that cuts off one or several terminal antheridia, each 
producing two sperms. Figure 35 shows three dwarf filaments of different 
ages. In the one on the right the single vegetativ-e cell has cut off a small, 
undivided, antheridial cell. In the middle filament a second antheridium 



48 



PLANT MORPHOLOGY 



has been formed by the vegetative cell, while the first antheridium has 
produced two sperms. In the male filament on the left two sperms have 
escaped from the upper antheridium, but two more have been formed in 
the lower one. 

Bulbochaete is a genus closely resembling Oedogonium, differing chiefly 
in having branches, most of the cells of which bear long one-celled hairs 

that are swollen at the base. 

Summary. The Oedogoniales are a small order 
differing from the Ulotrichales mainly in having 
a peculiar method of cell division and motile re- 
productive cells with a crown of cilia. The vege- 
tative body is multicellular and filamentous, the 
cells having one nucleus and a single chloroplast. 
Asexual reproduction occurs by zoospores, some- 
times by akinetes. All the members are heter- 
ogamous. 

5. Conjugales 

The Conjugales constitute a distinct and highly 
specialized order of green algae that occupy an 
isolated position. In fact, they are sometimes 
removed from the Chlorophyceae and made an 
independent class. All of them occur in fresh 
water. They include 38 genera and over 2,400 
species. Some representative genera are Closterium, 
Cosmarium, Mougeotia, Spirogyra, and Zygnema. 

Desmids. These algae are widely distributed 
in bogs, ponds, and small lakes, usually becoming 
abundant late in the season. They number about 
2,250 species. Closterium is a genus of nearly 200 
species, while Cosmarium has over 800. The des- 
mids are unicellular and the cells display a great 
variety of form. Like the diatoms, they have won 
the favor of microscopists by their great beauty. 
Desmids are typically solitary, but some develop 
into filamentous colonies. Many desmids have the power of movement, 
which appears to be caused by exudation of mucilage through pores in the 
cell wall. 

In most desmids the cell is organized into two symmetrical halves that 
are generally separated by a median constriction called the isthmus (Figs. 
36 and 37). In each half there is usually one large chloroplast (some- 
times two) with one or more pyrenoids. The chloroplast is often elabo- 
rately lobed. The nucleus lies in the isthmus. In Closterium, at each 




Fig. 35. A species of 
Oedogonium having 
dwarf male filaments, 
three of which have 
developed on the cell 
below the oogonium, 
X300. 



THALLOPHYTA: ALGAE 



49 



end of the cell, is a small group of calcium sulphate crystals that show 
Brownian movement. In some desmids, the outer surface of the cell wall 
displays warts, spines, ridges, or other markings, most of which show a 
regular arrangement. 

Asexual reproduction occurs mainly by fission, rarely by apian ospores. 
Zoospores have never been observed. In cell division the nucleus divides 



m 



J I 






ffii 






'•■^m* 



<* 




Fig. 36. Closterium, a common desmid. A, vegetative cell, showing nucleus at isthmus, a 
large lobed c-hloroplast with a row of pyrenoids in each half of the cell, and at each end a 
vacuole containing a few crjstals, X300; B and C, another species, showing two stages in 
conjugation, X200. 

first and then a cell wall is formed across the isthmus. Each of the two 
chloroplasts splits transversely. The daughter cells then separate and 
each forms a new half similar to itself. In sexual reproduction two cells 
come together and secrete a common mucilaginous sheath (Fig. 365, C). 
Their walls generally break at the isthmus. Then the protoplasts escape 
and fuse to form a zygote. In a few desmids each cell sends out a short 
tube. These meet, become continuous, and the two protoplasts fuse in 
the tube. The desmids are isogamous but their gametes, each represent- 



50 



PLANT MORPHOLOGY 






B 





D 




Fig. 37. Several desmids, showing variety of form. A, end view and B, front view of 
Staurastrum; C, Docidium; D. Cosmarium, E, Micrasterias; C, X250; others, X400. 



G^: 



mm 





Fig. 38. Single cells of Moiigeotia, showing the plate-like chloroplast as seen in side (A) and 
face (B) views, X500; C, conjugating filaments, with three zygotes formed in the conjugat- 
ing tubes. (C, after Wittrock.) 



ing an entire vegetative protoplast, are nonciliated. The zygote becomes 
thick-walled and, after a period of rest, its protoplast escapes and divides 
generally into two daughter protoplasts, each of which becomes a new 
individual. As a result of two successive divisions of the zygote nucleus, 
during which the reduction of chromosomes occurs, each daughter proto- 



THALLOPHYTA: ALGAE 



51 



plast has two haploid nuclei. Then one nucleus in each protoplast 
degenerates. 

Mougeotia. This alga consists of a deli(;ate unbranched filament. 
Each cell displays a nucleus and a peculiar, axial, plate-like chloroplast 
containing two or more pyrenoids (Fig. 38/1, B). The chloroplast can 
change its position in the cell, presenting its flat surface to dull light and 







BCD 

Fig. 39. Spirogyra. A, a vegetative cell, showing the central nucleus and the peripheral, 
band-like, spiral chloroplast with many pyrenoids, X500; B, C, D, stages in conjugation, 
X250. 

its edge to bright light. Reproduction occurs by fragmentation, aplano- 
spores, and by the conjugation of isogametes. The cells of two filaments 
lying parallel to each other put out short bud-like outgrowths that come 
into contact and form tubes. The protoplasts of two conjugating cells 
pass into one of these tubes and there fuse, producing a heavy-walled 
zygote (Fig. 38C). Upon germination, four cells are formed. Three of 
these die, the fourth producing a new filament. It is probable that the 
chromosome reduction takes place when the zygote germinates. 

Spirogyra. Spirogyra is a well-known green alga very common in 
ponds, lakes, and streams, where it forms slimy bright green masses on or 



52 



PLANT MORPIIOLOG Y 



beneath the surface of the water. It is a large genus of over 100 species. 
The vegetative bod}^ is an unl)ran('hed filament with cylindrical cells that 
are usually elongated. Each cell has a single nucleus suspended in the 
center by strands of cytoplasm (Fig. 39.4). It also has one or more 
peripheral, ribbon-like chloroplasts with many pyrenoids. The chloro- 
plasts have the form of spiral bands, the number in each cell depending 




G H 

Fig. 40. Nuclear changes in the zygote of Spirogyra longata {A to G) and germination of the 
zygote of Spirogyra neglecta (H). A, B, C, first meiotic division of fusion nucleus in the 
zygote; D and E, second division; F and G, degeneration of three of the haploid nuclei. 
(After Trondle.) 

on the species. Any cell may divide by the formation of a cross wall, thus 
resulting in growth of the filament. In some species the cross walls 
possess characteristic infoldings. 

In sexual reproduction the cells of the two filaments lying side by side 
put out lateral projections that come in contact (Fig. 395-Z)). The 
contiguous portions of the cell walls at the ends of these projections then 
break down and form tubes leading from one filament to the other. 
Through these conjugating tubes the protoplasts of one filament pass to 
fuse with those of the other filament, forming zygotes. An entire vege- 
tative protoplast thus becomes a large gamete. The zygote develops a 
heavy wall and goes into a resting stage. Upon germination, which 
usually occurs in the following spring, it directly produces a new filament 



THALLOPHYTA: ALGAE 



53 



(Fig. 40//). The zygote becomes diploid by the fusion of the two nuclei 
derived from the conjugating protoplasts. When germination takes 
place, the fusion nucleus undergoes two successive divisions that result in 
a reduction of chromosomes (Fig. 40A-G). Of the four haploid nuclei 
thus formed, three degenerate, leaving one to function. In this way the 
zygote gives rise to a haploid filament. 

Zoospores are never produced in Spirogyra. If conjugation fails to 
occur, a protoplast may round up and become a heavy-walled cell that, 
after a period of rest, gives rise to a new filament. Such a cell is often 





Fig. 41. A species of Spirogyra with lateral conjugation, gametic union occurring between 
adjacent cells of the same filament. A, B, C, development of conjugating tubes and forma- 
tion of zygote, X300. 

called an aplanospore but would be more appropriately designated as a 
gamete that develops without undergoing conjugation. 

Spirogyra, like the other Conjugales, is peculiar because an entire vege- 
tative protoplast becomes a single large gamete that is not ciliated and 
does not escape into the water. Although the gametes show no differ- 
entiation in size, the active ones are regarded as male and the passive ones 
as female. The ordinary type of conjugation is known as scalariform 
(ladder-like) conjugation. In a few species lateral conjugation occurs 
(Fig. 41). In this type conjugating tubes are developed between adjacent 
cells of the same filament. At its completion a zygote is formed in one 
of the conjugating cells. 

Zygnema. This is a genus closely related to Spirogyra and resembling 
it in many ways. Both forms grow in the same sort of places and look 
much alike to the naked eye. The filaments of Zygnema are unbranched 
and consist of cyUndrical, more or less elongated cells (Fig. 42). Each 
has two spherical chloroplasts between which, at the center of the cell, 
lies the nucleus. Each chloroplast has a single pyrenoid surrounded by 



54 



PLANT MORPHOLOGY 



radiating starch grains. As in Spirogyra, sexual reproduction takes place 
by the passage of isogametes, each representing an entire vegetative pro- 
toplast, through conjugating tubes and their fusion in the cells of one of 
the filaments. There is also the same degeneration of three of the haploid 
nuclei derived from the nucleus of the zygote. 





A B 

Fig. 42. Single cells of Zygiiema, X750. A, vegetative cell, showing central nucleus and 
two spherical chloroplasts, each with a pyrenoid surrounded by radiating starch grains; B. 
young zygote with four chloroplasts and the two gametic nuclei not yet fused. 

Summary. The Conjugales are an aberrant order of green algae show- 
ing no close relationship to any of the other orders. The plant body may 
be either unicellular or multicellular, in the latter case consisting of a 
simple unbranched filament. The cells are uninucleate and have one or 
more peculiar chloroplasts. The distinguishing feature of the order is the 
absence of all ciliated cells in the life history. No zoospores are produced, 
but aplanospores may occur. Sexual reproduction is accomplished by the 
conjugation of two noncihated isogametes, each derived from the entire 
protoplast of a vegetative cell. These either escape and fuse, unite in a 
conjugating tube, or pass through a conjugating tube and fuse in one of 
the cells. 

6. Siphonocladiales 

This is a group whose members are often distributed among other 
orders, although its characters are rather well defined. They are repre- 
sented in both fresh and salt water, but most of them are marine, being 
found principally in tropical and subtropical seas. Many of the marine 
forms are incrusted with lime. Representatives of the group have been 
found as fossils as far back as the Ordovician. The Siphonocladiales 
include about 37 genera and 450 species, the best-known genera being 
Cladophora, Sphaeroplea, and Acetabularia. 

Cladophora. Cladophora is a genus of about 150 species, world-wide in 
distribution. It is found in great abundance in streams, ponds, and lakes, 
usually attached to stones and piers. Some of its species are marine. 
The vegetative body is filamentous and much branched, its cells being 
elongated and cylindrical (Fig. 43A). A branch originates as an out- 
growth from the upper end of a cell lying near the end of a filament. Each 
cell is a coenocyte, containing many nuclei. The cytoplasm usually sur- 



THALLOFHYTA: ALGAE 



55 



rounds a large central vacuole. When young, a cell has a large, periph- 
eral, reticulate chloroplast with many pyrenoids. Later the chloroplast 
often appears to break up into numerous small chloroplasts, some of 
which have pyrenoids. 

Many quadriciliate zoospores are formed, usually in cells at or near the 
ends of branches (Fig. 43i^). The zoospores, which are uninucleate. 




Fig. 43. Cladophora. A, portion of plant, a branching filament, X65; B, a vegetative 
cell, a sporangium, and two escaped zoospores. X300. Each vegetative cell has many 
nuclei and a large, peripheral, reticulate chloroplast with a large number of pyrenoids. 



escape singly through a small pore in the cell wall. They develop into 
new filaments, but these, in turn, produce only isogametes. The gametes 
may arise in any vegetative cell. They escape into the water and swim 
by means of two ciha. The gametes pair and fuse, but fusion occurs, as a 
rule, only between gametes coming from different plants. The zygote, 
without undergoing a period of rest, gives rise to a new filament directly. 
This plant produces only zoospores. Although alike vegetatively, the 
gamete-producing plants are haploid and the spore-producing plants are 
diploid. The reduction of chromosomes occurs in connection with the 



56 



PLANT MORPHOLOGY 



formation of zoospores. Thus, as in Ulva, the life cycle of Cladophora 
involves a distinct alternation of generations of the isomorphic type. 

Sphaeroplea. This is a fresh-water alga that grows in wet meadows 
and occasionally in pools. Although widely distributed, it is not com- 
mon. The vegetative body consists of an unbranched filament with very 
long cylindrical cells, each containing numerous nuclei and chloroplasts 
(Fig. 44). The chloroplasts, some of which have pyrenoids, are parietally 
placed and grouped into wide annular bands of cytoplasm separated by 



W m m 




ii.!.iJU4^j5Sjp*.,uiu...r,i 







r 



-^ — ■ I n'* 







Fig. 44. Sphaeroplea annulina, X400. A, portion of a vegetative cell with ring-like 
bands of cytoplasm containing many small nuclei, chloroplasts, and pyrenoids; B, anther- 
idia producing sperms; C, portion of an oogonium with nianj- eggs ready for fertilization. 

wide vacuoles. The vegetative cells of Sphaeroplea do not produce any 
zoospores. Sexual reproduction is heterogamous, the two kinds of sex 
organs usually being borne in different filaments. Any vegetative cell, 
without undergoing a change in shape, may become an antheridium or an 
oogonium. The antheridium produces a large number of small biciliate 
sperms, while the oogonium gives rise to many large nonmotile eggs. 
The eggs are at first multinucleate, but later all the nuclei degenerate 
except one. The sperms escape through small pores in the cell wall, 
enter the oogonium through similar pores, and there fertilization takes 
place. Each zygote becomes thick-walled and, after undergoing a long 
resting period, gives rise usually to four biciliate zoospores. Each of 
these forms a new filament. The reduction of chromosomes occurs when 
the zygote germinates, and so the spores and vegetative filaments are 
haploid. 



THALLOPHYTA: AWAE 



57 



Acetabularia. This is a marine genus occurring in tropical and sub- 
tropical regions. It is called the mermaid's-wineglass. Acetabularia 
crenulata is a common species off the coast of Florida and throughout the 
West Indies. Its vegetative body, reaching a height of 6 to 9 cm., con- 
sists of a stalk bearing rhizoid-like holdfasts below and expanded above 
into a cup-like disk about 1 cm. in diameter (Fig. 45). The disk is com- 
posed of a whorl of elongated branches that are laterally coherent, each 
branch being a coenocyte. The plants are more or less incrusted with 
lime. At first the plant has a single nucleus that soon gives rise to many 
small nuclei. These pass up the stalk and 
enter the disk, which has now become divided 
into cells. 

Reproduction begins by the formation of 
a large number of aplanospores (cysts) within 
the fertile branches composing the disk. The 
aplanospores are at first uninucleate but later 
become multinucleate. They are liberated 
into the water and, after a resting period, 
each gives rise to a large number of biciliate 
isogametes that escape and fuse in pairs. 
The zygote germinates immediately to form a 
new plant. The vegetative plant is diploid, 
the reduction of chromosomes occurring when 
the nucleus of the aplanospore divides. 

Acetabularia has been widely used by students of genetics and develop- 
ment, especially in experiments on regeneration and polarity. 

Summary. The Siphonocladiales are multicellular algae with large 
multinucleate cells, these usually containing many small chloroplasts. 
The plant body is thus partially coenocytic. Vegetative growth takes 
place by cell division. Asexual reproduction usually occurs by zoospores, 
aplanospores, or akinetes. Sexual reproduction may be either isogamous 
or heterogamous. This order is related both to the Chlorococcales and 
to the Siphonales. 




Fig. 45. Acetabularia crenu- 
lata, natural size. 



7. Siphonales 

The Siphonales are a distinct group of mostly marine algae, only a few 
being found in fresh water. They are especially abundant in tropical 
seas. As in the Siphonocladiales, many marine forms secrete lime. 
Fossil members are known as far back as the Ordovician. The order 
includes 50 genera and about 350 species. Representative genera are 
Vaucheria, Codium, Bryopsis, and Caulerpa. 

Vaucheria. This well-known alga grows in felt-like masses in fresh 
water and on damp soil. Some of its species live in brackish water and 



58 



PLANT MORPHOLOGY 



some live in the ocean. The plant body consists of a sparsely branched 
coenocytic filament without any cross walls in the vegetative portion. It 
is attached by means of colorless rhizoid-like holdfasts. Numerous small 
nuclei and chloroplasts are scattered throughout the cytoplasm, which 
surrounds a large central vacuole. There are no pyrenoids or starch 
grains, but oil droplets are usually present in abundance. In this respect 
Vaucheria differs from the other Siphonales. 

Vaucheria displays three methods of vegetative reproduction, as 
follows: (1) A branch may be constricted at the base, thus producing a 




>^ 






T^i 



s^V*^ 



i^p. 






A C 

Fig. 46. Fawc/ieria, a coenocytic green alga, X250. A, an escaping zoospore covered with 
many cilia; B, a zoospore giving rise to a new vegetative filament; C, two oogonia of 
Vaucheria sessilis, each with a zygote ; also an antheridium that has discharged its sperms ; 
D, two sperms, more highly magnified. 

new plant body directly. (2) The tip of a branch may swell slightly and 
become cut off by a cross wall to form a club-shaped sporangium (Fig. 
46A). The multinucleate protoplast in the branch rounds up and 
becomes a large zoospore entirely covered by cilia. The cilia are in pairs 
and beneath each pair is a nucleus. For this reason the zoospore is 
regarded as compound. It escapes into the water through a terminal 
pore and, upon germination, gives rise to a new filament (Fig. 465). (3) 
The contents of an entire filament may break up into aplanospores, each 
developing a thick wall. 

Vaucheria is heterogamous. The antheridia and oogonia are not 
transformed vegetative cells but are developed on special branches of the 
filament (Fig. 46C). In most species a short branch, sooner or later cut 
off by a wall, becomes a globular oogonium. Its protoplast is organized 
as an egg, which becomes uninucleate. It is uncertain whether this is 



TH A LLOPH Y TA : A WA E 



59 



accomplished by the degeneration of all its nuclei except one, as some 
observers have claimed, or, as others contend, by the passage back into 
the filament of all but one nucleus before the wall is formed at the base 
of the oogonium. Arising close to the oogonium is a longer and more 
slender branch, its curved tip being cut off by a wall to form an anther- 
idium. In some species both kinds of sex organs are borne on the same 
branch, the antheridium being terminal and surrounded by two or more 
oogonia (Fig. 47) . The antheridium produces many small biciliate sperms 
that are liberated into the water through a terminal pore. A sperm 




*#5^^^ 






.$, 






'^ 






^ 



. *'■■ 



Fig. 47. Sex organs of Vaucheria 
geminata, two oogonia and an anther- 
idium borne on the same branch, 
X250. 




Fig. 48. Thallus of Codium fragile, one- 
half natural size. 



enters an oogonium through a terminal pore in its wall and fuses with the 
egg to produce a heavy-walled zygote. After remaining dormant, it 
gives rise to a new filament directly. The reduction of chromosomes 
probably occurs during germination of the zygote. 

Codium. Codium is a widely distributed marine alga that grows on 
rocks between tide lines. The thallus is dark green and spongy, consist- 
ing of thick cylindrical branches composed of a dense mass of interwoven 
filaments (Fig. 48) . It is anchored by means of a basal disk-Uke holdfast. 
Like other members of the order, the vegetative body is without cross 
walls. The cytoplasm is peripheral and has numerous small nuclei and 
chloroplasts. There is no asexual reproduction by means of spores. 
Two kinds of gametangia are produced, generally on different plants. 
They arise on the sides of large club-shaped branches that form a sort of 
cortex, and are cut off by a basal wall. The male gametangium, which is 
smaller than the female one, hberates many thousands of biciliate male 
gametes. In the female gametangium some of the nuclei degenerate, 
while others enlarge. Several hundred biciliate female gametes are 



60 



PLANT MORPHOLOGY 



organized. These escape through a terminal pore and are fertilized in 
the water. The zygote gives rise at once to a vegetative plant. The 
vegetative plants of Codhim are diploid and the reduction of chromosomes 
takes place in connection with the formation of the gametes. 

Bryopsis. Some of the marine Siphonales are highly branched, one of 
these being Bryopsis. The thallus is composed of a prostrate rhizome- 
like portion, anchored by rhizoids, and an upright feathery portion, the 




Fig. 49. A small portion of the vegetative body of Bryopsis, showing branches of limited 
growth, X75. 

latter consisting of an axis with branches of limited growth (Fig. 49). 
In the formation of a gametangium, a branch is cut off by a cross wall and 
gives rise to numerous bicilate gametes. A gametangium produces 
either male or female gametes and these are usually borne on different 
plants. The female gametes are about three times as large as the male 
ones. Both escape into the water, where they pair and fuse. The 
zygote secretes a cell wall and germinates immediately to form a new 
vegetative plant. There are no spores of any kind in the life cycle. The 
reduction of chromosomes occurs when the gametes are formed, and thus 
the vegetative plant is diploid. 

Caulerpa. This is a marine form of interest because of the high degree 
of differentiation of its coenocytic plant body (Fig. 50) . It consists of a 



THALLOPHYTA: ALGAE 



61 



creeping axis with root-like holdfasts and erect leaf-like shoots of various 
form. In some species the shoots reach a height of 30 cm. Cross walls 
are absent in the vegetative part of the plant, but the central cavity is 
traversed by numerous slender strands. Asexual reproduction occurs 
only by fragmentation, sexual reproduction by biciliate isogametes. 

Summary. The Siphonales are characterized by a completely coeno- 
cytic plant body that is usually much branched and often differentiated 
in form. Cross walls appear only in connection with the formation of 






ABC 

Fig. 50. Three species of Caiilerpa, a coenocyte with a high degree of structural differentia- 
tion, one-half natural size. A, Canlerpa prolifera; B, Caulerpa crassifolia; C, Caiilerpa 
macrodisca. 

r?productive organs. The vegetative body contains innumerable nuclei 
and small chloroplasts. It is really a single multinucleate cell. Asexual 
reproduction may be accomplished by fragmentation of the thallus, by 
zoospores, aplanospores, or akinetes. Sexual reproduction ranges from 
isogamy to heterogamy. This is a highly specialized order related both 
to the Chlorococcales and the Siphonocladiales. 

Summary of Chlorophyceae 

The Chlorophyceae are algae with only chlorophyll and its associated 
carotinoids in their plastids, these being present in the same proportions 
as in the higher plants. In vegetative organization they are highly 
diversified. Some are unicellular but most of them are multicellular, the 
thallus being most commonly filamentous, sometimes plate-like, and 
rarely massive. Some are partially or completely coenocytic. There is 
relatively little cellular differentiation. A definite cell wall composed of 
cellulose is nearly always present, this seldom becoming mucilaginous. 
The cells contain a well-organized nucleus (often more than one) and one 
or more distinct plastids. Pyrenoids are usually present. Reserve food 
is stored generally as starch, sometimes as oil. Asexual reproduction 



62 PLANT MORPHOLOGY 

occurs by fission (in some unicellular forms), fragmentation, or by zoo- 
spores, aplanospores, and akinetes. Sexual reproduction is either isoga- 
mous or heterogamous. In the heterogamous forms the sperms are 
ciliated, the eggs nearly always noncihated. Motile reproductive cells 
generally have two or four cilia, equal in length and apically attached. 
The zygote nearly always becomes a resting cell. 

Within the Chlorophyceae, three main evolutionary trends can be 
recognized. The occurrence of ciliated reproductive cells in practically 
all members, except the Conjugales, indicates that the common ancestor 
of the group must have been a form like Chlamydomonas. The Volvo- 
cales, retaining motility in vegetative cells, represent one hne of evolution. 
It emphasizes the ciliated colonial type of organization that culminates in 

Volvox. 

A second line of development, represented by the Chlorococcales, also 
emphasizes the colony but shows a loss of motility by the vegetative cells. 
A tendency toward the formation of multinucleate cells appears in this 
order. This leads to the development of coenocytic bodies, which 
reaches a climax in the Siphonales. Protosiphon is a connecting link 
between the Chlorococcales and Siphonales. Some regard the Siphono- 
cladiales as a transitional stage leading to the evolution of the Siphonales; 
others consider them an offshoot of that order, the incomplete formation 
of walls being a recent development. Still others think that at least some 
of the Siphonocladiales have arisen from the Ulotrichales. 

A third line of development within the Chlorophyceae is represented 
by the Ulotrichales, an order in which several different types of multi- 
cellular bodies have appeared. All of these grow by division of uni- 
nucleate vegetative cells. The Oedogoniales may represent an offshoot 
from this order, but a connection between the Conjugales and the 
Ulotrichales seems rather remote, the lack of ciliated cells and pecuHar 
type of sexual reproduction in the Conjugales being the chief obstacles. 
The Ulotrichales are of great interest in being the order of green algae 
most closely resembling the probable ancestors of the higher green plants. 
The occurrence of plate-hke forms is particularly significant, inasmuch 
as the vegetative body of the simpler bryophytes is a plate-like thallus. 



CHAPTER III 

THALLOPHYTA : ALGAE (CONTINUED) 

8. CHAROPHYCEAE 

The Charophyceae, or stoneworts, constitute a very isolated group of 
highly organized green thallophytes with uncertain affinities. Although 
often included in the Chlorophyceae, they are so distinct that they belong 
in a separate and coordinate class. The Charophyceae are multicellular 
plants in which the only pigments present are chlorophyll and its asso- 
ciated carotinoids, these occurring in essentially the same proportions as 
in the green algae. They include 6 genera and about 200 species, nearly 
all of which belong to Chara and Nitella. The stoneworts grow in 
streams, ponds, and lakes attached to the bottom. They also live in 
brackish water but not in the ocean. Most species of Chara extract 
calcium carbonate from the water and deposit it in their walls, thereby 
becoming rough and brittle. Fossils belonging to the Charophyceae 
have been identified in deposits of the Cretaceous and later geologic 
periods. There is some evidence of their existence even as far back as 
the Devonian. 

Vegetative Body. The vegetative body of the stoneworts consists of 
a slender cylindrical stem bearing many short branches in whorls (Fig. 
51 A). It grows erect and often reaches a height of 20 to 30 cm. The 
stem is attached to the substratum by means of colorless branched 
rhizoids. It is made up of short nodes and long unicellular internodes, 
the branches arising from the nodes. There are two kinds of branches: 
branches of unlimited growth, comprising the main axes, and branches of 
limited growth, the so-called leaves, in whose axils the main axes arise. 
All the cells contain numerous small spherical chloroplasts without 
pyrenoids. Reserve food is stored as starch. 

Both kinds of branches grow by means of an apical cell, hemispherical 
in shape, that cuts off a longitudinal series of segments by successive 
transverse walls (Fig. olB). Each segment again divides transversely 
into two cells, the lower one becoming the long internodal cell and the 
upper one the nodal cell. The latter, by vertical divisions, gives rise to 
a plate of cells that produce the branches. The internodal cell, often 
attaining a length of 10 cm., may become coenocytic by fragmentation of 
its nucleus. Its cytoplasm gives a striking demonstration of protoplasmic 
streaming. In Chara the internodal cells become ensheathed by cells that 

63 



64 



PLANT MORPHOLOGY 



arise from the nodes and form a one-layered cortex. Half of the cortical 
cells are derived from the node below and half from the node above, the 
two halves meeting in a zigzag line midway between the nodes. In 
Niiella the Internodes remain uncovered. 

Reproduction. No spores are produced in the Charophyceae. The 
nodes of the branches of limited growth bear unicellular branches and the 
sex organs, which are the most complex of all the algae. Most species are 





Fig. 51. Chara. A, upper portion of plant, showing branches of Hmited and unlimited 
growth, natural size; B, median longitudinal section through the stem tip, showing promi- 
nent apical cell and alternating nodes and internodes derived from it, X200. The large 
internodal cell below is being ensheathed by a layer of cortical cells arising from adjacent 
nodes. 



monoecious, an antheridium lying below an oogonium at the same node 
(Fig. 52). The antheridium is a stalked globular body that is brilliant 
red or yellow. It develops from a single initial cell that at first divides in 
three planes to produce octants. Each octant then undergoes two peri- 
clinal divisions to form an outer, a middle, and an inner cell. A jacket 
of eight triangular plate-like cells, called shields, is derived from the outer 
cells. The rapid enlargement of the shields results in the formation of a 
cavity within the antheridium. Projecting inward from the center of 
each shield is an elongated cell, the manubrium, that bears a rounded 
terminal cell, called a primary capitulum, which often divides in two. 
The manubria and primary capitula are derived from the middle and 



THALLOPHYTA: ALGAE 



65 



inner cells, respectively, of the young antheridium (Fig. 53.4). The 
primary capituliim forms about six secondary capitnla. Each of these 
gives rise to a pair of long filaments consisting of 100 to 200 small cells, 
from every one of which a sperm is liberated (Fig. 53B). At maturity, 
the entire antheridium falls apart. A single antheridium of Chara 
produces 20,000 to 50,000 sperms. These are coiled and bicihate, 
resembling the sperms of bryophytes. 




Fig. 52. Branch of Chara bearing an oogonium, with sterile jacket and crown, and an 
antheridium, with interlocking, shield-like wall cells, X50. 

An oogonium is an enlarged apical cell. It produces a single large egg. 
A unique feature of the oogonium is the presence of five elongated, 
spirally wound cells that arise below and completely surround it (Figs. 
52 and 53 A). At the top of the oogonium each jacket cell cuts off a small 
cell, these five cells forming a crown. In Nitella each spiral cell cuts off 
two crown cells, making ten in all. When the egg is ready for fertiliza- 
tion, the spirally twisted cells separate shghtly just below the crown, 
forming five slits through which the sperms enter the oogonium. After 
a sperm nucleus has united with the egg nucleus, the walls of the sur- 
rounding cells harden, the whole structure becoming nut-like. In this 
condition the zygote rests. Before germination, the fusion nucleus gives 
rise to four nuclei. Each probably has the haploid number of chromo- 
somes, although this has not been definitely established. Three of these 



06 



PLANT MORPHOLOGY 



nuclei degenerate. Upon germination, the zygote sends out a simple 
green filament and a colorless rhizoid. The adult shoot arises from this 
filament as a lateral branch. 

Summary. The Charophyceae are an aberrant group, standing apart 
from the other algae. They resemble the Chlorophyceae in containing 
an excess of chlorophyll over the carotinoids and in storing starch as 
reserve food. The vegetative body is distinctive, being an erect thallus 
differentiated into nodes and internodes and with two kinds of branches 





A B 

Fig. 53. Chara. A, longitudinal section of a young oogonium, invested by a sterile jacket, 
and a young antheridium, the latter consisting of a stalk cell, an outer layer of shield cells, 
four middle cells (manubria), and four inner cells (primary capitula), X200; B, a shield 
cell from a mature antheridium with manubrium projecting from it. At the tip of the 
manubrium is a primary capitulum to which are attached smaller secondary capitula, each 
bearing a pair of spermatogenous filaments. 

arising at the nodes. There is no reproduction by spores. The sex 
organs are multicellular and complex, both being enclosed by a jacket of 
sterile cells. In this respect the Charophyceae resemble the bryophytes, 
although the development of the sex organs in the two groups is very 
different. The sperms are also like those of bryophytes. 

9. PHAEOPHYCEAE 

The Phaeophyceae, or brown algae, are nearly all marine in distribu- 
tion, occurring along most seacoasts but reaching their greatest display in 
cool waters. They range in color from olive green to dark brown as a 
result of the presence in their cells of chlorophyll and an excess of carotin 
and a unique xanthophyll, fucoxanthin, which is brown. There are no 
unicellular brown algae. Their multicellular bodies may be filamentous, 
plate-like, or may reach massive proportions and be highly differentiated 
in form. They are always attached. The Phaeophyceae are a special- 
ized group, probably derived independently from flagellate ancestors and 
apparently not related to any of the higher plants. There is no satis- 



THALLOPHYTA: ALGAE 



67 



factory fossil evidence of their existence before the Jurassic. The 
Phaeophyceae number almost 1,000 species, nearly all of which are con- 
tained in six main orders: Ectocarpales, Sphacelariales, Cutleriales, 
Dictyotales, Laminariales, and Fucales. 

1. Ectocarpales 

The Ectocarpales include the simplest of the brown algae. They 
occur along all rocky seacoasts, growing attached to rocks, piers, and 
other plants. They include over 60 genera and 300 species, forming a 
diverse assemblage that is often broken up into 
several smaller orders. Of the many genera, 
perhaps the two that are best known are Ecto- 
carpus and Pylaiella. 

Ectocarpus. Ectocarpus is a simple brown 
alga, widely distributed along seacoasts, where it 
grows attached to rocks or to other algae. It is 
filamentous and usually much branched, the older 
portions sometimes being surrounded by rhizoid- 
like branches. Otherwise the body is strictly 
mono sipho nous, each branch consisting of a single 
filament. An alga composed of parallel bundles 
of filaments is said to be polysiphonous. Growth 
of the filaments occurs mainly by intercalary 
cell divisions. Each cell contains a single nucleus 
and a number of small brown plastids. 

Zoospores and isogametes are borne in spo- 
rangia and gametangia, respectively. These de- 
velop from the terminal cell of a short lateral 
branch, but may be either stalked or sessile. The 
sporangium is globular or somewhat elongated 
(Fig. 54A). It is unicellular and contains many 
(32 or 64) zoospores. It is at first uninucleate, 
becoming multinucleate and forming zoospores by 
cleavage of the cytoplasm. The gametangium 

is longer than the sporangium and often ovate or cylindrical (Fig. 545). 
It is divided by cell walls into many small cubical cells, in each of which 
an isogamete is formed. Both the zoospores and gametes are laterally 
biciliate, the cilia being of unequal length. The pairing gametes are 
generally of the same size but, in some species, one is slightly larger than 
the other and swims less vigorously. Where this slight tendency toward 
heterogamy exists, all the gametes in a gametangium are either smaller 
(male) or larger (female). As in all the brown algae, the zygote germi- 
nates without going into a resting stage. 




Fig. 54. A sporangium 
(A) and a gametangium 
(5) of Ectocarpus, X400; 
also a single escaped zoo- 
spore, more highly magni- 
fied. 



68 



PLANT MORPHOLOGY 



■1^ 



M. 



Ectocarpus displays a primitive type of alternation of generations and 
one that is not well established. Although all the plants of a species are 
alike vegetatively, some are gametophytes and some are sporophytes. 
The gametophytes, producing gametangia, are haploid. The zygotes 
give rise to sporophytes, which are diploid. These produce two kinds of 
sporangia. One is mvilticellular and looks like a gametangium but gives 

rise to diploid zoospores that develop into 
other sporophytes. The other is the uni- 
cellular sporangium already described. 
The division of the nucleus in the young 
unicellular sporangium is reductional, 
and so the zoospores that it produces 
are haploid. These haploid zoospores 
always give rise to gametophytes. Some- 
times gametes, without pairing and 
fusing, develop directly into other hap- 
loid plants. It is apparent that much 
variation occurs in the behavior of the 
spores and gametes. 

Pylaiella. This alga resembles Ecto- 
carpus in its habitat and general struc- 
ture. It differs chiefly in that the fila- 
ments are usually only slightly branched 
and any cell may become a sporangium 
or gametangium (Fig. 55). Conse- 
quently the reproductive organs are in- 
tercalary in position and usually appear 
in a linear series. They have the same 
structure as those of Ectocarpiis, the 
sporangia being unicellular and the gam- 
etangia multicellular. Sometimes mul- 
ticellular sporangia are produced on the 
plants bearing unicellular ones. Although Pylaiella is essentially 
isogamous, one of the pairing gametes is slightly larger than the other. 
An alternation of generations is seen also in this genus, the gamete- 
producing plants being haploid and the spore-producing plants diploid. 
The reduction of chromosomes occurs in the young unicellular sporan- 
gium. 

Summary. The thallus of the Ectocarpales is usually composed either 
of freely branching filaments or wholly or in part of a plate-like or solid 
body composed of interlacing filaments. In some forms the thallus is 
parenchymatous. Vegetative growth is mainly intercalary, often being 
confined to basal portions of the branches. Reproduction occurs typi- 



■C 



■% 



1» 



A B 

Fig. 55. A row of sporangia (A) and 
gametangia (B) oi Pylaiella, X500. 



THALLOPHYTA: ALGAE 69 

cally by zoospores borne in unicellular (and multicellular) sporangia and 
by motile isogametes borne in multicellular gametangia. There is an 
alternation of generations, the haploid plants being either similar to the 
diploid plants in size and vegetative structure, or much smaller and 
simpler. 

A few heterogamous forms with unicellular sex organs borne on minute 
gametophytes, but otherwise resembling the Ectocarpales, are now segre- 
gated into two small orders: the Sporochnales and Desmarestiales. Some 
authors also segregate into the Chordariales, Punctariales, and Dictyo- 
siphonales isogamous forms with multicellular gametangia but with dis- 
similar haploid and diploid plants. 

2. Sphacelariales 

The Sphacelariales are a small but distinct order related to the Ecto- 
carpales. They are all littoral algae numbering 10 genera and 60 species, 
chiefly tropical but also occurring in temperate regions. The two chief 
genera are Sphacelaria and Stypocaulon. 

Sphacelaria. This alga grows in small tufts attached to rocks and 
other algae. It occurs along both coasts of North America but is rather 
uncommon. The vegetative body is differentiated into a flat, plate-like, 
prostrate portion and a filamentous erect portion that is freely branched, 
the branches increasing in length by means of a large apical cell (Fig. 56). 
This cuts off a series of transverse segments that then divide both longi- 
tudinally and transversely to form a polysiphonous thallus. In most 
algae, growth is intercalary, which means that it occurs by division of all 
or many of its cells. Where there is an apical cell, all the cells of the body 
are descendants of it, even though some may later divide independently. 

The sporangia and gametangia of Sphacelaria are similar to those of 
Edocarpus, the sporangia being unicellular and the gametangia multi- 
cellular. Both are short-stalked and borne on the axes. The zoospores 
and gametes are laterally biciliate and, in some species, one of the pairing 
gametes is slightly larger than the other. As in Edocarpus, there is an 
alternation of vegetatively similar generations and the number of chromo- 
somes is reduced one-half in the young unicellular sporangium. A form 
of vegetative reproduction common in Sphacelaria involves the production 
of propagules. These are short, flattened, modified branches that become 
detached and give rise to new plants. 

Summary. The thallus of the Sphacelariales is filamentous, being 
monosiphonous near the tips and polysiphonous below. Growth takes 
place by means of an apical cell. Reproduction occurs by zoospores 
borne in unicellular sporangia and motile isogametes borne in multi- 
cellular gametangia. The order displays an isomorphic alternation of 
generations. 



70 



PLANT MORPHOLOGY 




gmm 



' ° " lyj ^.Ly. ' 



ti.ii4-j\'-i- 



m 



me 



® 



tm 




B 



Fig. 56. Sphacelaria, X200. A, tip of filament, showing large apical cell and segments 
derived from it; B, slightlj' older portion of thallus, showing development of branches. 



3. Cutleriales 

The Cutleriales are a very small order including only Cutleria, with 3 
species, and Zanardinia, with 1. Both genera occur in the Mediterranean 
Sea, while Cutleria has been reported also from Florida, the West Indies, 
and the Gulf of California. The Cutleriales are more advanced than the 
two preceding orders, although apparently related to them. 

Cutleria. The best-known species of Cutleria is found in the warmer 
parts of Europe. The plants grow just below the low-tide mark. Cut- 
leria displays a heteromorphic alternation of generations, the gametophyte 
and sporophyte being unlike \'egetatively. In fact, they are so different 
in general appearance that, before they were known to belong to the same 
Ufe history, they were placed in separate genera. The gametophyte was 
called Cutleria and the sporophyte Aglaozonia. The sporophyte is a 
small, flat, lobed disk several layers of cells in thickness and about 2 to 
5 cm. in diameter (Fig. o7E). The lower side bears numerous rhizoids. 
On the upper side are enormous numbers of elongated unicellular spo- 



THALLOFHYTA: ALGAE 



71 



rangia in crowded clusters. Each sporangium has a one-celled stalk and 
produces 8, 16, or 32 laterally biciliate zoospores (Fig. 57F). At first 
the sporangium has a single nucleus, the first two divisions of which 
result in a reduction of chromosomes. After three, four, or five simul- 
taneous free-nuclear divisions have occurred, uninucleate protoplasts are 




Fig. 57. Cidleria muUifida. A, gametophyte, one-third natural size; B, male gametan- 
gium, X600; C, two female gametangia, X600; D, an egg and a sperm in the living condi- 
tion; E, voung sporophyte 30 days after fertilization; F, two nearly ripe sporangia, XGOO; 
G, a zoospore in the living condition. {A, after Thuret; D, E, G, after Yamanouchi.) 

formed by cleavage of the cytoplasm. Thus the zoospores are haploid 
(Fig. blG). 

The gametophytes, which are produced by the zoospores, are either 
male or female but are ahke vegetatively (Fig. 57.4). They are erect, 
ribbon-like, and dichotomously branched, reaching a length of about 20 
cm. They are several layers of cells in thickness. The male and female 
gametangia, which are somewhat similar in appearance, are borne in 



72 PLANT MORPHOLOGY 

clusters on both sides of the thallus, intermixed with sterile hairs called 
paraphijses. Each has a short stalk and a number of gamete-producing 
cells. The male gametangium (antheridium) is a club-shaped organ 
consisting of over 200 small cells arranged in many tiers (Fig. 57 B). 
Each cell produces a single sperm. The female gametangium (oogonium) 
has fewer cells, about 20 to 60, each giving rise to an egg (Fig. 57C). The 
eggs are considerably larger than the sperms but both are laterally 
biciliate and free-swimming (Fig. 57 D). The eggs are less active than 
the sperms, however, and usually come to rest first. The zygote germi- 
nates at once, giving rise to a sporophyte. 

Zanardinia. This genus differs from Cutleria in several ways. The 
gametophyte and sporophyte are ahke vegetatively, both being disk- 
like, several layers of cells thick, and about 5 cm. or more in diameter. 
Each sporangium produces four large bicihate zoospores, the reduction of 
chromosomes occurring when they are formed. The gametophytes are 
monoecious, the two kinds of gametangia being intermixed. The male 
gametangium produces about 250 sperms, the female gametangium about 
12 to 36 eggs. The gametes resemble those of Cutleria, both being 
laterally biciliate. 

Summary. The Cutleriales have a flat plate-like thallus that may be 
either erect or prostrate. Its growth is entirely or partially intercalary. 
The zoospores are borne in unicellular sporangia, the gametes in multi- 
cellular gametangia. The group has well-marked heterogamy, but both 
the sperms and eggs are ciliated. A distinct alternation of generations is 
present, the gametophyte and sporophyte being either vegetatively 
similar (Zanardinia) or dissimilar {Cutleria). 

4. Dictyotales 

The Dictyotales are a distinct group of brown algae occupying a some- 
what intermediate position with respect to the other groups. They are 
found in both tropical and temperate seas but always grow in warm 
waters. There are 18 genera and about 100 species. Dictijota, Padina, 
and Zonaria are well-known members. 

Dictyota. Although found along both the Atlantic and Pacific coasts 
of North America, this genus does not occur north of about 35° latitude. 
The plants grow attached to rocks in tidepools and are always submersed. 
The vegetative body consists of a thin, flat, dichotomously branched 
thallus with a basal holdfast (Fig. 58). It is composed of three layers of 
cells: an upper and a lower layer of small photosynthetic cells with a layer 
of large colorless cells between them. The thallus grows by means of a 
large apical cell, one of which lies at the tip of each branch (Fig. 59) . The 
sporophyte and gametophyte are alike vegetatively, and so alternation of 
generations is isomorphic. 



THALLOPHYTA: ALGAE 



73 



Numerous unicellular sporangia are scattered over both surfaces of the 
sporophyte (Fig. 60). Each sporangium, borne on a one-celled stalk, 
produces four nonmotile spores (apian ospores). In connection with the 
formation of four free nuclei from the single nucleus of the young spo- 
rangium, the number of chromosomes is reduced one-half. Two of the 




Fig. 58. Dictyota binghamiae. Portion of plant showing dichotomous branching, three- 
fourths natural size. 




Fig. 59. Longitudinal section of a bifurcating thallus of Dictyota dichotoma, cut parallel to 
its flat surface. The branch tip on the left shows a large undivided apical cell, while the one 
on the right has just undergone a second dichotomy. 

spores from each sporangium give rise to male plants and two to female 
plants. 

Like other members of the order, Dictyota displays well-developed 
heterogamy. The antheridia are borne in clusters of about 100 to 300 on 
both surfaces of the male plants (Fig. 61 A). The clusters are surrounded 
by several rings of sterile cells. Each antheridium is composed of a stalk 



74 



PLANT MORPHOLOGY 




Fig. CO. Dictyota dichotoma. Cros.s section of thallus with a sporangium, showing three 
of the four spores. (After Mottier, Textbook of Botany, The Blakiston Company.) 





B 

Fig. 61. Sex organs of Dictyota dichotoma. A, cross section of thallus with group of 
antheridia; B, cross section of thallus with group of oogonia. {After Mottier, Textbook of 
Botany, The Blakistori Company.) 



THALLOPHYTA: ALGAE 



75 



cell and about 1 ,500 small cells, each of which produces a sperm. Although 
the sperms are laterally biciliate, one cilium is very short. The oogonia 
are borne in groups of about 25 to 50 on both sides of the female plants 
(Fig. 615). The groups are not surrounded by sterile cells. Each 




Fig. 62. Zonaria farlowii. A, portion of thallus with numerous groups of sporangia, 
Xl}i; B, young sporangium with eight free nuclei; C, mature sporangium, the eight 
aplanospores cut off by walls; D, young antheridia; E, young oogonia; F, mature antheridia; 
G, two mature oogonia; B to (?, X300. (After Haupt.) 

oogonium consists of a small stalk cell and a single large nonmotile egg. 
The eggs are discharged into the water and there fertilized. The zygote 
gives rise to the sporophyte without undergoing any resting period. 

Zonaria. Zonaria has about the same distribution along both coasts of 
North America as Dictijota. The thallus consists of an erect fan-like 
cluster of thin flat branches arising from a stalk-like portion that is 



76 



PLANT MORPHOLOGY 



% 



m 



JWii 



;*■ i 



attached by a disk-shaped mass of rhizoids (Fig. Q2A). It grows by- 
means of a row of apical cells extending around the distal margin of each 
branch. The mature thallus is about eight layers of cells in thickness. 

The diploid sporophytes bear groups of sporangia 
intermixed with paraphyses. Each sporangivim, lack- 
ing a stalk cell, gives rise to eight large haploid aplan- 
ospores (Fig. 625, C). These produce the gameto- 
phytes, which are either male or female and resemble 
the sporophytes vegetatively. The antheridia and 
oogonia are, in general, similar to those of Dictyota 
(Fig. 62Z)-G). The zygote produces a sporophyte. 

Summary. The thallus of the Dictyotales is flat, 
plate-like, and erect. It grows by means of a single 
apical cell or a marginal row of apical cells. Repro- 
duction occurs by aplanospores, four or sometimes 
eight being developed in a unicellular sporangium, 
and by heterogametes. Small biciliate sperms are 
borne in multicellular antheridia and large nonmotile 
eggs are borne singly in unicellular oogonia. A 
distinct alternation of generations is present, the gam- 
etophyte and sporophyte being similar vegetatively. 

5. Laminariales 

The Laminariales comprise the kelps, the largest 
of the brown algae. They are widely distributed 
throughout temperate and arctic regions, occurring 
mainly in cool waters and making their greatest dis- 
play along shores bordering the North Pacific Ocean. 
Most of the Laminariales grow below the low-tide 
line. They include about 30 genera and 100 species. 
Some of the best-known members are Laminaria, 
Macrocijstis, Nereocystis, Postelsia, and Egregia. 
Laminaria, with 30 species, is the largest genus. 

Laminaria. Common along both coasts of North 
America, in cooler waters, are various species of 
Laminaria. Some are not more than 30 cm. long, 
while others reach a length of 9 to 12 m. They live 
attached to rocks just below the low-tide line. Alter- 
nation of generations is heteromorphic. The large 
vegetative plant is a sporophyte (Fig. 63). It consists of a long blade 
and a thick leathery stipe anchored by means of a branching basal 
holdfast. According to the species, the blade may be entire or divided 
lengthwise into segments. The cells of the stipe show a differentia- 



i € 



Fig. 63. Laminaria, 
a small kelp with a 
blade, stipe, and 
holdfast, about one- 
half natural size. 



THALLOPHYTA: ALGAE 



77 



tion into an outer cortical region of photosynthetic tissue and a central 
pith that usually contains storage cells. Many of the central cells are 
elongated and have pores in their end walls, thus resembling the sieve 
tubes of vascular plants. Vegetative growth is not apical but results 



WMMMM^'^M 




Fig. 64. Sporangia of Lamiiiaria, intermixed with paraphyses, X400. 




Fig. 65. Gametophjtes oi Laminaria yendoana. A and B, male gametophytes, X 1,200; 
C, a sperm, X 1,200; D, E, F, female gametophytes, X800; a, antheridia, some empty; e, 
egg; o, oogonia; s, young sporophytes arising from the fertilized egg. (After Kanda.) 



from the activity of a meristem situated at the junction of the blade and 
stipe. The meristem forms a new blade each year, replacing the old one, 
which dies off. 

Numerous unicellular, club-shaped sporangia, intermingled with long 



IP 



78 



PLANT MORPHOLOGY 



sterile cells (paraphyses) , arise in large pat ches on both sides of the thallus 
(Fig. 64). They produce 32 or 64 small, laterally biciliate zoospores. 
The reduction of chromosomes results from the division of the nucleus of 
the young sporangium. After four or five simultaneous free-nuclear 
divisions have taken place, the contents of the sporangium undergoes 
cleavage into uninucleate protoplasts, the zoospores. These are liberated 




Fig. 66. Apical portion of a plant of Macrocystis pyrifera, one-fifth natural size. 



and develop into minute male and female gametophytes (Fig. 65). The 
sperms are laterally biciliate and are borne singly in antheridia that arise 
at the ends of short, branched filaments. The female plant usually con- 
sists of only a few cells, one of which becomes an oogonium. This pro- 
duces a single nonmotile egg that is extruded through a terminal pore, to 
which it remains attached. The zygote germinates at once, giving rise 
to the large sporophyte. 



THALLOPHYTA: ALGAE 



79 



Other Kelps. As in Laminaria, the bodies of nearly all the other kelps 
are differentiated into holdfast organs, stout stalks, and flat blades often 
much divided into narrow segments. Air bladders are freciuently present. 
Reproduction is similar in all members of the order. The greatest variety 
and largest of the kelps occur along 
the Pacific coast of North America, 
where they live in water 10 to 30 m. 
deep, their stalks attached to rocky 
reefs and their blades often floating 
on the surface. Macrocijstis may 
reach a length of 30 to 50 m. A sin- 
gle plant consists of a stalk with 
many blades, each blade having a 
float (Fig. 66). Another large kelp 
is Nereocijstis, with a large hollow 
bulb at the end of a thick stalk and 
a number of blades arising from the 
bulb (Fig. 67). It reaches a length 
of 25 to 30 m. Postelsia, known as 
the "sea palm," has a stout stalk up 
to 60 cm. long bearing at its tip nu- 
merous branches terminating in nar- 
row blades (Fig. 68). Egregia, the 
"feather-boa kelp," has a long stalk 
that bears two rows of lateral blades 
and floats, the blades producing 
sporangia being much narrower 
than the sterile ones. 

Summary. The vegetative body 
of the Laminariales is highly differ- 
entiated both externally and inter- 
nally. It consists of a massive 
thallus usually with a holdfast, stipe, 
and one or more blades. Growth 
is due to an intercalary meristem. 
The large plant body is a sporo- 
phyte bearing unicellular sporangia 
that contain many zoospores. The 
gametophytes are microscopic, dioecious, and heterogamous. The sperms 
are biciliate and produced singly in unicellular antheridia. The eggs are 
nonmotile and borne in unicellular oogonia. The Laminariales have a 
heteromorphic alternation of generations. 




Fig. 67. Young plant of Nereocystis 
luetkeana, one-quarter natural size. 



80 



PLANT MORPHOLOGY 




Fig. 68. A sea palm {Postelsia palmaefonnis) growing on a rock exposed at low tide, about 
one-quarter natural size. 



6. Fucales 

The Fucales, commonly known as rockweeds, are a highly specialized 
order of brown algae standing apart from the others. They are widely 
distributed throughout tropical and temperate regions, most of them 
growing along rocky seacoasts in the intertidal zone. They comprise 32 
genera and 325 species, representative forms being Fucus, Pelvetia, 
Ascophyllum, and Sargassum. 

Fucus. Fucus is widely distributed in cool waters, being represented 
along both the eastern and western coasts of North America. The 
thallus, rarely exceeding a meter in length, is coarsely ribbon-like and 



THALLOPHYTA: ALGAE 



81 



repeatedly forked, with a basal stalk arising from a disk-like holdfast 
(Fig. 69). It is rather tough and leathery. In some species air bladders, 
giving buoyancy to the plant, are conspicuous. Growth occurs by means 
of an apical cell that occupies a notch at the end of each branch. The 




Fig. 69. 
size. 



Fucus furcatus. Portion of thallus, showing conceptacles, two-thirds natural 



apical cell is complex, having the form of a truncated quadrangular 
pyramid and cutting off cells in three planes. When the thallus branches, 
the apical cell divides vertically into two nearly equal parts, each of which 
becomes the apical cell of a new branch. Internally the thallus is differ- 
entiated into a firm outer cortex of photosynthetic tissue and a central 
colorless pith that is rather spongy. The only method of asexual repro- 
duction is by fragmentation of the thallus. There are no spores of any 
kind. 



82 



PLANT MORPHOLOGY 



Within the swollen tips of some of the branches are numerous flask- 
shaped pits or chambers, called conceptades, each with a pore-like opening 
(Fig. 70). Sperms and eggs are produced inside the conceptades, the 
sperms in antheridia and the eggs in odgonia. The antheridia are oval 




^^ 



Fig. 70. Longitudinal section of a conceptacle of Fucus furcatus, showing oogonia in 
various stages of development, small branching filaments bearing antheridia, and numerous 
paraphyses, X 100. 



and sac-like; they appear on special branching filaments that arise from 
the Avail of the conceptacle (Fig. 7L4). Each antheridium produces 64 
small, laterally biciliate sperms. The antheridium is unicellular and, 
when young, is uninucleate. The number of chromosomes is reduced 
one-half when its nucleus divides. Free-nuclear divisions continue until 
there are 32 nuclei. Then the cytoplasm undergoes cleavage to form an 
equal number of uninucleate protoplasts, each of which divides again to 



THALLOPHYTA: ALGAE 



83 



produce two sperms. The sperms escape from the antheridium in a 
mass surrounded by a membrane that soon disappears. 

The eggs of Fucus are borne in groups of eight inside the oogonia, which 
are large oval or globular cells, each of which has a one-celled stalk (Fig. 
71B-D). The young oogonium has a single nucleus, the division of which 
is reductional. Three simultaneous divisions result in the formation of 
eight free nuclei. Cytoplasmic cleavage follows and an egg is organized 
around each nucleus. The eggs are extruded from the oogonium in a 







A -^""-^ D ' ^^- E 

Fig. 71. Sex organs oi Fucus furcatus. .-i, antheridial filament, X 320; B, young oogonium 
with four nuclei, X160; C, longitudinal section of an older oogonium with eight nearl,\' 
mature eggs, X160; D, mature oogonium, X160; E, escaped egg of Fucus vesiculosus sur- 
rounded b>' numerous sperms, X 240. (E, after Thuret.) 



group surrounded by a membrane that soon ruptures. In Ascophylhun 
four eggs are organized in an oogonium, in Pelvetia two, and in Sargassum 
only one. In all the genera of Fucales, however, eight nuclei always arise 
in the oogonium, the nonfunctional nuclei either being extruded or 
degenerating. Thus Fucus represents the primitive condition from which 
the other genera, by progressive reduction, have been derived. 

Depending on the species, the antheridia and oogonia of Fucus may 
occur in the same conceptacle, in different conceptacles on the same plant, 
or on different plants. In addition to the sex organs, the conceptacles 
contain numerous unbranched sterile filaments (paraphyses), some of 
which often project through the pore. Both the sperms and eggs escape 
from the conceptacles into the water but only the sperms are motile. The 
sperms surround the eggs in such vast numbers that they cause them to 
rotate (Fig. HE). After fertilization has taken place, the zygote sur- 



84 



PLANT MORPHOLOGY 



rounds itself with a cell wall and divides at once to produce a new vege- 
tative thallus. The reduction of chromosomes occurs when the nucleus 
of the young antheridium and that of the young oogonium divide. Thus 
from the four-nucleate stage to maturity the sex organs are haploid, the 
diploid condition arising at fertilization. 

Although Fucus has no alternation of gametophyte and sporophyte 
plant bodies, there is a brief haploid phase and a prolonged diploid phase. 
Some botanists interpret the vegetative body of the Fucales as a sporo- 
phyte, the antheridia as microsporangia, 
and the sperms as microspores (small 
zoospores). They interpret the oogonia 
as megasporangia and the eggs as mega- 
spores (large aplanospores). Then, to 
explain the sexual fusion, the microspores 
and megaspores are said to function di- 
rectly as gametes. This interpretation 
implies that a gametophyte generation 
was once well developed and has become 
so reduced that it comprises only the 
haploid nuclei in the gametangia and 
the gametes themselves. 

Sargassum. This is a very large genus 
whose 250 species are widely distributed 
throughout tropical and subtropical seas, 
especially in the Southern Hemisphere. 
The vegetative body is more highly de- 
veloped than that of Fucus, having 
distinct branches, leaf-like blades, and 
often small stalked air bladders as well (Fig. 72). Sargassum may live 
either in an attached or a floating condition. Like other rockweeds, it 
grows chiefly along seacoasts, but frequently plants are torn loose from 
the rocks and carried for hundreds of miles out to sea. The Sargasso Sea 
is a vast eddy lying west of the Canary Islands. Here great floating 
masses of "gulfweed," transported by the Gulf Stream from the West 
Indies and tropical America, accumulate and propagate themselves 
by fragmentation of the thallus. 

Summary. The Fucales have a coarse, ribbon-like thallus that grows 
by means of an apical cell. Spores are not formed. The order displays 
well-developed heterogamy. The sex organs are unicellular, the anther- 
idia producing numerous biciliate sperms, the oogonia producing one, 
two, four, or eight nonmotile eggs that escape before fertilization. The 
sex organs are borne in internal cavities (conceptacles). The Fucales are 
without a distinct alternation of generations. 




Fig. 72. Small portion of a plant 
of Sargassum, showing differentiation 
into stem, leaf-like blades, and 
berry-like air bladders, natural size. 



THALLOPHYTA: ALGAE 85 

Summary of Phaeophyceae 

The Phaeophyceae are algae having in their plastids an excess of 
carotin and a brown xanthophyll pigment (fucoxanthin) over the chloro- 
phyll. All of them are multicellular, the thallus being filamentous, 
plate-like, or massive, often with differentiated tissues. The cells contain 
a definite nucleus, generally several or many plastids, and a distinct cell 
wall. Reserve food is stored chiefly as laminarin (a dextrin-like carbo- 
hydrate) or oil. Except in the Fucales, zoospores are produced or, in the 
Dictyotales, aplanospores. Gametic reproduction may occur either by 
isogametes or heterogametes. In the heterogamous forms the eggs may 
be ciliated but are generally nonciliated. All motile reproductive cells 
are laterally biciliate, the cilia being unequal in length. No resting cells 
are formed. Most members exhibit an alternation of generations, the 
Fucales, with only a diploid plant body, being a notable exception. The 
gametophyte and sporophyte are either similar or dissimilar vegetatively. 

10. RHODOPHYCEAE 

Like the Phaeophyceae, the Rhodophyceae, or red algae, are almost all 
marine in distribution but, as a rule, live in deeper and warmer waters 
than the brown algae. They include the majority of the seaweeds. 
Most of the Rhodophyceae are rose red or violet, but some are dark 
purple, reddish brown, or olive green. In addition to chlorophyll and its 
associated carotinoids, a red pigment, phycoerythrin, is present in the 
cells. This more or less obscures the chlorophyll. Many of the Rhodo- 
phyceae also contain a small amount of phycocyanin, the blue pigment 
of the Cyanophyceae. Except for several unicellular forms, whose 
inclusion in the group is doubtful, all the red algae are multicellular. 
Their bodies are not large, most of them being less than 30 cm. in length, 
while only a few are as long as 1 m. They are rather varied in form, how- 
ever, being fdamentous, ribbon-like, or plate-like, but never massive. 
They are always attached. Some are heavily impregnated with lime. 
Lime-secreting forms are known as fossils as far back as the Ordovician. 
The Rhodophyceae are the most highly specialized of all the algae. They 
are probably not related to any of the higher plants except, perhaps, to 
some of the fungi. They include about 3,000 species. 

There are seven orders of Rhodophyceae. These, together with one 
or more representative genera, are as follows: (1) Bangiales — Bangia, 
Porphyra,'Porphyridium; (2 )Nemalionales — Nemalion, Batrachospermum; 
(3) Gelidiales — Gelidium; (4) Cryptonemiales — Corallina, Lithothamnion; 
(5) Gigartinales — Plocamium, Gracilaria, Chondrus, Gigartina; (6) Rhody- 
meniales — Rhodymenia; (7) Ceramiales — Callithamnion, Ceramium, Poly- 
siphonia, Delessaria. 



86 



PLANT MORPHOLOGY 



Porphyra. Poiyhyra is a typical member of the Baii^iales, the most 
primitive order of Rhodophyceae. It is widely distributed along rocky 
seashores, occurring on both coasts of North America. It grows in the 
intertidal zone on rocks and other algae. The thallus is plate-like and 
attached by means of a small basal holdfast (Fig. 73A). It is only one or 









S^'as). 



E 



•<^^j 








Fig. 73. Porphyra perforata. A, thallus, one-half natural size; B, vertical section of 
vegetative portion of thallus; C and D, vertical sections of thalli with carpogonia and 
developing carpospores; E, surface view, showing liberation of carpospores; F, amoeboid 
carpospores; G, vertical section through portion of a thallus liberating spermatia. {From 
Gilbert M. Smith.) 

two layers of cells in thickness and, in most species, is less than 50 cm. 
long. The cells lie in a tough gelatinous matrix derived from their walls. 
They are without apparent cytoplasmic connections (Fig. 73B). Each 
cell has a nucleus that divides by a primitive type of mitosis. It also 
has a single large plastid with a central pyrenoid. 

Some species of Porphyra are monoecious but most of them are dioe- 
cious. The antheridia develop directly from the vegetative cells. A cell 
undergoes repeated divisions in three planes until 64 or sometimes 128 
small cells are formed. The walls gelatinize and free the protoplasts, 
which function as male gametes (Fig. 73G). Such naked, nonmotile male 



THALLOPHYTA: ALGAE 87 

cells, a feature of all the red algae, are called spermatia. The female sex 
organs, or carpogonia, also arise from ordinary vegetative cells hut without 
undergoing division, the protoplast functioning directly as an egg (Fig. 
730. 

A spermatium, carried by water currents to the carpogonium, enters 
and fuses with the egg. The zygote divides at once to form a group of 
spores, usually 8 or 16, that are freed by the breaking down of the surround- 
ing cell walls (Fig. 7SC-E). These naked cells are carpospores and, like 
the gametes, are nonciliated. The freed carpospores exhibit an amoeboid 
movement (Fig. 73F). After coming to rest, each carpospore forms a cell 
wall and develops into a new thallus. The reduction of chromosomes 
occurs when the zygote germinates, and so the vegetative plant is haploid. 

As compared with the higher orders of Rhodophyceae, the Bangiales 
have a simple type of nucleus and cells without evident protoplasmic 
continuity. They display intercalary rather than apical growth. The 
carpogonium either lacks a trichogyne or has a very short one. The 
zygote is transformed directly into carpospores. An alternation of 
generations is not present. 

Porphyridium is a unicellular alga whose relationships are uncertain. 
It forms a reddish gelatinous layer on damp soil and moist walls. It was 
formerly placed in the Cyanophyceae but, because it has a true nucleus 
and a distinct plastid, is now included in the Rhodophj^ceae. The cells 
are spherical and surrounded by a mucilaginous matrix. Fission is the 
only know^n method of reproduction. 

Nemalion. Although showing a considerable advance over Porphyra, 
this form is much simpler than members of the higher orders. Nemalion 
is wddely distributed, growing on rocks between the high- and low-tide 
lines. The thallus, up to 60 cm. in length, consists of a slimy mass of 
branching filaments that are interwoven to form a worm-like cylinder. 
This is composed of a central core of slender colorless filaments from 
which tufts of larger chlorophyllose filaments radiate outward. The 
cells of the latter have a small nucleus and a large plastid with a con- 
spicuous pyrenoid. As in all the algae except the Bangiales, growth is 
apical. Furthermore, the vegetative protoplasts are connected by a 
conspicuous strand of cytoplasm that passes through a pore in the cell 
wall. 

Nemalion is monoecious, the sex organs occurring at the ends of short 
branches. The antheridium consists of a single small cell that is budded 
off laterally from an antheridial branch (Fig. 74A). Its protoplast, the 
spermatium, is discharged into the water. The female organ, called a 
procarp, consists of two parts, the carpogonium and the trichogyne (Fig. 
745). The protoplast of the carpogonium functions as an egg. The 
trichogyne is a long thread-Uke cell at the upper end of the carpogonium. 



88 



PLANT MORPHOLOGY 



Its nucleus degenerates. The nonmotile male cell, or spermatium, is 
carried by water currents. After coming in contact with the trichogyne, 
it becomes binucleate. Both of the male nuclei may enter the trichogyne 
but only one passes into the carpogonium, where it fuses with the egg 
nucleus. The other male nucleus does not function. 




Fig. 74. Nemalion Inhricitm, X 700. A, portion of plant with four antheridial branches 
consisting of groups of small cells, each producing a single spermatium; B, a carpogonial 
branch, terminating in a carpogonium with its slender trichogyne to which a spermatium 
is attached; C, a cjstocarp, composed of fertile filaments that cut off terminal carpospores. 

Following fertilization, many short filaments, called gonimohlasts, arise 
from the carpogonium and at the tip of each a carpospore is organized. 
After a carpospore is shed, another may be cut off from the cell behind it. 
It is only in the Bangiales that carpospores are produced by direct division 
of the zygote. In Nemalion the carpogonium, fertile filaments, and 
carpospores collectively form the cystocarp (Fig. 74C). The carpospores, 
upon being set free as naked, nonmotile cells, develop into sexual plants. 



THALLOPHYTA: ALGAE 



89 



The reduction of chromosomes occurs just after fertihzation, when the 
fusion nucleus in the carpogonium divides. Thus the gonimoblasts, 
carpospores, and sexual plants are hapioid. There is no true alternation 
of generations. 

Batrachospermum. This is a widely distributed fresh-water alga, 
growing in streams attached to rocks along the bottom. The plants 
are blue-green, olive green, violet, or reddish. The variation in color is 
primarily a result of differences in light 
intensity, plants growing in shallow water 
being greener than those in deeper water. 
Bafrachospcrnmm is related to Nemalion 
but differs from it in several respects. 
The vegetative body consists of long 
branching filaments of unUmited growth 
bearing whorls of dwarf branching fila- 
ments of limited growth (Fig. 75). The 
long filaments consist of an axial row of 
cells which, in the older portions of the 
body, is covered by a layer of small- 
celled filaments that form a sheath 
around it. The cells of the sheath arise 
from the basal cells of the dwarf fila- 
ments. Growth occurs by means of an 
apical cell. 

The sex organs are borne at the ends 
of some of the dwarf filaments and re- 
semble those of Nemalion (Fig. 76). 
After coming in contact with the trichogyne, the spermatium remains 
uninucleate instead of becoming binucleate. Moreover, following fer- 
tilization, the cells at the base of the carpogonium send out loose fila- 
ments that grow up around and invest the cystocarp while the carpo- 
spores are being produced. 

The germinating carpospore gives rise to a branching filamentous body 
that is much simpler than the gamete-producing plant. This plant, 
which represents a juvenile stage in the life cycle, may multiply by mono- 
spores, which are formed singly within sporangia at the ends of short 
lateral branches. Finally a special branch appears that becomes a 
gamete-producing plant. As in Nemalion, the chromosome number is 
reduced one-half when the fusion nucleus divides in the fertilized 
carpogonium. Conseciuently, the juvenile plant is not a sporophyte and 
there is no alternation of generations. 

Polysiphonia. Polysiphonia is a widespread genus of about 150 species. 
It is abundant along the Atlantic coast of North America but is less com- 




FiG. 75. Small portion of the 
vegetative body of Batrachospermum, 
showing dwarf filaments arising in 
whorls from the cylindrical main axis, 
X400. 



90 



PLANT MORPHOLOGY 



mon along the Pacific coast. It grows in tide pools on rocks and on other 
algae. It is a more highl}^ developed but more typical red alga than any 
of the others that have been discussed. The plant body, reaching a 
length of 25 cm., is filamentous and polysiphonous, being made up of an 








F G H 

Fig. 76. Batrachosperrrmm. A, antheridial branch with globular antheridia, one of which 
has liberated its protoplast; B, young carpogonial branch, the terminal cell forming the 
carpogonium and trichogyne; C, mature procarp with nucleated carpogonium and swollen 
trichogyne; D, later stage, showing spermatium united with tricliogyne and male nucleus 
fusing with carpogonial nucleus; E, completed fusion of male and female nuclei; F, develop- 
ment of gonimoblasts from carpogonium, a sterile branch arising on the left; G, further 
development of gonimoblasts and sterile filaments; H, formation of carpospores; A to F, 
X960; G and H, X720. {After Kylin.) 

axial row of elongated cells surrounded and completely covered by several 
rows of smaller peripheral cells that are cut off from the central cells by 
longitudinal divisions. Growth occurs by means of an apical cell. 

The reproductive features of Polysiphofiia are complex. Nonmotile 



THALLOPHYTA: ALGAE 



91 





A C 

Fig. 77. Polysiphonia. ^, portion of plant bearing tetraspores, X 160; B, portion of male 
plant bearing clusters of antheridia, X200; C, portion of female plant with cystocarp con- 
taining carpospores; also a single carpospore, X80. 



spores are formed in groups of four in a one-celled sporangium (Fig. 77 A). 
They are called tetras'pores. Chromosome reduction occurs in connection 
with the formation of the tetraspores, and so each is haploid. Upon 
germination, two tetraspores from each 
sporangium produce male plants and two 
produce female plants. These sexual plants 
are like the tetrasporic ones in general 
appearance. 

The antheridia occur in dense clusters on 
special lateral branches of the male plants 
(Fig. 775). In their formation, a number 
of cells arise laterally from the cells of the 
axial filament, giving rise to a simple mono- 
siphonous branch. Each cell of this branch 
develops two-celled lateral branches. An 
oblique division of the terminal cell of each 
branch produces a unicellular antheridium 
(Fig. 78). The antheridium discharges its 
protoplast, which functions as a nonmotile 
male cell, or spermatium. Other antheridia 
may then be budded off the same terminal 
cell. 

Besides the carpogonium and trichogyne, 
the procarp includes several other cells as 
well. It arises from a large 'pericentral cell 
that first produces a row of four cells, the terminal one becoming the 




Fig. 78. Diagram of antheridial 
branch of Polyalphonia, showing 
three stages in the development 
of an antheridium (o), whose 
protoplast functions as a male 
cell. {After Yamanouchi.) 



92 



PLANT MORPHOLOGY 



carpogonium (Fig. 79.4, B). The nucleus of the carpogonium divides 
into two nuclei, one of which passes into the trichogyne and finally dis- 
integrates, while the other remains in the carpogonium and functions as 
an egg nucleus. The pericentral cell also gives rise to a group of auxiliary 
cells, one of which crowds in between the pericentral cell and the carpo- 
gonium (Fig. 79C). The entire structure comprises the procarp. 

The free-floating spermatium, coming in contact with the trichogyne, 
remains uninucleate. The male nucleus enters the trichogyne, passes 





Fig. 79. Diagrams showing development of procarp of PoJysiphonia. A, early stage: 
B, later stage, the pericentral cell (p) having produced four cells, the terminal one forming 
the carpogonium (c) and trichogyne (0 ; C, mature stage, a group of auxiliary cells having 
developed from the pericentral cell. (After Yamanouchi.) 

into the carpogonium, and fuses with the female nucleus. A passageway 
to the pericentral cell now is opened through the intervening auxiliary 
cell and the fusion nucleus passes through. Then all the cells of the 
procarp unite and the fusion nucleus divides to form many nuclei. Lobes 
into which these nuclei pass are put out from the procarp and then the 
carpospores are cut off. The whole structure comprises the cystocarp 
(Fig. 77C). The usual envelope of sterile cells is formed around it. 

After escaping from the cystocarp, a carpospore gives rise to a tetra- 
sporic plant. A stalked sporangium is produced laterally from an axial 
cell, pushing through the cortical cells. The tetraspores give rise to 
sexual plants. Polysi'phonia displays an isomorphic alternation of 
generations. The sexual plants, with haploid cells, are gametophytes. 
The tetrasporic plant, with diploid cells, is a sporophyte. The sporophyte 
generation, however, beginning with the zygote, includes also the cysto- 



THALLOPHYTA: ALGAE 93 

carp and carpospores. The gametophyte generation begins with the 
tetraspore. 

Summary. In the Rhodophyceae both chlorophyll, with its associated 
carotinoids, and a red pigment (phycoerythrin) are present in the plastids. 
With only a few possible exceptions, all members are multicellular, the 
thallus being most commonly filamentous but often plate-like. The cells 
contain a definite nucleus (sometimes more than one), one or more 
plastids, and a cell wall that is often gelatinous. Reserve food is stored 
chiefly as "fioridean starch." Reproduction occurs by means of aplano- 
spores and heterogametes. The female organ is a carpogonium. Carpo- 
spores arise from the zygote, either directly (in the Bangiales) or indi- 
rectly (in the other Rhodophyceae). Except in the Bangiales and 
Nemalionales, both carpospores and tetraspores are produced, the latter 
by a sporophyte. All reproductive cells are nonciliated. No resting 
cells are formed. A distinct alternation of generations is a feature of all 
red algae except the Bangiales and Nemalionales. The gametophyte and 
sporophyte are similar vegetatively. 

COMPARISON OF THE CLASSES OF ALGAE 

The most important distinguishing characters of the ten classes of 
algae are as follows: 

Cyanophyceae. Cells containing, in addition to chlorophyll and 
carotinoids, a blue pigment (phycocyanin) and frequently a red pigment 
(phycoerythrin) also. Pigments not confined to definite plastids. 
Reserve food stored as glycogen. Plant body unicellular, generally 
colonial. Cells without a well-organized nucleus. Cell walls usually 
forming abundant mucilage. Reproduction by fission, never by zoo- 
spores or gametes No ciliated cells. 

Euglenophyceae. Cells with green plastids containing an excess of 
chlorophyll over the carotinoids; frequently colorless. Reserve food 
stored as paramylon. Unicellular and usually solitary; sometimes 
colonial. Cell walls almost always absent. Free-swimming or, when 
colonial, attached. Reproduction by fission, rarely by isogametes (?). 
Motile cells with one or two ciha attached anteriorly, equal or unequal. 

Chrysophyceae. Cells with golden brown plastids containing chloro- 
phyll and an excess of carotinoids; sometimes colorless. Food stored as 
oil or leucosin. Unicellular and often colonial, rarely multicellular. 
Cell walls almost always absent. Free-swimming or sometimes attached. 
Reproduction by fission, and sometimes by zoospores, rarely by isoga- 
metes (?). Motile cells with one or two cilia attached anteriorly, equal 
or unequal. 

Dinophyceae. Cells with yellow-brown plastids containing chloro- 
phyll and an excess of carotinoids, or colorless, storing food as starch or 



94 PLANT MORPHOLOGY 

oil. I^nicelhilar and mostly solitary, rarely multicellular. Sometimes 
naked but usually with sculptured cell walls. Nearly all free-swimming. 
Keproduction by fission, and sometimes by zoospores, rarely by isoga- 
metes (?). Motile cells generally with two laterally attached cilia, one 
lying in a transverse groove. 

Xanthophyceae. Cells with yellow-green plastids containing a larger 
proportion of carotinoids than chlorophyll. Reserve food stored as oil 
or leucosin. Unicellular (and uninucleate), (;oenocytic, or multicellular. 
Cell walls often absent, when present usually consisting of two over- 
lapping halves. Reproduction by fission or by spores and isogametes. 
Motile cells with two unecjual cilia attached anteriorly. 

Bacillariophyceae. Cells with golden-brown plastids containing an 
excess of carotinoids over chlorophyll and storing food mainly as oil. 
Unicellular and either solitary or colonial. Cell w^all consisting of two 
overlapping valves, highly silicified. Reproduction by fission, auxo- 
spores, and isogametes. Motile cells rare. 

Chlorophyceae. Cells with plastids containing a greater proportion of 
chlorophyll than carotinoids. Reserve food usually stored as starch. 
Unicellular (and uninucleate), coenocytic, or multicellular. Unicellular 
forms solitary or colonial. Reproduction by fission or by spores and 
either isogametes or heterogametes. Motile cells generally with two 
or four cilia attached anteriorly. 

Charophyceae. Cells with plastids containing a greater proportion of 
chlorophyll than carotinoids; usually storing food as starch. Multi- 
cellular. Reproduction by heterogametes formed in complex multi- 
cellular sex organs of a distinctive type. Sperms with two equal cilia 
attached anteriorly. 

Phaeophyceae. Cells with plastids containing chlorophyll and an 
excess of carotin and a brown pigment (fucoxanthin). Reserve food 
occurring chiefly as laminarin or oil. Multicellular. Reproduction by 
spores and either isogametes or heterogametes. Motile cells laterally 
biciliate, the cilia of unequal length. 

Bhodophyceae. Cells with plastids containing chlorophyll, carotinoids, 
and a red pigment (phycoerythrin). Reserve food stored chiefly as 
"floridean starch." With rare exceptions multicellular. Reproduction 
by spores and heterogametes, these never ciliated. 

GENERAL CONCLUSIONS 

The algae constitute the simplest and oldest group of green plants. 
Their bodies are adapted, both in structure and function, to live in water. 
Although knowledge is lacking concerning the nature of the first plants 
to have Uved on the earth, they must have been aquatic and may have 
been similar to some of the existing blue-green algae. These plants are 



THALLOPHYTA: ALGAE 95 

unicellular, have a very primitive cell structure, and reproduce by fission. 
Because they live in a variety of habitats, including hot springs, they 
may have lived on the earth before conditions were favorable for the 
existence of other green plants. 

The algae are not a homogeneous assemblage l)ut embrace a number 
of groups representing divergent lines of descent, all of which probably 
have had a common origin. Advanced students of the algae try to trace 
these lines of evolution, but we shall be concerned mainly with certain 
general tendencies and with the progress that the group as a whole has 
made. 

Development of Multicellular Bodies. The unicellular plant body 
obviously represents the simplest condition of structural organization 
and, necessarily, also the oldest. It is characteristic of all the blue-green 
algae, flagellates, dinoflagellates, diatoms, and many of the green algae. 
Unicellular plants may be either solitary or colonial, the latter condition 
having arisen from the tendency of cells, following division, to remain 
together for a while before separating. In the evolution of the algae, 
close association of cells in a colony may have led to a dependence of the 
cells on one another, with the resultant establishment of a multicellular 
body. It is significant that, among the algae, no sharp distinction exists 
between highly organized colonies and simple multicellular plants. This 
intergradation strongly indicates that multicellular plants have been 
derived from unicellular ones through the formation of colonies. 

Although the multicellular bodies of algae are very diverse in form, they 
may be referred to three main types: filamentous, plate-like, and massive. 
The filamentous type is most common, probably because it seems best 
adapted to aquatic life. In such a body all the cells are in direct contact 
with water and the absorbing surface is very large. Thus the absorption 
of gases is greatly facilitated. The massive type of body, as exemplified 
by many brown algae, is adapted to withstand the buffeting action of 
waves and water currents along rocky seacoasts. In the simple multi- 
cellular algae growth is intercalary, every cell having the power of divi- 
sion. In branching forms growth is often limited to definite regions, such 
as the terminal cell of each branch. In many brown and most red algae 
growth occurs by means of an apical cell that cuts off a series of posterior 
segments. 

An important evolutionary tendency exhibited by the algae is for 
certain cells to become structurally differentiated in response to special 
functions. It occurs in both colonial and multicellular forms. A simple 
expression of this tendency is seen in those filamentous algae having the 
basal cell modified as a holdfast. In many branching filaments the cells 
of the branches are smaller than those of the main filament. The forma- 
tion of sporangia and gametangia represents a specialization of certain 



96 PLANT MORPHOLOGY 

cells for reproduction. Differentiation becomes marked among the 
brown algae, especially in the Laminariales and Fucales, where the body- 
consists of distinct vegetative organs within which simple tissues may be 
formed. A highly differentiated vegetative body is also characteristic of 
the Charophyceae and many marine Siphonocladiales and Siphonales. 

Asexual Reproduction. In reproduction, as in vegetative structure, 
the algae show a progressive advance. Most unicellular forms increase 
in number by fission, which is merely reproduction by cell division and is 
obviously the most primitive method in the plant kingdom. Among 
multicellular forms cell division results in growth and, to make repro- 
duction possible by other means than fragmentation, cells must be Ub- 
erated from the parent. The spores of algae are merely detached cells 
with the capacity of directly producing a new plant. They result not 
only in a multiplication of individuals but in their widespread distribu- 
tion. Spores may be formed from a cell with or without previous division 
of its protoplast. The commonest kind of spores in the algae are zoo- 
spores — naked cells ' with cilia. Nonmotile spores with a cell wall 
(aplanospores and akinetes) are generally formed in response to adverse 
environmental conditions, to which they are very resistant. Obviously 
they have been derived from zoospores that have lost the power of loco- 
motion. The same may be true of the nonmotile spores of the red algae. 
Fission and spores produced by a haploid plant body are a means of 
accompHshing vegetative or asexual reproduction because no reduction 
of chromosomes is involved. This is the only kind present in the blue- 
green algae, flagellates, dinoflagellates, many diatoms, and a few green 

algae. 

Like vegetative spores, the spores produced by two successive divisions 
of a diploid cell, involving a reduction of chromosomes, are usually 
regarded as asexual. In reality, however, they belong to the sexual life 
cycle, since meiosis is always a necessary conseciuence of a previous 
gametic union. Although such spores are functionally equivalent to the 
zoospores and aplanospores produced by a haploid plant body, they are 
not homologous with them, and should be designated as meiospores. 
Meiospores are produced by the zygote in such green algae as Ulothrix 
and Oedogonium, and by the sporophyte in all algae with an alternation 
of generations. 

In most green algae any ordinary vegetative cell is capable of producing 
spores. In nearly all the brown and red algae, however, spores are not 
borne in transformed vegetative cells but in sporangia, which are cells 
specialized for reproduction. Sporangia differ from ordinary vegetative 
cells in size or shape and sometimes are restricted to definite parts of the 

body. 

Sexual Reproduction. Sexual reproduction is accomplished by 
gametes and represents a distinct advance over reproduction by vegeta- 



THALLOPHYTA: ALGAE 97 

tive spores. Its essential feature is the fusion of two cells to form a 
zygote. It seems certain that originally gametes were derived from 
vegetative zoospores that had become too small to form a new plant 
directly. This is shown by the fact that in Ulothrix and many other 
isogamous algae zoospores and gametes intergrade, the smaller spores 
often germinating but producing dwarf filaments. The derivation of 
gametes from zoospores is shown also by the striking similarity between 
them in form and in the number and arrangement of their cilia. Any 
peculiarity in the spores is duplicated in one or both of the gametes, as in 
Oedogonium, the brown algae, etc. With few exceptions (notably the 
Conjiigales, Charophyceae, and Fucales), gametic reproduction has not 
replaced reproduction by vegetative spores but is supplementary to it. 
In nearly all the green algae the zygote is a resting cell, accjuiring a heavy 
wall and carrying the plant through a period of severe conditions into the 
next growing season. In fact, the formation of gametes is often induced 
by the advent of such conditions. In the brown and red algae the zygote 
germinates at once. 

Originality, in the evolution of the algae, both of the fusing gametes 
were ciliated and of the same size. This condition of isogamy is retained 
by the yellow-green algae, diatoms, and many of the simpler green and 
brown algae. In such forms as Pandorina, one of the pairing gametes is 
slightly larger and less active than the other. In Cutleria both gametes 
are ciliated, but the female gamete is considerably larger than the male. 
In Dictyota and Fucus the female gamete (egg) is increased in size still 
more and, although extruded into the water, is nonciliated, only the 
male gamete (sperm) being motile. Finally, in Oedogonium and many 
other algae, the large nonmotile egg is not liberated but is fertilized 
within the oogonium by the small motile sperm. Thus the evolution of 
heterogamy from isogamy has been gradual. 

After sexual reproduction had become established, one gamete retained 
its motility and small size, while the other sacrificed its motility for an 
increased nutritive capacity. The advantage of heterogamy lies in the 
greater amount of reserve food that comes to be stored in the zygote. 
This advantage is reflected by the occurrence of heterogamy in many 
green algae, most brown algae, all stoneworts, and all red algae, as well 
as in all plants above the thallophyte level. It should be emphasized 
that heterogamy has arisen independently in the various groups of algae 
where it occurs. 

The production of gametes in ordinary vegetative cells is characteristic 
of Ulothrix, Oedogonium, and most other green algae. A more advanced 
condition is seen in Vaucheria, the Charophyceae, and nearly all the 
brown and red algae, where the gametes are borne in gametangia or sex 
organs, which are specialized for reproduction, a function lost by the 
other cells of the body. This tendency parallels the production of spores 



98 PLANT MORPHOLOGY 

in sporangia. The sporangia remain unicellular but the gametangiaof 
some algae have become multicellular by the formation of cross walls, 
as in Ectocarpus and Cidleria. In Diciyota the antheridia are multicel- 
lular and the oogonia are unicellular. 

Alternation of Generations. In nearly all the green algae the vegeta- 
tive plant, of which there is but one kind, gives rise to gametes and is 
haploid. Here the diploid condition, which always results from fertiliza- 
tion, is restricted to the zygote itself, since the reduction of chromosomes 
takes place when it germinates. This reduction always involves the 
formation of four haploid nuclei. In Oedogonium each of the four zoo- 
spores (meiospores) coming from the zygote contains one of these nuclei. 
In Spirogyra three of the nuclei degenerate and the zygote gives rise 
directly to a haploid vegetative body. In Coleochaete four haploid cells 
are formed by the zygote, but each divides one, two, or three more times 
before zoospores are organized. In the two lower orders of red algae 
(Bangiales and Nemalionales) an analogous condition exists in the forma- 
tion of carpospores. 

In some of the algae, notably in the diatoms, Acetahularia, C odium, 
Bryopsis, and the Fucales, there is only one kind of vegetative body and 
it is diploid, the reduction of chromosomes occurring in connection with 
the formation of gametes, or in several nuclear divisions immediately 
preceding their formation. This is also the condition in animals. 

Some botanists recognize an alternation of generations wherever there 
is a diploid and a haploid phase in the life history, even though one or 
the other is represented by only one cell — in other words, wherever there 
is sexual reproduction. Such a broad use of the term makes it almost 
meaningless. In algae displaying a true alternation of generations, a 
more or less prolonged growth phase intervenes between fertilization 
and meiosis, as well as between meiosis and fertilization. Here the life 
history involves two distinct and independent vegetative bodies, a 
haploid body (gametophyte) producing gametes and a diploid body 
(sporophyte) producing spores. The gametophyte arises from a spore, 
the sporophyte from a zygote. The reduction of chromosomes occurs 
when the spores are formed. A true alternation of generations is found 
in only a very few green algae, such as Ulva and Cladophora, in all brown 
algae except the Fucales, and in all red algae except the Bangiales and 
Nemalionales. The alternation may be isomorphic, with both genera- 
tions alike vegetatively, as in Ulva, Cladophora, Ectocarpus, Dictyota, 
and Polysiphonia, or it may be heteromorphic, with both generations 
unlike vegetatively, as in Cutleria and Laminaria. In all algae possessing 
a true alternation of generations both gametophyte and sporophyte are 
free-living; one is never dependent upon the other. 

Interrelationships. It is not possible to arrange the classes of algae 
in such a way as to indicate their true relationship. The secjuence in 



THALLOPHYTA: ALGAE 99 

which these groups have been presented is merely one denoting an ever- 
increasing complexity in vegetative and reproductive structures. It 
does not denote descent of one group from the one preceding it in the 
series, although in some cases this may be true. Each group merely 
stands for a different degree of progress from what was originally a 
primitive condition. 

The most important evidence concerning the interrelationships of 
plant groups is derived from paleobotany. The dearth of fossils belong- 
ing to groups below the pteridophyte level is so great, however, that 
practically all conclusions regarding phylogeny must be based on the 
comparative structure and development of existing plants. This means 
that such conclusions, even though well substantiated, are largely 
speculative. 

The Cyanophyceae are the most primitive group of autotrophic plants. 
Except for the presence of chlorophyll, they are strikingly like the bac- 
teria. Which of these groups appeared first on the earth is very uncer- 
tain, but is unimportant in connection with the present discussion. Both 
groups are at a very low level of structural organization. The classes 
consisting mainl}^ of flagellates show a considerable advance over the 
Cyanophyceae by their well-organized nuclei, definite plastids, and 
ciliated cells. In the absence of transitional forms, any direct connection 
between the Cyanophyceae and flagellates is difficult to visualize. It is 
easier to think of the flagellates as having arisen directly from the bac- 
teria. A direct relationship between the Cyanophyceae and any of the 
higher algal classes is also unlikely, although there is some evidence of 
this in the case of the Rhodophyceae. Ciliated cells are not present in 
either group; some members of each have both phycocyanin and phyco- 
erythrin; and a few primitive Rhodophyceae have a nuclear structure but 
little advanced over that of the Cyanophyceae. It is primarily the 
absence of ciliated cells that would seem to preclude the possibility of a 
relationship between either group and the flagellates. 

That the Xanthophyceae and Chlorophyceae have arisen independ- 
ently from a flagellate ancestry is strongly indicated by the presence of 
naked, free-swimming reproductive cells in the life history and by the 
occurrence of intermediate forms. The derivation of the Bacillario- 
phyceae and Phaeophyceae directly from flagellate ancestors is less evi- 
dent. The only connection between the diatoms and flagellates is the 
presence of ciliated reproductive cells in a few diatoms. The origin of 
the Phaeophyceae is obscure because the group is without unicellular 
members. Yet their motile reproductive cells suggest that they may 
have arisen from brown, laterally biciliate flagellates. There is also the 
possibility of a direct connection between the Phaeophyceae and Chloro- 
phyceae. The Charophyceae are an isolated group, yet seem to repre- 
sent a specialized offshoot from the Chlorophyceae. 



CHAPTER IV 
THALLOPHYTA: FUNGI 

Fungi are dependent (heterotrophic) thallophytes. Lacking chloro- 
phyll, they are unable to carry on photosynthesis and hence must obtain 
their food from an external source. Many are saprophytes, living on 
dead organic matter; others are parasites, obtaining nourishment from 
the bodies of living plants or animals, the organism attacked being the 
host. At least some of the fungi may have evolved directly from the 
algae through loss of power to carry on photosynthesis. Because of 
their relation to the decomposition of organic matter and to disease, 
fungi are of tremendous economic importance. The fungi comprise the 
five classes Schizomycetes, Myxomycetes, Phycomycetes, Ascomycetes, 
and Basidiomycetes. To these might be added the class Lichenes. 

1. SCHIZOMYCETES 

The Schizomycetes, or bacteria, are similar in many respects to the 
Cyanophyceae, differing from them chiefly in their smaller size and lack 
of chlorophyll. In fact the two groups are often combined into a single 
group, the Schizophyta. The bacteria are at once the smallest and 
simplest of all known organisms, unless the viruses are to be considered 
as living. They are also the most widely distributed, occurring under 
all conditions where life may exist — in fresh and salt water, in soil, in 
the air, and in the living and dead bodies of other organisms. Like the 
Cyanophyceae, they are a very ancient group and must have been among 
the first forms of life to have existed on the earth. The bacteria com- 
prise about 1,500 species. Some common genera are Streptococcus, 
Micrococcus, Sarcina, Bacterium, Bacillus, Pseudomonas, Microspira, 
Spirilhim, Cladothrix, and Beggiatoa. 

Structure and Reproduction. Like the blue-green algae, bacteria are 
unicellular plants that reproduce by fission. Their cells are of three 
general types: spherical (coccus) forms, rod-shaped {bacillus) forms, and 
curved or spiral (spirillum) forms (Fig. 80). Some are nonmotile, while 
others bear cilia, by means of which they move rapidly. The cilia 
may cover the entire cell or may be restricted to one or both ends, where 
they occur either singly or in tufts. The rod-shaped types average about 
2.5ii in length,! while many of the spherical forms are only about 0.5^ 
in diameter. 

' The unit of microscopic measurement is the viicron, abbreviated ju- It is one- 
thousandth of a millimeter (0.001 mm.), approximately equivalent to 1/25,000 inch. 

100 



THALLOPHYTA: FUNGI 101 

The cells of bacteria are so simple that they might almost be said to 
be structureless. A mass of homogeneous protoplasm is surrounded by 
a thin cell wall, generally composed chiefly of chitin, a nitrogenous sub- 
stance, whereas the cell walls of green plants are composed mainly of 
cellulose, a carbohydrate. Commonly the cell wall becomes mucilagi- 
nous, forming a slimy sheath or capsule. There is no organized nucleus 
but merely some scattered granules of a chromatin-like material that 
can be revealed by staining. In some bacteria these granules are aggre- 
gated to form a distinct central group. Other granules may also be 
present; these generally represent reserve food. 



8B 0®^) 







i 

D '^ E F 

Fig. 80. Group of common bacteria, X 1,500. A, Sarcina lutea; B, Bacillus subtilis; C, 
Bacillus typhosus; D, Spirillum cholerae; E, Streptococcus pyrogenes; F, Spirillum undulatum,. 

In some bacteria the two cells separate following division, while in 
others they remain together in colonies. Spirillum forms are nearly 
always solitary. In the coccus forms the colonies may be cubical, 
plate-like, chain-like, filamentous, or irregular. In the bacillus forms 
the divisions occur only in one plane, and so the colonies are always 
filamentous. In Beggiatoa, a sulphur bacterium, the filaments are as 
highly organized as those of Oscillatoria. In Cladothrix, an iron bac- 
terium, the filaments exhibit false branching. 

As in the Cyanophyceae, cell division takes place by the formation of 
an inward-growing cell wall. Under favorable circumstances, cell divi- 
sion in many bacteria may occur as frequently as every 20 minutes, so 
that, in the course of 24 hours, a single cell may give rise to billions. Such 
a rate of multiplication is soon checked, however, by the exhaustion of 
the food supply or by the accumulation of poisonous waste products of 
metabolism. Although all bacteria are active only in the presence of 
moisture and other favorable conditions, if these fail, many bacteria can 
pass into a resting stage and remain inactive for a long time. Bacteria 
on dust particles in the air are in a dormant state and can resist desicca- 



102 PLANT MORPHOLOGY 

tion and great extremes of temperature. In some bacilli the resting cell 
becomes an endospore. Here the protoplast rounds up inside the cell 
cavity and invests itself with a new cell wall, the old wall eventually 
disappearing. Endospores are extremely resistant. With the return of 
favorable conditions, they again become active vegetative cells. Thus 
"spore formation" in bacteria does not result in reproduction but merely 
in survival during a period of stress. 

Although the bacteria are said to be without sexual reproduction, 
there is some evidence that it may occur at least in certain bacteria, since 
there seems to be Mendelian segregation resulting from mixtures of 
different types. 

Activities. Most bacteria are either saprophytic or parasitic and in 
both cases food is absorbed through the cell wall. Some can live either 
as saprophytes or as parasites, while a few can make their own food 
without the aid of chlorophyll or light. Aerobic bacteria require free 
oxygen in respiration, while anaerobic bacteria obtain oxygen from organic 
compounds. 

Most diseases of animals and many diseases of plants are caused by 
parasitic (pathogenic) bacteria, the disease itself being merely a response 
on the part of the host to the presence of the parasite. A disease mani- 
fests itself by symptoms, which are abnormalities in structure or function. 
Well-known human diseases of bacterial origin are typhoid fever, tuber- 
culosis, diphtheria, pneumonia, cholera, and tetanus. Some bacterial 
diseases of plants are pear blight, cabbage rot, cucurbit wilt, and crown 
gall. The disease may be caused by direct attack of the bacteria on the 
host tissues, by the liberation of toxins, or by both. 

The decomposition of dead organic matter is accomplished chiefly 
by saprophytic bacteria. They break up organic compounds into sim- 
pler substances through a series of intermediate steps, a succession of 
different bacteria being involved. The ultimate products of decom- 
position are such simple substances as water, carbon dioxide, ammonia, 
hydrogen sulphide, etc. Bacteria of decay cause fermentation and 
putrefaction. They play an important part in the economy of nature 
by returning to the air and soil substances that may again be used by 
other organisms. 

All plants require nitrogen in order to synthesize proteins, but only 
the nitrogen-fixing bacteria and a few other forms are able to use the 
nitrogen of the air directly. Practically all green plants are dependent 
for nitrogen upon its compovnids, particularly nitrates. Some of the 
nitrogen-fixing bacteria, such as Clostridium and Azotobacter, live free in 
the soil and are saprophytic on organic matter, while Rhizobium is para- 
sitic in the roots of various Leguminosae, such as clover, alfalfa, peas, 
beans, etc. These bacteria combine atmospheric nitrogen with oxygen 



THALLOPHYTA: FUXGI 103 

and other elements, particularly potassium, sodium, or calcium, and form 
nitrates, which may later be utilized by green plants. The root of the 
legume responds to the presence of these parasitic bacteria by forming 
local enlargements called tubercles or nodules. 

Nitrifying bacteria also live in the soil but form nitrates in a different 
way. The decomposition of organic matter by bacteria of decay yields 
ammonia (NH^). This is oxidized, first to nitrites (NO2 compounds) 
by Nitrosomonas, and then to nitrates (NO3 compounds) by Nitrohacter. 
An interesting fact about these bacteria is that, although lacking chloro- 
phyll, they are able to synthesize carbohydrate food from water and 
carbon dioxide (or carbonates). They derive their energy, not from 
sunlight, but from the oxidations that they carry on. With respect to 
their nutrition, these bacteria, like green plants, are autotrophic, even 
though they do not carry on photosynthesis. The process by which 
they make their own food is sometimes called chemosynthesis. Such 
autotrophic bacteria may have preceded all other forms of life on the 
earth. In addition to the nitrifying bacteria there are other kinds that 
are autotrophic. They oxidize sulphur, hydrogen sulphide, free hydro- 
gen, methane, or iron compounds. Beggiatoa is a colorless filamentous 
form that oxidizes hydrogen sulphide (H2S) to form water and sulphur, 
storing the sulphur as yellow granules inside its cells. It is found in 
sulphur springs. Certain iron bacteria oxidize ferrous iron compounds 
to ferric hydroxide (FeOHs), which accumulates to form a kind of iron 
ore. These bacteria are common in bogs. 

Denitrifying bacteria live in the soil, especially where poorly drained. 
They convert nitrogen salts into gaseous nitrogen. This escapes into 
the air and so causes a loss of soil fertility. 

Myxobacteria. The myxobacteria are a group of peculiar organisms 
that live as saprophytes on animal refuse. Their cells resemble those of 
true bacteria but form remarkable complex colonies held together by 
mucilage. Some of the myxobacteria form stalked sporangia that are 
often brightly colored. Some exhibit slow creeping movements. In 
these respects the group resembles the myxomycetes. 

Summary. The Schizomycetes are the simplest of all plants. All of 
them are unicellular, the cells being either solitary or in colonies. A defi- 
nite cell wall is present, generally composed of chitin rather than of 
cellulose, and usually breaks down to form mucilage. The protoplast 
shows Httle organization, a nucleus being represented only by scattered 
granules of chromatin. Reproduction occurs by fission. Some bacteria 
move by means of cilia, while others are nonmotile. In some species a 
resting cell (endospore) may form inside a vegetative cell, becoming 
invested with a new cell wall. The Schizomycetes are closely related to 
the Cyanophyceae, differing from them chiefly in the lack of chloro- 



104 PLANT MORPHOLOGY 

phyll, presence of cilia in some members, and character of the rest- 
ing cell. 

2. MYXOMYCETES 

The Myxomycetes, or slime molds, are peculiar organisms that, like 
the flagellates, are claimed by both botanists and zoologists, the latter 
calling them Mycetozoa (fungus-animals). They are a widely dis- 
tributed group, living in damp, shady places as saprophytes on humus, 
decaying wood, bark, fallen leaves, etc. All lack chlorophyll. The 



^,:Mm 


















<■-)■:■' !» 



't- 













I 



Fig. 81. Plasmodium of Didymitim, a slime mold, X30. {From Gilbert M. Smith.) 

Myxomycetes number over 400 species. Some of the common genera 
are Lycogala, Stemonitis, Fuligo, Arcyria, and Trichia. 

Plant Body. The vegetative body of a myxomycete is a Plasmodium, 
which is a naked mass of multinucleate protoplasm (Fig. 81). The 
nuclei, like those of the higher plants, are well organized. The Plas- 
modium is without definite form and may attain a diameter of several 
centimeters, or even a meter in some myxomycetes. The Plasmodium 
moves by the formation of pseudopodia and engulfs solid particles of food 
as it passes over them, digesting them within food vacuoles. In these 
respects it resembles an amoeba. It also absorbs food material in solu- 
tion through the plasma membrane. Depending on the species, the 
Plasmodium may be white or some shade of yellow, orange, red, brown, 
or violet. The Plasmodium tends to move toward moisture but shows an 
avoiding reaction to light, appearing at the surface of its substratum 
only at night. In times of drought it retracts itself into a waxy mass and 



THA LLOPH VTA: F UNGl 



105 



hardens, forming a sclerotium. In this condition the organism may 
remain dormant for months, or sometimes even for years, becoming 
active again in the presence of water. 

Reproduction. Although the myxomycetes are animal-hke in their 
vegetative state, their reproductive features are distinctly plant-like. 
When reproduction is to occur, the entire Plasmodium comes to the 





B 




^^'^\'^~Pi^Z ~^ 







F G 

Fig. 82. Group of common slime molds, showing sporangia arising from the Plasmodium. 
j4, Hemitrichia ovata, XlO; B, Craterum leucocephalum, XIO; C, Arcyria incarnata, X5; 
D, Stemonitis herbatica, X2; E, Diachea leucopoda, XIO; F, Lycogala epidendrum, XI; G, 
Fuligo septica, X J'^ . 

surface of its substratum and contracts into a cushion-like mass. As this 
hardens, it forms one or more sporangia that are usually brown or yellow 
(Fig. 82). In some myxomycetes the entire Plasmodium may be con- 
verted into a single giant sporangium, called an aethalium, but, more 
commonly, a number of small separate sporangia are formed. These 
may be either sessile or stalked. Throughout the various genera the 
sporangia exhibit much diversity in form, but are commonly spherical, 
oval, or cjdindrical. The sporangium contains many nuclei and the 
remains of the Plasmodium, the latter usually forming a network of 
tough strands known as the capillitium (Fig. 83). In the meshes of this 




106 PLANT MORPHOLOGY 

network innumerable spores are formed, each one being uninucleate. 
The spores have cellulose walls and are scattered by the wind. In 
their dispersal the wall erf the sporangium ruptures irregularly at the 
apex and the capillitium performs hygroscopic movements. 

The myxomycetes display considerable variation with respect to the 
development of the Plasmodium from a spore. Commonly the proto- 
plast escapes from the spore wall and becomes a zoospore, developing 
one long cilium and one very short one, both anteriorly attached. Some- 
times two to eight zoospores are produced. The zoospore may ingest 

food and undergo multiplication by fission. 
After a period of free swimming, the cilia are 
retracted and the protoplast becomes amoe- 
boid. These amoeboid cells, called myx- 
amoebae, may also take in food and divide 
repeatedly, or they may pass into a resting 
stage. Finally, however, they fuse in pairs. 
Then, instead of forming resting zygotes, a 
number come together to form a multi- 
nucleate Plasmodium. In the fusion of the 
small amoeboid cells in pairs, the two nuclei 
Fig. 83. Portion of capii- Unite, but there are no subsequent nuclear 
litium of stemonitis with fusions. Consequently the nuclei of the plas- 

spores in its meshes, X 250. i • i • i 

modmm are diploid. They undergo repeated 
divisions as the Plasmodium increases in size. Reduction of chromosomes 
to the haploid number occurs just prior to the formation of spores in the 
sporangium. 

Summary. The Myxomycetes combine features found in both plants 
and animals. The body is a naked mass of multinucleate protoplasm 
(a Plasmodium) that displays amoeboid movements and engulfs solid 
food particles. In a quiescent state it gives rise to sporangia containing 
numerous walled spores from which eventuall}^, although not directly, 
a new Plasmodium is formed. Sexual reproduction occurs by a fusion 
of similar amoeboid gametes. Certain aspects of the life history suggest 
a relationship to the flagellates. Any possible connection with the true 
fungi is very uncertain. 

Other Slime Fungi. In addition to the Myxomycetes, or slime molds, 
there are two other groups of slime fungi that deserve some attention. 
These are the Acrasieae and the Labyrinthuleae. Many mycologists 
include all three groups in a separate assemblage, the Myxothallophyta, 
and place them outside and below the fungi. They have certain impor- 
tant characters in common: simple, naked, nucleated, amoeboid cells 
resembling protozoans but plant-like in their reproduction by the forma- 
tion of spores. The interrelationships of the three groups of slime fungi 



THALLOPHYTA: FUNGI 



107 



are not well understood, but are not assumed to be close. They have 
proliably been derived from protozoan ancestors and have evolved along 
independent lines. 

Acrasieae. These simple forms are saprophytes on soil, decaying wood, 
and animal refuse. The vegetative body is a myxamoeba, a naked cell 
with a nucleus and a contractile vacuole. It reproduces by fission and 
in the presence of unfavorable conditions may encyst. Eventually, a 
number of myxamoebae come together without fusing to form a pseudo- 
plasmodium in which each m3^\amoeba retains its individuality. Not 




S HRS 

Fig. 84. Three-dimensional graph showing the development of the fruiting body of 
Dictyostelium discoideum in height, time, and position. A, aggregation of a mass of 
individual myxamoebae; B to D, formation of the pseudoplasmodium; E toG, migration of 
pseudoplasmodium; H to N, formation of fruiting body with disk, stalk, and spherical spore 
mass. {From J. T. Bonner.) 

only is a multinucleate plasmodium lacking, but no zoospores are pro- 
duced. The pseudoplasmodium assumes a definite form, usually elongat- 
ing and varying in length from several tenths of a millimeter to a milli- 
meter or more. 

The subsequent behavior of the pseudoplasmodium is remarkable 
in that it migrates over the substratum, apparently by a gliding of the 
amoebae over one another. After coming to rest, the pseudoplasmodium 
is transformed into a fruiting body consisting of a basal disk, a vertical 
stalk, and a terminal spherical region that is converted into a mass of 
spores (Fig. 84). In some species the fertile region consists of a series 
of spherical spore masses arranged at the ends of whorled branches. 
All these complex changes are accomplished by movements of individual 
myxamoebae to their proper place in the fruiting body, where each 
becomes the type of cell appropriate for its position, such as a disk cell, 
stalk cell, or spore. The spores have a cell wall. Upon germination, 
each spore gives rise to a myxamoeba. 



108 PLANT MORPHOLOGY 

Labyrinthuleae. In this little-known group the vegetative cell is 
spindle-shaped with tufts of pseudopodia at the ends. When the cells 
come in contact, their pseudopodia generally fuse, the union of numerous 
cells producing a net-like structure called a net-plasmodium. The 
individual cells, retaining their identity, appear to glide along the threads 
of the net in limited movements. During this stage they feed, increase 
in size, and undergo repeated division. In dividing, the cells become 
constricted at the middle and then separate, but are held together by a 
protoplasmic strand. At the close of the vegetative stage, the cells 
collect into sessile or stalked masses and become encysted. In some 
species the spores have cell walls, in others not. Later the spores germi- 
nate, freeing one to four spindle-shaped cells with polar pseudopodia. 

3. PHYCOMYCETES 

The Phycomycetes, or alga-like fungi, comprise the first group of "true 
fungi" (Eumycetes), as the higher fungi are often called in contrast to 
the bacteria and myxomycetes. All the true fungi have a definite nucleus 
and nearly all have a characteristic plant body called a mycelium. This 
is composed of branching filaments, each branch being a hypha. The 
hyphae may be either loosely or compactly interwoven. With few excep- 
tions, the Phycomycetes are characterized by an absence of cross walls 
in the mycelium, and so, as in Vaucheria, the plant body is a coenocyte. 
Their spores are borne in indefinite numbers within a sporangium. The 
origin of the Phycomycetes is not clear. They may have evolved either 
from colorless flagellates or, through loss of chlorophyll, from the Chloro- 
phyceae, a group which they resemble in both vegetative and reproduc- 
tive features. A number of Phycomycetes cause diseases of economic 
plants, such as cranberry gall, brown rot of lemon, downy mildew of 
grape, and late blight of potato. The group is a relatively small one, 
numbering about 1,000 species. These are included in seven main 
orders: Chytridiales, Monoblepharidales, Plasmodiophorales, Sapro- 
legniales, Peronosporales, Mucorales, and Entomophthorales. 

1. Chytridiales 

The Chytridiales are the simplest of the Phycomycetes. Nearly all 
of them are parasitic, many living on fresh-water algae and others 
attacking seed plants growing in moist situations. The order includes 
about 65 genera and 300 species, the best-known forms being Chytridium, 
Olpidium, and Synchytrium. 

Chytridium. A common species of Chytridium attacks the green alga, 
Oedogonium. A uniciliate zoospore comes in contact with an oogonium 
of the host, loses its cihum, and sends into the host cell a tube through 
which food is absorbed. This tube represents a weakly developed myce- 



THALLOPHYTA: FUNGI 



109 



Hum. The external part of the fundus then becomes transformed into a 
sporangium, its protoplast undergoing cleavage into many zoospores 
(Fig. S5A). A zoospore may penetrate a zygote of the host and, by the 
secretion of a thick wall, become a resting spore. When the zygote 
germinates, the resting spore of the fungus sends out tubes that give rise 
to terminal sporangia. 

Olpidium. This fungus grows on many different hosts, some of which 
are fresh-water algae. One species, Olpidium brassicae, attacks young 




A B 

Fig. 85. Chytridiales. A, sporangium of Chytridium olla attached to zygote of Oedo- 
gonium; B, sporangia of Olpidium brassicae in root of cabbage seedling; also two zoospores of 
same to the left. {A, after Campbell; B, after Woronin.) 

cabbage plants. A uniciliate zoospore comes to rest on the host, with- 
draws its cilium, and secretes a cell wall. It sends a short tube into the 
host and the protoplast enters one of the cells. At first the protoplast is 
naked and amoeboid. It enlarges and becomes multinucleate, finally 
occupying the whole cell cavity. Then it forms a cell wall and becomes 
a sporangium. A tube is sent to the surface of the host and numerous 
uniciliate zoospores escape through it (Fig. 855). Sexual reproduction 
occurs by means of isogametes that are formed like the zoospores but 
escape and fuse in pairs. The zygote sends a short tube into a host cell, 
after which its protoplast enters, enlarges, and secretes a thick wall. 
After resting over the winter, it gives rise to a number of uniciliate 
zoospores. 

Synchytrium. This form attacks the epidermal cells of various seed 
plants, such as cranberry, primrose, hog peanut, filaree, and many others. 
A disease called cranberry gall is caused by Synchytrium vaccinii, while 



no 



PLANT MORPHOLOGY 



the destructive l)lack wart of the potato is caused by Stjnchytrium endo- 
bioticum. A uniciHate zoospore comes in contact with a young epidermal 
cell of the host and enters it. Without forming a cell wall, the protoplast 
of the fungus enlarges and lives symbiotically with the protoplast of the 
epidermal cell, not killing it but causing it and the adjacent cells of the 




C D 

Fig. 86. Stages in the development of the sporangia of Synchytrium decipiens. A, greatly 
enlarged fungous protoplast in leaf of host after having destroyed one of its epidermal cells; 
B, division of large nucleus of fungus to form many small free nuclei; C, cleavage of proto- 
plast into many small uninucleate cells; D, separation of small cells to form sporangia, 
each of which has become multinucleate; E, enlarged portion of same; A to D, X125; E, 
X500. 



host to enlarge. A small gall or blister forms on the surface of the host, 
this serving as a means by which an infected plant can be recognized. 
Blisters usually appear both on the leaves and stems. 

Finally, the infected epidermal cell dies. Then the fungus secretes 
a wall about itself and goes into a resting stage (Fig. 86^). Later 
its nucleus undergoes repeated divisions and progressive cleavage of the 
cytoplasm from the surface inward results in the formation of many 
protoplasts, each of which secretes a wall (Fig. 865, C). These cells 
may be multinucleate when formed but, if uninucleate, they soon become 
multinucleate by additional free-nuclear divisions (Fig. 86Z), E). Each 



THALLOPHYTA: FUNGI 



ni 



of these cells becomes a sporangium and, when conditions are favorable, 
gives rise to a number of naked zoospores (usually 8 to 12) that escape. 
Frequently, however, the resting cell arising from a vegetative protoplast 
divides to form a number of gametangia rather than sporangia. Each 
of these produces many isogametes that, after escaping, fuse in pairs. 
The zygote invades a host cell and then goes into a resting stage, forming 
a thick wall. Later it gives rise to many zoospores. Both the zoospores 
and gametes of Synchytnum are uniciliate. 

Summary. Most of the Chytridiales are unicellular fungi with either 
no mycelium or a ver}^ poorly developed one. Generally all or most of 
the vegetative body develops into a sporangium or gametangium. 
Reproduction occurs by uniciliate zoospores or isogametes. Because 
they possess the simplest tj^pe of sexual reproduction known among the 
fungi, the Chytridiales are regarded by some mycologists as primitive 
forms, while others consider them to be degenerate Phycomycetes. 

2. Monoblepharidales 

The Monoblepharidales are a very small order containing 2 genera: 
Monoblepharis, with 6 species, and Monoblepharella, with 2 species. 




ABC 

Fig. 87. Monoblepharis sphaerica. A, end of hypha with young oogonium and a young 
antheridium just below it; B, sperms escaping and approaching the mature oogonium with 
its single egg; C, zygote with empty antheridium below it. (After Cornu.) 

Monoblepharis is a saprophyte on decaying aquatic vegetation. It has 
a well-developed mycelium that produces sporangia and sex organs. 
The sporangia are terminal club-shaped cells containing many vmiciliate 
zoospores. Sexual reproduction is heterogamous. The oogonium is a 
globular cell, cut off by a wall commonly at the end of a hypha (Fig. 87). 
Its protoplast rounds up and becomes a large uninucleate egg. The 
antheridium usually arises immediately below the oogonium as a short 



112 



PLANT MORPHOLOGY 



slender branch that is cut off by a basal wall. It gives rise to a number 
of uniciliate sperms that escape and swim in the water. A sperm enters 
the oiigoniiiin through a terminal pore and unites with the egg. The 
]\Ion()l)lophari(hiles are remarkal)le in being the only fungi with swimming 
sperms. According to the species, the zygote may mature either inside 
or outside the oiigonium. It becomes a thick-walled resting cell, later 
producing a new mycelium. 

3. Plasmodiophorales 

This order comprises 8 genera and 23 species, of which the best known 
is Plasmodiophora brassicae, a parasite attacking cabbages and other 
crucifers and causing a disease known as clubroot. Another member of 
the group, Spongospora subterranea, is responsible for a disease of potatoes 





A B 

Fig. 88. Section of a portion of a cabbage root, showing two stages in the development of 
Plasmodiophora brassicae within the cells, X250. A, plasmodium completely filling a cell; 
B, spore formation. 

called powdery scab. The Plasmodiophorales, once regarded as para- 
sitic myxomycetes, are now generally considered to belong to the lower 
phj^comycetes. 

When cabbages are attacked by Plasmodiophora, the root undergoes 
a marked enlargement. The cells of the root are invaded by biciliate 
zoospores. The cilia, attached anteriorly, are of unequal length.^ The 
zoospores lose their cilia and become amoeboid, migrating directly 
through the cell walls of the host. An amoeboid cell (myxamoeba) gives 
rise to a multinucleate plasmodium (Fig. 88A). This soon undergoes 
cleavage into many uninucleate, walled cells, each of which is said to 
form four or eight biciliate isogametes that pair and fuse. The amoeboid 
zygote enlarges, becomes multinucleate, and migrates into another cell 
of the root, which it finally fills. The diploid nuclei of the young plas- 
modium continue to divide until the two reduction divisions have 
occurred. Then a number of small spores are formed, each with a cell 

1 Until recently it was thought that the zoospores were uniciliate and, chiefly on 
this basis, Plasmodiophora and related forms were classified as a family under the 
Chytridiales. 



THALLOPHYTA: FUNGI 



113 



wall (Fig. 88B). These are liberated by decay of the host. Upon 
germination, each gives rise to a zoospore. 

4. Saprolegniales 

The Saprolegniales, or water molds, are an order of aquatic fungi, 
usually occurring in ponds and streams. Most of them are saprophytic 
on plant or animal remains lying in the water, while a few are parasitic. 
Many also occur on damp soil. The order includes 20 genera and about 
120 species, representative members being Saprolegnia and Achlya. 



/ 



\ 




A B 

Fig. 89. Saprolegnia. A, a sporangium and three escaped zoospores, X350; B, an 
oogonium with many eggs and with two antheridia in contact with it, X350. 

Saprolegnia. This common water mold usually lives on dead insects, 
fishes, tadpoles, and other aquatic animals. Sometimes it attacks 
living fishes and fish eggs. The vegetative body consists of a delicate, 
branching, coenocytic mycelium that penetrates the food supply. Some 
of the hyphae form terminal sporangia, each of which is a slender elon- 
gated cell, cut off by a basal wall, and giving rise to many uninucleate 
zoospores (Fig. 89A). These are developed by progressive cleavage of 
the cytoplasm within the sporangium. The zoospores escape singly 
into the water through a terminal pore in the sporangial wall. In Achlya 
they escape as a mass. 

The zoospores of Saprolegnia are oval and have two equal cilia attached 
apically. After swimming for a while, they become quiescent, form a 
cell wall, and go into a dormant stage. After about 24 hours, the proto- 
plasts escape and again become motile, but this time the spores are 
kidney-shaped and laterally biciliate. Finally they settle down and, 
on a suitable substratum, each produces a new mycelium. The occur- 



114 PLANT MORPHOLOGY 

rence of two types of zoospores is very puzzling and its significance has 
never been satisfactorily explained. 

Saprolegnia is heterogamoiis, forming sex organs on special branches 
of the mycelium (Fig. 89B). The oogonium is a spherical cell that pro- 
duces several eggs, sometimes many, rarely only one. At first they are 
multinucleate but, by degeneration of the extra nuclei, become uni- 
nucleate. The antheridium is a slender curved tube that arises just 
below the oogonium or, in most species, from an adjacent hypha. Each 
oogonium may be surrounded by several antheridia. Both kinds of sex 
organs are cut off from the vegetative mycelium by a basal wall. No 
sperms are organized. Instead, the tip of the antheridium comes in 
contact with the oogonium and sends into one or more of the eggs a fer- 
tilization tube through which some of the cytoplasm and a male nucleus 
pass. This nucleus unites with the egg nucleus, resulting in fertilization. 
The zygote secretes a heavy wall and usually remains dormant for several 
months, finally producing a new mycelium. In Achlya it has been shown 
that the reduction of chromosomes occurs when the zygote germinates. 
In some species of Saprolegnia the antheridia are nonfunctional, while 
in others antheridia are not even formed. Nevertheless the eggs become 
thick-walled and later germinate, thus developing by parthenogenesis. 

In Achlya, which is dioecious, the appearance of sex organs is caused by 
hormone-like substances. These are secreted into the water by the male 
and female plants and stimulate production of sex organs of the opposite 
sex. A hormone produced by the male plants causes oogonia to appear 
on the female plants, while a hormone produced by the female plants 
results in the appearance of antheridia on the male plants. 

Summary. The Saprolegniales are chiefly saprophytes. They are 
aquatic fungi Avith a well-developed mycelium. They produce biciliate 
zoospores in persistent sporangia. All of them are heterogamous. The 
oogonium contains one or more eggs that are fertilized by a male nucleus 
coming from the antheridium through a fertilization tube. In most 
members the entire oogonial protoplast enters into the formation of eggs. 
The absence of swimming sperms in an exclusively aquatic order is a 
noteworthy feature. 

5. Peronosporales 

The Peronosporales, or downy mildews, are mostly parasites that 
attack various seed plants, the mycelium living within the intercellular 
spaces of the host. The order includes about 12 genera and 150 species, 
representative members being Pythium, Albugo, Phytophthora, Plasmo- 
para, and Peronospora. 

Albugo. This fungus lives as a parasite on a number of different seed 
plants. A common species, Albugo Candida, attacks various members of 



THALLOPHYTA: FUNGI 



115 



the Cruciferae, such as radish, turnip, mustard, and shepherd's-purse, 
causing a disease known as white rust of crucifers. The rayceHum, which 
may hve in almost any part of the host, ramifies throughout the inter- 
cellular spaces and sends short button-Uke haustoria into the living cells. 
Here and there beneath the epidermis the mycelium gives rise to com- 
pact clusters of erect sporangiophores from the ends of which thin-walled, 




A C 

Fig. 90. Sporangia and sex organs of Albugo. A, cross section of a small portion of the 
stem of shepherd's-purse, showing sporangiophores of Albngo Candida arising beneath the 
epidermis and giving rise to multinucleate sporangia, X 600; B, sex organs of Albugo Candida, 
a fertilizing tube from an antheridium penetrating an oogonium with a single nucleus in the 
ooplasm, X750; C, oogonium of Albugo portulacae with multinucleate ooplasm, X500. 



globular, multinucleate sporangia are cut off in chains (Fig. 90.4). 
These push up the epidermis and form a white blister on the surface of 
the host. These blisters may appear on the leaves, stems, floral parts, 
or fruits. Finally, the epidermis is ruptured and the sporangia are 
carried by the wind to uninfected hosts. Here they give rise to 12 or 
more laterally biciliate zoospores that escape, swim about for a while, 
encyst, and finally produce new mycelia. When a spore germinates, it 
produces a hypha that enters the host through a stoma. 

The sex organs of Albugo Candida, appearing later in the season than 
the sporangia, are formed on the mycelium deep within the host tissues. 



116 PLANT MORPHOLOGY 

The oogonium is a globular multinucleate cell, cut off by a cross wall from 
the swollen end of a hypha (Fig. 905). The cytoplasm becomes differ- 
entiated into a peripheral zone, the periplasm, and a central denser 
region, the ooplasm, which becomes the egg. At first both regions are 
multinucleate, but later all nuclei degenerate except a single nucleus in 
the ooplasm. The antheridium, appearing on a separate hypha, is a 
slender multinucleate cell. After coming in contact with the oogonium, 
it sends into it a fertilizing tube that extends into the egg, where a male 
nucleus and a small amount of cytoplasm are discharged. Following 
fusion of the male and female nuclei, the periplasm is used up in the 
formation of a heavy wall around the zygote. The zygote is finally freed 
by decay of the host tissues and, after undergoing a period of rest, gives 
rise to more than one hundred biciliate zoospores, each of which may, 
under appropriate conditions, produce a new mycelium. 

Albugo bliti, a species common on the pigweed {Amaranthus) , differs 
from Albugo Candida in several respects. Periplasm and ooplasm are 
differentiated but the latter remains multinucleate. The entire contents 
of the antheridium are discharged into the egg and each male nucleus 
fuses with a female nucleus. In Albugo portulacae, which lives on the 
common purslane (Portulaca), multinucleate pairing and fusing also occur 

(Fig. 90C). 

Other Downy Mildews. The Peronosporales include genera that 
bear sex organs like those of Albugo, but differ in the way their sporangia 
and spores are formed. Some of these are of considerable economic 
importance. A species of Pythium is frequently the cause of a disease of 
seedlings known as damping-off. It is particularly common in green- 
houses and other warm, moist places. Pythium is intermediate between 
the Saprolegniales and Peronosporales in that it produces zoospores in 
both permanent and detachable sporangia. 

Phyfophthora infestans causes a serious potato disease called late blight, 
while another species, Phytophthora cifrophtJiora, is responsible for the 
brown rot of lemon. Plasmopara viticola causes downy mildew of the 
grape, a very destructive disease. In both Phytophthora and Plasmopara 
the internal mycelium sends erect sporangiophores to the surface of the 
host (Fig. 91). Instead of forming blisters, as in Albugo, the sporangio- 
phores push out through the stomata and bear solitary terminal sporangia 
on branches. The sporangia, which are shed without opening, are carried 
by the wind to uninfected hosts, where each produces several biciliate 
zoospores. These form a new mycelium within the leaf. 

Peronospora is a large genus of about 60 species, some of which are 
parasitic on various garden vegetables, such as cabbage, spinach, onion, 
pea, etc. It is of interest in that, in many species, no zoospores are pro- 
duced, the detachable sporangia giving rise to new mycelia directly. 



THALLOPHYTA: FUNGI 



117 



Summary. The Peronosporales are almost all internal parasites on 
seed plants. They have a well-developed mycelium and small multi- 
nucleate sporangia that, with few exceptions, are borne on erect spo- 
rangiophores. The sporangia are almost always detachable and, after 
dispersal by the wind, give rise to biciliate zoospores or, in some cases, 
to a new mycelium directly. All members are heterogamous. The 
oogonium produces only one egg, in the 
formation of which the outer portion of 
the oogonial protoplast is not included. 
The male nucleus reaches the egg through 
a fertilization tube developed by the 
antheridium. 

6. Mucorales 

The Mucorales are the black molds, most 
of which are terrestrial saprophytes living 
on decaying vegetable and animal matter. 
There are about 30 genera and 400 species, 
common representatives of the group being 
Rhizopus, Mucor, and Pilobolus. The 
largest genus is Mucor, with about 50 
species. 

Rhizopus. The common black mold that 
grows on moist stale bread is Rhizopus 
nigricans. It also occurs on fruits, vege- 
tables, jelly, and other decaying organic 
matter. The mycelium consists of a white 
fluffy mass of profusely branched coenocytic 
hyphae. These grow horizontally over the 
substratum, sending into it tufts of short 

root-like haustoria through which food is absorbed (Fig. 92.4). Erect un- 
branched sporangiophores arise in clusters from the mycelium at places 
where the haustoria are formed. Each sporangiophore produces a large, 
globular, terminal sporangium. In its development, the tip of the spo- 
rangiophore enlarges as additional cytoplasm and nuclei pass into it (Fig. 
93.4). Soon the peripheral part of the enlarging sporangium becomes 
denser than the central portion and a line of vacuoles appears between 
them (Fig. 93B). These two regions are then separated by a cleavage 
furrow, arising from below, and finality by a dome-shaped wall. This 
projects into the sporangium to form a columella (Fig. 92B). 

The portion of the sporangium lying between the columella and the 
outer wall now undergoes a process of progressive cleavage, whereby it 
becomes divided into numerous small, multinucleate protoplasts by 




Fig. 91. Plasmopara viticola on 
the stem of grape. Sporangio- 
phores bearing numerous spo- 
rangia are emerging through a 
stoma, X200. 



118 



PLANT MORPHOLOGY 



furrows that start at the surface and grow inward (Fig. 9ZC-E) . Finally, 
each protoplast secretes a cell wall and becomes a minute, black, multi- 
nucleate spore (Fig. 93F). The spores, produced in enormous numbers, 
are liberated into the air by rupture of the sporangial wall. Upon reach- 
ing a suitable supply of food, they give rise to new mycelia. The replace- 
ment of zoospores by aerial spores is a notable feature of the Mucorales. 




Fig. 92. Rhizopus nigricans. A, horizontal branch of mycelium producing haustoria and 
sporangia, X 15; B, a mature sporangium, showing central columella, X 150; C, D, E, stages 
in sexual reproduction, resulting in the formation of a heavy-walled zygote, X 150. 



Sexual reproduction occurs in Rhizopus only under special conditions 
(Fig. 92C-E'). A short lateral branch is put out by each of two hyphae 
lying parallel to each other. Their tips come in contact, enlarge, and 
from each a multinucleate cell is cut off by a cell wall. Although ordi- 
narily of the same size, often one cell is slightly larger than the other. 
Finally, the wall between the cells is dissolved and their contents fuse to 
form a zygote. Many of the nuclei become associated in pairs and fuse, 
the others disintegrating. The zygote enlarges and becomes a thick- 
walled resting cell. The two conjugating cells are usually interpreted as 
gametangia and their contents as large compound isogametes. It has 
been observed in other Mucorales, but not in Rhizopus, that the zygote. 



THALLOPH i TA : F UNGI 



119 




Fig. 93. Development of the .sporangium of Rhizopus nigricans. A, young sporangium; 
B, appearance of small vacuoles between outer and inner parts of sporangium; C, enlarge- 
ment and fusion of vacuoles to form columella cleft; appearance of cleavage furrows at 
outer surface; D, enlarged view, showing early cleavage furrows and scattered nuclei and 
vacuoles; E, sporangium completely cut off from columella; cleavage further advanced; F, 
mature sporangium. (After Swingle.) 



120 PLANT MORPHOLOGY 

upon germination, gives rise to a short hypha bearing a terminal sporan- 
gium. This contains many aerial spores. Meiosis occurs during the 
first two divisions of the fusion nucleus in the zygote. 

Although all the mycelia of Rhizopus appear to be ahke, gametic repro- 
duction does not take place unless two sexually differentiated myceha, 
designated as plus and minus strains, come together. This may happen 
very infreciuently for, when a mycelium produces spores, all the resulting 
mycelia belong to the same strain and conjugation does not take place 
between them. Molds with sexually differentiated strains are said to be 
heterothaUic, while those without such differentiation are homothaUic. In 
homothallic species conjugation may take place between any two hyphae, 
even those of the same mycelium. In some of the heterothaUic Muco- 
rales, when a sporangium is formed at the end of a hypha arising from the 
zygote, a segregation of strains occurs, so that some of the spores in the 
sporangium produce plus myceha and others minus myceha. In other 
heterothaUic species this sporangium contains spores of one kind or the 
other, but not both kinds. In Rhizopus nigricans it is not known where 
the segregation of strains takes place. 

Pilobolus, which lives on barnyard refuse, is an interesting mold with a 
peculiar method of spore dispersal. As the sporangium ripens, the por- 
tion of the sporangiophore just below it enlarges and becomes very 
turgid. Finally it bursts suddenly, shooting out the entire sporangium 
with considerable force, sometimes to a distance of 2 m., and always 
toward the brightest source of light. 

Summary. The Mucorales are largely saprophytic fungi with a well- 
developed mycelium. They produce no zoospores, asexual reproduction 
occurring by aerial spores borne in sporangia. Sexual reproduction is 
isogamous, conjugation occurring between the entire contents of two 
multinucleate gametangia. 

7. Entomophthorales 

The Entomophthorales constitute a small group of fungi, most of 
which are parasitic on insects. The order includes 6 genera and about 50 
species, the best-known genera being Empusa and Entomophthora. A 
common species, Empusa muscae, attacks the housefly. The mycelium, 
which is feebly developed, penetrates the body of the host and eventually 
kills it. Then it sends out numerous sporangiophores, from each of 
which a single multinucleate sporangium is cut off (Fig. 94). This is 
forcibly discharged into the air and, upon coming in contact with an unin- 
fected fly, produces a new mycelium. Although it becomes detached and 
functions directly as a spore, the sporangium of Empusa corresponds to 
the sporangium of the Mucorales. In Entomophthora the sporangiophores 



THALLOPHYTA: FUNGI 



121 



are branched and the sporangia uninucleate. Sexual reproduction seems 
to be absent in Emqmsa muscae but, in several other species, as in the 
Mucorales, it occurs by the conjugation of multinucleate protoplasts, each 
representing the whole contents of a gametangium. 




c@ 



©■) 



\®( 



"t; 




B 

Fig. 94. Development of the sporangium of Empusa muscae, X600. A, hyphal body 
elongating to form a sporangiophore; B, migration of nuclei to apex; C, formation of 
multinucleate sporangium at tip of sporangiophore. 



4. ASCOMYCETES 

The Ascomycetes, or sac fungi, constitute the largest group of fungi. 
They differ from the Phycomycetes in having a septate mycelium, that is, 
one divided by cross walls into cells. They are also characterized by the 
production of spores in a sac-like structure called an ascus. This is a cell 
that at first contains two nuclei. These fuse and the fusion nucleus 
typically gives rise to eight nuclei by three successive divisions, the first 
two of which are reductional. From these haploid nuclei, eight walled 
ascospores are then organized. In all except the lowest orders, the asci are 
enclosed by a definite fruiting body, the ascocarp, composed of interwoven 
hyphae. The relationships of the Ascomycetes are obscure. They maj^ 
have been derived either from the Phycomycetes or from the Rhodophy- 
ceae. The group is of immense economic interest, many members causing 
serious plant diseases, such as peach leaf curl, brown rot of stone fruits, 
black knot of plum, apple scab, and bitter rot of apple. There are about 
25,000 species of Ascomycetes. These are included in nine main orders: 
Protoascales, Protodiscales, Plectascales, Perisporiales, Pezizales, Helvel- 
lales, Tuberales, Pyrenomycetales, and Laboulbeniales. 



122 



PLANT MORPHOLOGY 



1. Protoascales 

The Protoascales include the yeasts and other simple forms, most of 
which are regarded as degenerate Ascomycetes. They number about 500 
species. These are mainly saprophytes but some are parasites on animals. 
A few of the saprophytic; forms have a mycelium. In the yeasts, which 
are unicellular fungi, a mycelium ordinarily is not developed. Yeasts are 
of economic value in breadmaking and in the preparation of alcoholic 
beverages. The best-known genus is Saccharomyces. Some yeasts repro- 
duce by fission but most of them reproduce by budding (Fig. 95). A bud 




Fig. 95. Saccharomyces cerevisiae. Cells in the living condition, showing reproduction by 
budding, X 1,500. 



arises as a small outgrowth, usually at one end of the cell. The nucleus 
divides to form two nuclei, one of which goes into the bud. The bud 
enlarges and becomes abstricted from the parent cell. It may either 
separate at once or remain attached and produce another bud. In this 
way short chains may be formed. 

In many yeasts, under conditions unfavorable for vegetative activity, 
the contents of any cell may divide to form four or, in some species, eight 
thick-walled spores, thus becoming a simple ascus. In some yeasts a con- 
jugation of two cells precedes the formation of ascospores. The develop- 
ment of an ascus directly from the zygote is a feature occurring only in the 
Protoascales. 

There is considerable variation in the life history of different yeasts, and 
even in the same yeast under different environmental conditions. Thus 
the ascospores may enlarge to form vegetative cells that undergo a long 
period of multiplication, or they may conjugate at once. The zygote may 
become an ascus directly, or may give rise to vegetative cells that later 
become asci. Under unfavorable conditions, vegetative multiplication 
may be omitted. If no conjugation occurs, the ascospores are formed by 
parthenogenesis. 



THALLOPHYTA: FUNGI 



123 



Yeasts present three different types of life cycles. The first may be 
illustrated by Schizosaccharomyces odosporus, a fission yeast. Here the 
vegetative cells are haploid, and eight spores arise in the cell formed by the 
conjugation of two cells (Fig. 96). The zygote is the only diploid cell in 
the life history, meiosis occurring when its nucleus divides. In the second 
type of life cycle, the vegetative cells are diploid. Two ascospores unite 
and the zygote, without undergoing meiosis, gives rise to vegetative cells 
that multiply and finally produce ascospores. Meiosis occurs when the 
spores are formed, and so they are the only haploid cells in the life history. 






E F G H 

Fig. 96. Schizosaccharomyces octosporiis. A to D, conjugation of two cells, the two nuclei 
uniting to form a single nucleus; E to G, three successive divisions of the fusion nucleus to 
form eight nuclei; H, formation of eight ascospores. (After Guilliermond.) 

The third type of life cycle, represented by Saccharomyces cerevisiae, is 
more complicated. Here the vegetative cells are either haploid or diploid. 
When two haploid cells conjugate, the zygote gives rise to a large number 
of diploid vegetative cells by budding. Meiosis occurs when one of these 
cells forms four ascospores. The spores give rise to haploid vegetative 
cells that multiply by budding. These are smaller than the diploid 
vegetative cells. 

Yeasts live in sugar solutions and are the principal agents in causing 
alcoholic fermentation. They use as food only a small part of the sugar 
that they absorb. The rest is broken down into carbon dioxide, ethyl 
alcohol, and small amounts of other substances. This process of fermen- 
tation is accomplished by the production of an enzyme called zymase. It 
is most active in the absence of free oxygen and serves as a means of 
releasing energy when the ordinary type of respiration cannot be carried 
on. 

2. Protodiscales 

The Protodiscales, numbering less than 100 species, are internal para- 
sites attacking seed plants, especially trees. The only genus is Taphrina. 
A common species, Taphrina deformans, causes a disease of peaches 



124 



PLANT MORPHOLOGY 



known as peach leaf curl, while Taphrina pruni produces a disease of the 
domestic plum called plum pockets, in which the fruit becomes shriveled. 
Taphrina cerasi attacks branches of the cherry, causing brush-like deform- 
ities known as witches'-brooms. 

The mycelium of Taphrina grows in the intercellular spaces of the 
host and sends to the surface groups of asci that arise just beneath the 
cuticle (Fig. 97). Each ascus contains eight ascospores. The asci are 
crowded to form a layer, called the hymenium, but are Avithout accom- 
panying sterile hyphae. Moreover, an ascocarp is not developed and 




Fig. 97. Taphrina deformans. Cross section of portion of peach leaf, showing layer of asci 
and ascogenous cells on the surface, X500. 

there is no formation of sex organs. The cells of the mycelium are 
binucleate. The two nuclei in the young ascus fuse, three successive 
divisions result in the formation of eight free nuclei, and from these the 
eight ascospores are organized. Upon germination, the ascospores, which 
are haploid, may give rise to one or more uninucleate cells by a process 
that resembles budding in yeasts. In some species these cells, which 
are called conidia, are formed while the ascospores are still within the 
asci. The ascospores, or the conidia produced by them, infect new host 
plants, a hypha penetrating the cuticle and pushing its way between the 
epidermal cells. The germinating spore may become binucleate by divi- 
sion of its nucleus, or a pair of ascospores or conidia may conjugate, a 
nucleus passing from one to the other. The binucleate condition is then 
transmitted to the cells of the vegetative mycelium. 

3. Plectascales 

The Plectascales include the blue and green molds, saprophytes that 
are abundant everywhere, occurring on bread, cheese, jelly, fruits, vege- 
tables, meat, leather, etc. The order includes over 30 genera and 800 
species. The two commonest genera are Aspergillus and Penicillium, 
the latter numbering over 500 species. One species, Penicillium notatum, 



THALLOPHYTA: FUNGI 



125 



produces a substance, called penicillin, that has remarkable germicidal 
properties. It has recently come into prominence as a valuable agent 
in the treatment of many infections and diseases caused by certain bac- 
teria, particularly cocci. Its great advantage over many other drugs lies 




Fig. 98. Branching conidiophores of PeniciUium producing chains of conidia, X800. 






B E 

Fig. 99. Aspergillus niger. A to E, successive stages in the development of a conidiophore 
and its conidia, as seen in optical section, X400. 

in its almost complete nontoxicity to the human body. Substances like 
penicillin are called antibiotics.^ 

The mycelium of the Plectascales produces special branches, called 
conidiophores, that cut off chains of spores, or conidia, enormous numbers 

1 Most antibiotics, including streptomycin, aureomycin, and Chloromycetin, are 
derived from actinomycetes, a group of organisms of which some are mold-like and 
others bacteria-like. They are variously classified with the Fungi Imperfecti, the 
bacteria, or as a distinct group of fungi. Some are parasites but most are saprophytes 
prevalent in the soil. 



126 



PLANT MORPHOLOGY 



of which are liberated into the air. Upon coming in contact with a suit- 
able food supply, the conidia produce new mycelia. In Penicillium the 
conidia arise from the ends of branched conidiophores (Fig. 98). In 
Aspergillus the conidia are abstricted from the ends of short hyphae that 
radiate from the enlarged tip of a conidiophore (Fig. 99). 






A B C 

Fig. 100. Development of the ascocarp of Aspergillus. A, sex organs; B, sterile hyphae 
enclosing the sex organs; C and D, later stages, showing the development of asci. (From a 
Turtox classroom chart.) 




Fig. 101. Section through a mature ascocarp of Aspergillus, showing the completely 
enclosed asci, X 500. 



The sex organs are represented by two short, spirally twisted filaments, 
the contents of which appear to fuse (Fig. 100). Then ascogenous hyphae, 
bearing numerous small asci, arise from one of the filaments. These are 
intermixed with and surrounded by sterile hyphae, those on the outside 
forming a minute, globular, closed ascocarp. A fruiting body of this 
type is known as a cleistothecium. There is no definite hymenium in the 



THALLOPHYTA: FUNGI 



127 



Plectascales, the asci being irregularly scattered throughout the mass of 
sterile hyphae (Fig. 101). 

4. Perisporiales 

The Perisporiales, or powdery mildews, are superficial parasites attack- 
ing many kinds of seed plants, such as grape, lilac, willow, rose, squash, 
bean, pea, apple, grasses, and numerous others. They number about 
500 species. Common genera are Sphaerotheca, Erysiphe, Uncinula, 
Podosphaera, Microsphaera, and Phyllactinia. The mycelium lives on 




Fig. 102. Erysiphe graminis growing on surface of grass leaf, showing haustoria in 
epidermis of host and conidia in various stages of development, X500. 

the surface of the leaves, forming whitish patches. Short haustorial 
branches are sent into the epidermal cells and through them food is 
absorbed. During the summer the mycelium produces erect conidio- 
phores, which give rise to chains of conidia (Fig. 102). These are very 
abundant and result in a rapid spread of the fungus to uninfected hosts. 
In the autumn closed ascocarps (cleistothecia) appear. They are minute, 
spherical, dark brown or black bodies with long appendages that, in some 
genera, are branched at the tip (Fig. 103). Inside the ascocarps are the 
asci, each usually with eight ascospores. The ascocarps, scattered by 
the wind, survive the winter. During the next season the ascospores pro- 
duce new mycelia. 

The character of the appendages produced by the ascocarps is impor- 
tant in distinguishing genera from one another. Thus in Sphaerotheca 
and Erysiphe the tips of the appendages are undivided, while in Podo- 
sphaera and Microsphaera they are dichotomously divided. In Uncimda 
the tips of the appendages are hooked or curved, while in Phyllactinia 
they are straight but the appendages are swollen at the base so as to 
form an enlarged plate. 



128 PLANT MORPHOLOGY 

The sex organs arise from uninucleate cells formed at the tips of special 
branches of the mycelium, all the (;ells of which are uninucleate (Fig. 
104). The antheridium, slightly smaller than the oogonium (ascogo- 
nium), comes in contact with it. The intervening cell wall is dissolved 
and the male nucleus passes over to fuse with the female nucleus. Ster- 
ile hyphae, arising from the cell beneath the oogonium, form a closed 
ascocarp. Following fertilization, the fusion nucleus gives rise to three 




Fig. 103. Ascocarp of Microsphaera alni with characteristic appendages, crushed slightly 
so that three asci, each with eight ascospores, have appeared, X 250. 

to five (often more) free nuclei and then transverse walls come in, form- 
ing a short row of cells. All of these are uninucleate except the penul- 
timate cell, which is binucleate. In Sphacrofheca and Podosphaera this 
cell directly forms a solitary ascus in which the two nuclei fuse, while 
in the other genera it gives rise either to a row of cells, each of which 
becomes an ascus, or to ascogenous hyphae that, in turn, produce the 
asci. Although in Sphacrotheca and Podosphaera the ascocarp has only 
one ascus, in the other genera it contains a basal layer of several parallel 
asci. The development of the ascus takes place in the regular way, 
except that it frequently contains less than eight ascospores. Eight 
nuclei are formed as usual, but some are not organized into spores. The 
asci are generally not intermixed with sterile hyphae. 

If the male and female nuclei actually fuse in the oogonium, the fusion 



THALLOPHYTA: FUNGI 



129 



in the young ascus involves two diploid nuclei, necessitating a double 
reduction of chromosomes in the two meiotic divisions that immediately 
follow. This beha\ior has been disputed by some investigators, who 
assert that the only nuclear fusion occurs in the young ascus and involves 
haploid nuclei, some claiming that the male and female nuclei remain 




F G 

Fig. 104. Sphaerothera castagnei. A, antheridial and oogonial branches in contact; B, 
antheridial branch cut off by a wall; C, antheridial cell separated from stalk cell; D, union 
of male and female nuclei in oogonium; E, oogonium with zygote nucleus and two layers 
of investing hyphae derived from cell just below; F, multicellular ascogonium, the penulti- 
mate cell, with two nuclei, becoming the ascus; G, young ascus with fusion nucleus and two 
ascogonial cells below it. {After Harper.) 



distinct in the oogonium, others that the antheridium is nonfunctional 
and a male nucleus does not enter the oogonium. If these views are cor- 
rect, the fusion nucleus in the young ascus is diploid and divides meiot- 
ically in the usual way. 

5. Pezizales 

The Pezizales, or cup fungi, grow mostly on decaying wood or humus, 
but some are parasitic on seed plants. They are a large order of approxi- 
mately 5,000 species. The principal genera include Pyronema, Peziza, 
Ascoholus, Lachnea, and Sclerotinia. 

Pyronema. This is a saprophyte on soil, especially where it has been 
burned over. The mycelium grows as a white fluffy layer on the surface. 



130 



PLANT MORPHOLOGY 



It bears well-developed sex organs. The female organ resembles the 
procarp of Nemalion. It consists of a globular, multinucleate basal por- 
tion, the ascogonium, and an elongated curved cell, the trichogyne, arising 
from its upper end (Fig. 105.1). The antheridium, which is terminal, 
club-shaped, and multinucleate, arises from an adjacent hypha. It comes 
in contact with the tip of the trichogyne, whose nuclei degenerate, and 
discharges its contents into it. The wall at the base of the trichogyne 





A B 

Fig. 105. Pyronema conftuens. A, ascogonium and trichogyne with antheridium in con- 
tact with its tip and discharging nuclei into it. Antheridium is curved around trichogyne 
and appears in section to be cut in two. B, somewhat diagrammatic section of a young 
ascocarp, involving two ascogonia from which ascogenous hyphae and paraphyses have 
arisen. Asci are shown in various stages of development. (After Harper.) 

disappears and the male nuclei migrate into the ascogonium, where multi- 
nucleate pairing of male and female nuclei occurs. The nuclei do not 
fuse, however, until an ascus is formed. 

Following fertilization, the ascogonium is cut off from the trichogyne 
by a new wall and branching ascogenous hyphae arise from it (Fig. 1055). 
These give rise to asci. Sterile hyphae (paraphyses) grow up from the 
mycelium and intermingle with the asci, the entire group of fertile and 
sterile hyphae becoming surrounded by a fleshy ascocarp. Ordinarily 
several sets of sex organs enter into the formation of a single ascocarp. 
The ascocarp of Pyronema is disk-shaped, red or yellow, and only 2 or 3 
mm. in diameter. The asci and paraphyses form a definite layer, the 
hymenium, that covers its upper surface. A broadly open ascocarp is 
called an apothecium, a type of fruiting body that is characteristic of the 
Pezizales. 

The origin of the asci is somewhat complex (Fig. 106). The paired 



THALLOPHYTA: FUNGI 



131 



nuclei of the ascogonium pass into the ascogenous hyphae, where they 
multiply. The members of each pair remain together as walls are 
formed. The terminal cell of a branch that is to become an ascus bends 
back to form a hook and its two nuclei divide simultaneously. Three 










Fig. 106. Origin of the ascus in Pyronema confluens. A, hook formation at tip of ascog- 
enous hypha; B, simultaneous division of nuclei; C, formation of uninucleate terminal and 
basal cells and of binvicleate penultimate cell; D, fusion of nuclei in penultimate cell to form 
an ascus; also migration of nucleus of basal cell into terminal cell; E, same stage except that 
nucleus of terminal cell has migrated into basal cell; F, later stage showing development of 
hook from basal cell; G, development of three hooks and an ascus from binucleate tip 
of an ascogenous hypha. (After Claussen.) 

cells are now cut off by walls. The terminal and basal cells are uninu- 
cleate but the middle one (the penultimate cell) has two nuclei of opposite 
sex, these being the descendants of a male and female nucleus that came 
from the fertilized ascogonium. The two nuclei may now fuse and the 
middle cell become an ascus, or the nucleus from the terminal cell may 
migrate into the basal cell and another hook may be formed. This 



132 



PLANT MORPHOLOGY 



behavior may be repeated a number of times. Each cell in which a 
nuclear fusion occurs may become an ascus, the fusion nucleus under- 




FiG. 107. Ascocarps of Peziza growing on decaying wood, natural size. 

going the usual three successive divisions to produce eight ascospore 
nuclei. The significance of hook formation, which occurs in many 

Ascomycetes, is a puzzle. 

Peziza. Peziza is one of the best-known 
cup fungi, including about 150 species. It is 
a common saprophyte on rich humus or on 
decaying wood. The mycelium, which is ex- 
tensive and much branched, penetrates the 
substratum and gives rise on the surface to 
smooth, fleshy, cup-like ascocarps 1 to 5 cm. 
or more in diameter (Fig. 107). These are 
generally bright red, brown, or gray. As in 
Pyronema, the ascocarp is lined with a layer 
of parallel asci and paraphyses, these consti- 
tuting the hymenium (Fig. 108). Each ascus 
contains eight ascospores. Upon germination, 
these produce new mycelia. In Peziza the 
ascocarp apparently arises directly from the 
mycelium without any formation of sex organs. 
Sclerotinia. A parasitic cup fungus, Sclero- 
tinia fructicola, attacks plums and peaches, 
causing a disease known as brown rot of stone 
fruits. The twigs, flowers, and fruits become 
infected with the mycelium. As the fruit turns brown and decays, great 
numbers of conidia are formed on the surface. These are cut off in chains 
from the ends of short conidiophores. The conidia carry the fungus to 




Fig. 108. Several mature 
asci of Peziza, each with eight 
ascospores, intermixed with 
paraphyses, X 250. Some 
young asci are arising below. 



THALLOPHYTA: FUNGI 



133 



uninfected trees. The fungus is usually carried over the winter on dried 
diseased fruits, called "mummies," that remain on the tree and furnish a 
fresh source of conidia the following spring. Brown cup-like ascocarps, 
which are rare, resemble those of Peziza and may be formed early in 
the season on mummified fruits lying on the ground. 

Summary. The Pezizales are mostly saprophytes but some are para- 
sites. All have a well-developed mycehum. The asci, accompanied by 
paraphyses, form a hymenial layer that lines an open, disk-like or cup- 
like ascocarp, the apothecium. This may be fleshy or leathery and ses- 
sile or stalked. Some members have well-developed sex organs, the asci 
arising from the fertilized ascogonium. In other members the asci arise 
directly from the mycelium, sex organs being absent. 

6. Helvellales 

The Helvellales are related to the Pezizales, being distinguished from 
them mainly by the form of the ascocarp, which is also an apothecium but 




Fig. 109. Ascocarps of Morchella (A) and Helvella {B), natural size. 

is more highly differentiated. The Helvellales are saprophytes that grow 
chiefly on humus. They number about 300 species. The best-known 
genera are Morchella and Helvella. 

Morchella. The common edible morel (Morchella esculenta) has a 
much-branched mycelium growing in rich humus soil. On it are formed 
compact masses of hyphae that develop into fleshy ascocarps of character- 
istic form. These come to the surface of the soil, where they often attain 



134 PLANT MORPHOLOGY 

a height of 15 to 20 cm. A mature ascocarp of Morchella is differentiated 
into a thick hollow stalk and a conical cap (Fig. 109A). The surface of 
the cap contains numerous depressions lined with a hymenium consisting 
of parallel asci and paraphyses. 

Helvella. The mycelium is subterranean and composed of hyphae 
with multinucleate cells. It gives rise to fleshy ascocarps that push 
upward to the surface of the ground, there reaching a height of about 5 cm. 
These are differentiated into a stout stalk and a saddle-shaped cap, the 
outer surface of which is covered with a hymenium consisting of parallel 
asci and paraphyses (Fig. 109B). The asci contain eight ascospores and 
discharge them into the air with considerable force. 

7. Tuberales 

The Tuberales are the well-known truffles, esteemed as a gastronomic 
delicacy. There are nearly 300 species, the representative genus being 
Tuber. The mycelium is subterranean, especially in woods, some forming 
the mycorrhiza of forest trees. Truffles occur in California and in various 
parts of southern and central Europe. Their life history is incompletely 
known. The ascocarp is fleshy and matures underground. It is more or 
less globular, its diameter rarely exceeding 8 cm. It is usually open when 
young but later nearly or completely encloses the asci. The ascocarp is 
thus a modified apothecium. The hymenium may surround a large cen- 
tral cavity or it may form irregular folds that divide the cavity into 
chambers. 

8. Pyrenomycetales 

The Pyrenomycetales, or black fungi, are a large order of about 450 
genera and 14,000 species that are generally segregated into three smaller 
orders, the Hypocreales, Dothideales, and Sphaeriales. They include 
saprophytes that live on decaying wood, humus, etc., and parasites that 
attack various seed plants. Some representative genera are Nectria, 
Claviceps, Plowrightia, Venhiria, Xylaria and Neurospora. 

Nectria. This large genus of about 250 species grows on living or dead 
wood. It is responsible for several important fungous diseases. One of 
the most destructive of these, canker of woody plants, is caused by Nectria 
cinnabarina and Nectria galligena. They attack a great variety of shrubs 
and trees, but not conifers. The fungus gains entrance through wounds 
in the stem. The cortex becomes infected and its cells are immediately 
killed. This results in a wound that gradually enlarges. Sometimes 
enough cork tissue is developed around the infected area to close the 
wound, but usually this is not possible and the trunk is finally girdled. 
During the summer the mycelium produces large, pinkish, disk-like 
masses, or stromata, that break through the bark and give rise to large 



THALLOPHYTA: FUNGI 



135 



numbers of conidiophores, the conidia being carried by the wind to new 
hosts. Later in the season small, red, flask-shaped ascocarps, called 
perithecia, are developed on the stromata (Fig. 110). 

Claviceps. The common ergot disease of rye and other grasses is caused 
by Claviceps purpurea. Its damage to the rye is usually slight, but the 
eating of diseased grain by animals results in a paralysis and other serious 
conditions. A drug derived from the fruiting bodies of this fungus, called 
ergotine, has important uses in medicine. The ovaries of the rye are 
infected by ascospores in the early summer and become hypertrophied, a 







Fig. 110. Stroma of Nectria cinnabarina on bark of Rihes, showing two perithecia with 
young asci and paraphyses, X75. 

mycelium developing within. The formation of conidia soon follows. 
The conidia are minute cells abstricted from the tips of short conidio- 
phores. As they are formed, a sweet liquid is exuded from the spikelet. 
This attracts insects, which carry the conidia to uninfected flowers. 
Later the mycelium hardens to form a compact sclerotium, which replaces 
the ovary of the flower. The sclerotia are elongated, slightly curved, 
purplish bodies that project from the ears of the rye. Many of them 
eventually fall to the ground, where they pass the winter. In the spring 
the sclerotium produces several or many globular, stalked stromata, which 
are compact mycelial masses containing numerous flask-shaped, deeply 
embedded perithecia (Fig. 111). The entire stroma is cream-colored at 
first, becoming grayish violet. Each perithecium is fined with a hyme- 
nium consisting of many asci and paraphyses. The ascospores, which are 
needle-shaped, are discharged forcibly and dispersed by the wind. 

The sex organs of Claviceps are borne on hyphae lying below the surface 
of the stroma. The ascogonium is broader than the antheridium and 
both are multinucleate. The contents of the antheridium enters the 



136 



PLANT MORPHOLOGY 



ascogonium, which then gives rise to ascogenous hyphae. As mPyronema, 
the asci arise as a result of hook formation at the tips of the ascogenous 
hyphae. Their development occurs in the typical manner. 

' Plowrightia. This is another parasitic genus, its best-known species, 
Plowrightia morbosa, causing a destructive disease of the plum and cherry 
known as black knot. The mycelium passes the winter under the bark of 




Fig. 



B C 

111. Claviceps purpurea. A, stalked stromata arising from a sclerotium, X4; B, 



longitudinal section through a stroma, showing the embedded perithecia, X30; C, a 
perithecium with young asci and paraphyses, X 250. 

a branch or twig. In the spring it breaks out on the surface to form an 
elongated gall or knot consisting of both mycelium and hypertrophied host 
tissue (Fig. 112). Leaves and fruits are not attacked. The elongated 
knots, often reaching a length of 12 cm. or more, are developed mostly on 
one side of the stem, which becomes more or less deformed. In early 
summer the mycelium within the knot gives rise to innumerable short 
conidiophores that form a velvety layer on the surface. The conidia, 
distributed by the wind, spread the fungus to other hosts. Later in the 
season conidium formation ceases and the knot becomes hard and black, 
forming a stroma in which hundreds of perithecia appear (Fig. 113). 
These are small flask-shaped organs, embedded in the stroma, and lined 
with a hymenium consisting of asci and paraphyses. The ascospores 



THALLOPHYTA: FUNGI 



137 



mature and are liberated during the following spring. Like the conidia, 
they directly infect new hosts. 

Venturia. Venturia inaequalis is the cause of an apple disease known 
as apple scab. It affects chiefly the leaves and fruits, producing brown 
spots that become scaly as a result of cork formation. The mycelium 
grows between the cuticle and the epidermis. It forms large numbers of 




Fig. 112. Galls produced on cherry twigs by the black-knot fungus, Plowrightia morbosa, 
natural size. 



conidiophores that break through to the surface (Fig. 114A). Conidia, 
abstricted from their tips, spread the fungus during the summer to other 
apple trees. In the autumn, after the infected leaves fall to the ground, 
the mycelium becomes saprophytic and produces perithecia in the follow- 
ing spring (Fig. 1 145). These appear on the lower side deeply embedded 
within the leaf tissues. Sex organs are produced, but the ascocarp begins 
to develop before fertilization has occurred. The ascogonium is long, 
coiled, and multinucleate. It has a trichogyne with which the anther- 
idium comes in contact. Following fertilization, the ascogonium gives 
rise to ascogenous hyphae from which asci are developed as a result of 
hook formation at their tips. The ascocarps (perithecia) are dark brown 
and flask-shaped when mature, discharging the ascospores forcibly. 



138 



PLANT MORPHOLOGY 





A B 

Fig. 113. Plorvriyhtia morhosa. A, section of stroma bearing young perithecia, X50; B, 
a single peritheciuni with young asci and paraphyses, X 150. 





Fig. 114. Venturia inaequalis. A, conidiophores arising on lower side of apple leaf, X 600; 
B, section of mature perithecium on old apple leaf, X 250. 



THALLOPHYTA: FUNGI 



139 



Xylaria. Xylaria is a large genus of about 200 species. It is a common 
saprophyte, the myeehum Uving in decaying wood. It produces sclerotia 
from which black, club-shaped, often branched stromata arise. At first 
these are covered with a mass of white conidiophores from which small 
oval conidia are abstricted. Later the stromata produce numerous 
embedded, flask-shaped perithecia lined with a hymenium (Fig. 115). 

Neurospora. This is the pink bread mold, a form much used experi- 
mentally in genetics. The mycelium produces conidia in branched chains. 
Perithecia are rarely formed. They are 
dark-colored, pear-shaped, and without 
paraphyses. Like Rhizopus, Neurospora 
is heterothallic and sexual reproduction 
occurs only when a plus and a minus 
strain come together. The young peri- 
thecium contains a coiled ascogonium 
from which trichogynal hyphae grow out. 
If these come in contact with spermatia, 
conidia, or hyphae of the opposite strain, 
the perithecia mature and asci are pro- 
duced. Two nuclei of opposite sex fuse 
in the young ascus, the fusion nucleus 
undergoes three divisions of which the 
first two are meiotic, and eight ascospor^ 
are formed in the usual way. Experi- 
ments have shown that sexual differen- 
tiation occurs in connection with asco- 
spore formation, usually during the first 
meiotic division but sometimes during 

the second. As a result, four ascospores in each ascus will produce plus 
mycelia and four minus mycelia. Other genetic characters behave 
similarly. 

Summary. The Pyrenomycetales include both saprophytes and para- 
sites. They are characterized by a flask-shaped ascocarp (a perithecium) 
with a small opening at the top. It is Uned with a hymenium composed 
of parallel asci and paraphyses. The perithecia may arise singly on the 
mycelium, in small groups, or may be embedded in a compact mycelial 
mass, the stroma. Sex organs are present in some members. 

Differences in the character of the perithecia and stromata provide a 
basis for splitting up this large order into three smaller orders. 

1. Hypocreales. These forms have soft, bright-colored perithecia with 
a definite wall. The perithecia may occur singly or in a stroma, which is 
also bright-colored. They include Nectria and Claviceps. 

2. Dothideales. Members of this group have black stromata in which 




Fig. 115. Longitudinal section 
through a perithecium of Xylaria, 
showing asci arising from the 
hymenium, X 100. 



140 PLANT MORPHOLOGY 

the perithecia, lacking independent walls, are developed as stromatal 
cavities. Plowrightia belongs here. 

3. Sphacriales. The Sphaeriales have dark-colored perithecia with a 
distinct wall. The perithecia may be free or embedded in the substratum 
or in stromata that are firm, leathery or brittle, and dark-colored. Here 
belong Venturia, Xylaria, and Neurospora. 

9. Laboulbeniales 

The Laboulbeniales comprise an order of about 50 genera and 1,200 
species. They are parasitic on insects, especially aquatic ones. As a rule 
the mycelium grows on the surface of the host and is very small, usually 
less than 1 mm. in length. The Laboulbeniales are of particular interest 
because their sex organs are remarkably like those of the red algae. The 
antheridium is unicellular and produces a nonmotile male cell, the 
spermatium. The ascogonium has a trichogyne and auxihary cells. 
Ascogynous hyphae arise from the fertilized ascogonium, small perithecia 
are formed, and the asci bud out from the auxiliary cells. The whole 
process resembles cystocarp formation in the red algae. 

5. BASIDIOMYCETES 

The Basidiomycetes, or club fungi, comprise the highest group of fungi. 
They resemble the Ascomycetes in having a mycelium with cross walls. 
They are characterized by the production of spores externally on a club- 
like structure known as a hasidiiOm. This arises from the swollen end of 
a hypha and may consist of either four cells or one. Four slender branches 
(sterigmata) arise from the basidium, each forming a hasidiospore at its 
tip. The young basidium contains a nucleus derived from the fusion of 
two nuclei. Two successive divisions, which are reductional, result in the 
formation of fourhaploid nuclei, each passing into one of the basidiospores. 
In the higher members the basidia are borne on a distinct fruiting body, 
the hasidiocarp, composed of interwoven hyphae. The Basidiomycetes 
are related to the Ascomycetes and are generally regarded as having been 
derived from them. Some are of great economic importance, particularly 
the smuts, rusts, and mushrooms. The Basidiomycetes number about 
20,000 species. They embrace seven principal orders: Ustilaginales, 
Uredinales, Auriculariales, Tremellales, Exobasidiales, Hymenomycetales, 
and Gasteromycetales. '^ 

1. Ustilaginales 

The Ustilaginales, or smuts, are parasites that live on various her- 
baceous seed plants. They attack chiefly floral organs, particularly those 
of grasses. They are most destructive to oats, less so to wheat and corn. 
The smuts number about 500 species. The principal genera are Ustilago 
and Tilletia. 



THALLOPHYTA: FUNGI 



Ul 



Ustilago. The life history of the corn smut (Ustilago zeae) will be 
described. The mycelium ramifies throughout the stem and leaves of the 
corn plant and in its vegetative condition does not seem to do much dam- 
age. It lives in the intercellular spaces, sending short haustoria into the 
host cells. When flowers appear, some of the ovaries become packed 
with the mycelium and, as a consequence, become greatly swollen and 
distorted. Swellings may also appear in other parts of the plant. Later 
the mycelium divides up into countless numbers of black spores, called 





Fig. 116. Ustilago zeae. A, an ear of corn infected with smut, some of the grains of which 
are greatly enlarged and filled with chlamydospores, one-half natural size; B, a germinating 
chlamydospore, the four-celled basidium producing basidiospores, X 1,400. 

chlamydospores, which form large powdery masses (Fig. 116^4). A 
chlamydospore is a heavy-walled cell representing merely a transformed 
cell of the vegetative mycelium. 

A chlamydospore may germinate at once but, as a rule, falls to the 
ground and remains dormant until the following spring. Then it sends 
out a short filament of three or four cells that lives saprophytically on 
organic matter in the soil (Fig. 1165). Thin-walled basidiospores are 
bud(ied off each cell of the filament, often in great numbers. This fila- 
ment is a basidium but, because of the large number of spores produced, 
is not a typical one. In some smuts, however, only one spore is budded 
off each of the four cells of the basidium. The basidiospores infect 
young corn plants in the spring. 

The cells of the vegetative mycelium are binucleate, as are the j^oung 
chlamydospores. But before the chlamydospore is mature the two nuclei 



142 



PLANT MORPHOLOGY 



fuse, thus establishing the diploid condition. The fusion nucleus divides 
reductionally in the young basidium and four cells are formed by the 
appearance of transverse walls, thus separating the four haploid nuclei. 
When a basidiospore is budded off, two nuclei are formed, one of which 
passes into the spore while the other remains in the basidium. The 
latter may divide again, if another spore is budded off, and this may be 
repeated many times. These haploid basidiospores produce on the young 
corn plant mycelia of limited extent and with uninucleate cells. When 
two mycelia of opposite sex come together within the host, a union of 
cells takes place without a fusion of nuclei. The binucleate cells formed 
in this way give rise to a mycelium that spreads throughout the host, 
eventually producing chlamydospores. 

2. Uredinales 

The Uredinales, or rusts, are destructive parasites. They attack a 
great variety of vascular plants, including ferns, conifers, and angio- 
sperms, being especially common on grasses. The mycelium lives in the 
intercellular spaces, particularly of the leaves. There are about 3,000 
species of rusts, the most important genera being Puccinia, Uromyces, 




Fig. 117. Puccinia graminis. Section through a uredinium on a leaf sheath of wheat, 
showing uredospores in various stages of development, X200. 



Gymnosporangium, Phragmidium, Cronartium, Coleosporium, and Metkrnp- 
sora. The largest genus, Puccinia, has about 700 species. 

Puccinia. The common wheat rust {Puccinia graminis) is the best- 
known member of the order. Its life history is very complicated, involv- 
ing two different hosts and several kinds of mycelia and spores, all with 
a definite relation to one another. 

The mycelium that lives on the wheat is an internal parasite, extend- 



THALLOPHYTA: FUNGI 



143 



ing throughout the entire body of the host. It does not directly kill the 
host cells, but lives on their food materials, which it absorbs by means 
of haustoria. During the late spring and early summer numerous spores 
are produced. They break through the epidermis of the leaves, groups 
of them, known as uredinia, appearing on the surface as reddish brown 
streaks or lines (Fig. 117). These spores are called uredospores. Each 
consists of a stalked binucleate cell with a rather thick cell wall. They 
are scattered by the wind, directly infecting other wheat plants, and are 
chiefly responsible for the rapid spread of the disease, especially during 
a wet season. Successive crops of uredospores may be produced through- 
out the summer. 




Fig. 118. Puccinia graminis. Section through a telium on a leaf sheath of wheat, showing 
teliospores in various stages of development, X 200. 

Later in the season, at harvest time or thereabouts, the same mj^celium 
that produced the uredospores earlier now gives rise to elongated groups 
of black spores called teliospores. These groups, known as telia, appear 
chiefly on the stems and leaf sheaths (Fig. 118). The teliospores are also 
stalked but are two-celled and heavy-walled. At first each cell has two 
nuclei, but the members of each pair fuse as the spore matures. The 
teliospores do not germinate until the next spring, thus carrying the 
fungus over the winter. Upon germination, one or both cells of the 
teliospore gives rise to a short filament. This filament is the basidium 
(Fig. 119.4). It consists of four cells, each of which sends out a short 
branch, called a sterigma, bearing a small terminal basidiospore. The 
basidiospores cannot infect wheat plants. They are carried by the wind 
to leaves of the common barberry {Berberis vulgaris), where they germi- 
nate and produce an extensive internally parasitic mycelium. It is 
mainly this species that is susceptible to infection by the basidiospores of 
wheat rust. Most other barberries are immune. 

The mycelium produced by the basidiospores on the barberry develops 
spermogonia (pycnidia), small flask-shaped organs appearing on the upper 



^44 PLANT MORPHOLOGY 

side of the leaves (Fig. 119^). In these organs small cells, called sper- 
matia (pycnospores), are formed by abstriction from the ends of slender 
hyphae. The spermatia are exuded from the spermogonia in drops of a 
sweet liquid. This attracts insects that aid in their dissemination. Soon 




Fig. 119. Pucciniagraminis. Stages on the barberry. ^, four basidiospores arising from 
a basidium produced by a teliospore, X300; B, leaf of common barberry with groups of 
aecia, natural size; C, enlarged view of group of aecia, X 10; D, longitudinal section of 
aecium with numerous aeciospores arising in chains, X200; E, longitudinal section of 
spermogonium producing numerous small spermatia, X200. {A, after Chamberlain.) 

after the appearance of the spermogonia, the mycelium on the barberry 
produces larger, cup-like structures that appear in clusters on the lower 
side of the leaves (Fig. 119S and C). These aecia, or cluster cups, con- 
tain large numbers of aeciospores, which arise in chains from the bottom 



THALLOPHYTA: FUNGI 145 

of the cup (Fig. 119/)). The chains consist of alternating spores and 
sterile cells, the latter disintegrating. The aeciospores cannot infect the 
barberry. Instead, carried by the wind, they infect wheat plants during 
the late spring and summer, thus completing the life cycle. 

The cells of the mycelium produced by the basidiospores on the bar- 
berry, as well as the spermatia, are uninucleate, but the aeciospores are 
binucleate. The binucleate condition appears to arise by the spermatia 
coming in contact with special receptive hyphae of the opposite sex. 
These extend from the basal cells of the young aecium to the orifice of 
the spermogonium, through which they project. A spermatium enters a 
receptive hypha and passes down into the basal cell, which then becomes 
binucleate. Each binucleate basal cell gives rise by repeated division to 
a chain of aeciospores. The binucleate condition is carried over by the 
aeciospores to the mycelium on the wheat and to the uredospores and 
young teliospores produced by it. 

The fusion of the two nuclei in each cell of the teliospore introduces the 
uninucleate condition. When the teliospore germinates, the diploid 
nvicleus in each of its cells undergoes two successive meiotic divisions that 
result in the formation of four haploid nuclei. Each of the four cells in 
the basidium receives one of these nuclei, which then passes into a 
basidiospore. The uninucleate basidiospores, being haploid, produce a 
haploid mycelium on the barberry. Thus, although there is an alternat- 
ing haploid and diploid phase in the life history of Puccinia, the latter is 
not initiated by a nuclear fusion, as in most plants, but by the coming 
together in the same cell of two nuclei that retain their identity through- 
out a large number of cell divisions. Eventually the nuclear fusion 
occurs, but is then followed by the reduction divisions, which mark the 
beginning of the haploid phase. 

Other Rusts. The wheat plant is attacked not onl}^ by Puccinia 
graminis, its most destructive rust, but by several related species. One 
of these is Puccinia coronata, whose alternate host is the buckthorn 
{Rhamnus); another is Puccinia rubigo-vera, which produces the aecial 
stage on blueweed (Echiuni) and other Boraginaceae. All three species 
may attack other grasses than wheat, such as barley, oats, rye, and 
various meadow grasses, producing the same morphological type of 
mycelium and spores on each kind of grass but a different physiological 
strain. 

Many rusts have a shorter life cycle than Puccinia graminis. All rusts 
produce teliospores and these always give rise to basidia and basidio- 
spores, but one or more of the other spore forms may be missing. Thus 
the aecia may be omitted, the uredospores, the aecia and spermatia, or 
the aecia, spermatia, and uredospores. If a rust requires two different 
and unrelated hosts to complete its life cj^cle, it is said to be heteroecious; 



146 PLANT MORPHOLOGY 

if all stages are passed on the same host, or on closely related hosts, it is 
autoecious. Gymnosporangium juniperi-virginianae, a heteroecious rust, 
has no uredospores. It develops the telial stage on the red cedar {Junip- 
erus virginiana), or related species, and the aecial-spermogonial stage on 
the apple, pear, and quince (Pyrus). A heteroecious rust of great eco- 
nomic importance is Cronartium ribicola, the white pine blister rust. Its 
uredospore-teliospore stage is passed on various species of currants and 
gooseberries (Ribes), its aecial-spermogonial stage on the white pine 
{Pinus strobus) and related species. The damage to white pines has 
been so great that it has resulted in their virtual extinction in many parts 
of the country. Puccinia asparagi is an autoecious rust, producing 
uredospores, tehospores, aeciospores, and spermatia on the asparagus. 
Pxiccinia malvacearuvi, another autoecious rust, has a very short life cycle, 
producing only teliospores on the hollyhock and other Malvaceae. 

3. Auriculariales 

The Auriculariales are the ear fungi, an order of about 15 genera and 
over 100 species. They are chiefly saprophytes growing on bark and 
decaying wood. The representative genus is Auricularia. The myce- 
lium produces brightly colored, gelatinous, ear-shaped bodies, each being 
a basidiocarp. When dry, the basidiocarps become wrinkled and hairy. 
The inner surface is lined with a hymenium consisting of basidia inter- 
mixed with paraphyses. As in the Uredinales, the basidia are four-celled 
and have sterigmata. Each basidium produces four basidiospores. This 
order may be regarded as transitional between the lower and higher 
Basidiomycetes. 

4. Tremellales 

The Tremellales, or trembhng fungi, are somewhat similar to the 
Auriculariales. They include 18 genera and nearly 100 species, the best- 
known genus being Tremella. The mycelium lives in decaying wood and 
bark, producing gelatinous basidiocarps. These are indefinite in form 
and more or less wavy or folded. The hymenium occurs on the upper 
surface. The basidia are characteristic, being longitudinally divided into 
four cells instead of transversely divided. Each basidium bears four 
basidiospores on long sterigmata. 

5. Exobasidiales 

The Exobasidiales are internal parasites attacking particularly mem- 
bers of the Ericaceae, such as blueberries, cranberries, huckleberries, 
azaleas, etc. There are about 30 species, nearly all belonging to the 
genus Exobasidium. Galls composed of mycelium and host tissue are 
produced on stem tips, leaves, and floral organs. The basidia are formed 
under the epidermis and, when they break through, cover the host with a 



THALLOPHYTA: FUNGI 



147 



whitish bloom. There is no formation of basidiocarps, the basidia arising 
directly from the mycelium. In this and succeeding orders the basidium 
is one-celled. The young basidium has two nuclei that fuse, two succes- 
sive nuclear divisions follow, and four basidiospores are developed, each 
at the end of a sterigma. 

6. Hymenomycetales 

This large order of approximately 15,000 species is usually split up 
into several smaller orders, but here will be regarded as one homogeneous 
group. Most of the members are saprophytic on humus, bark, decaying 
wood, etc. Some are parasitic on trees, often 
causing considerable damage. The Hymeno- 
mycetales have complex basidiocarps with 
basidia in a definite hymenial layer that be- 
comes freely exposed. The basidia are one- 
celled and bear four basidiospores, each at the 
end of a slender sterigma. 

Families. The families of Hymenomyce- 
tales are distinguished from one another on 
the basis of the form of the basidiocarp and 
the position of the hymenium. The principal 
families are as follows: 

1. Thelephoraceae. These forms produce 
simple basidiocarps appearing on tree trunks. 
Some resemble leathery incrustations with the 
hymenium on the smooth upper surface, while some are bracket-like 
with the hymenium on the lower surface. Others have the hymenium 
on the outside of a funnel-like basidiocarp. The representative genus 
is Thelephora, with about 150 species. 

2. Clavariaceae. The coral fungi produce erect, fleshy basidiocarps 
that are usually branched like coral, the hymenium covering the surface 
of the branches (Fig. 120). They are commonly white or yellowish, but 
sometimes are more brightly colored. In some forms the basidiocarps are 
club-shaped and unbranched, with a complete hymenial covering. The 
principal genus is Clavaria, wdth about 250 species. 

3. Hydnaceae. These are the tooth fungi, the hymenium being borne 
on tooth-like or spine-like processes that generally point downward. 
The simpler forms occur as rounded masses or thin sheets of indefinite 
form. Some are more or less branched. Others have a stalk and an 
umbrella-like pileus that bears teeth on its lower side. The main genus 
is Hydnum, with about 150 species. 

4. Polyporaceae. The pore fungi bear a number of tubes or grooves 
lined with a hvmenium. The basidiocarp may be crustaceous, the tubes 




Fiu. 120. A coral fungus 
{Clavaria), natural size. 



148 



PLANT MORPHOLOGY 




Fig. 121. Agaricus campestiis, four-fifths natural size. A, mature basidiocarp, showing 
pileus, stipe, and annulus; B, view of underside of pileus with stipe removed, showing the 
radiating gills; C, young basidiocarp before the pileus has expanded; D, young basidiocarp 
cut in half, showing velum attached to stipe. 



opening on its upper side. Ordinarily, however, the basidiocarp is 
bracket-like or umbrella-like, the tubes opening on its lower side. The 
texture of the basidiocarp may be leathery, fleshy, or hard and woody. 
Some of the largest genera are Merulius, Porta, Fomes, Polyporus, Poly- 
stichuSy and Boletus. Polyporus, the largest genus, has about 500 species. 



THALLOPHYTA: FUNGI 



149 



MeruUus lacrymans is the dry-rot fungus, a species attacking woodwork 
and structural timbers. It often causes great destruction to wooden 
buildings. 

5. Agaricaceae. This is the large family of gill fungi, a group to which 
the common mushrooms and toadstools belong. The basidiocarp may be 
bracket-like but more commonly is umbrella-hke. It is usually fleshy, 
rarely leathery in texture. In this family the hymenium covers blade- 
like radiating plates known as gills. Of the numerous genera, a few 
common ones are Coprinus, Agaricus, Amanita, Lepiota, Hypholoma, 
Russula, and Marasmius. The largest genus is Marasmius, with about 
450 species. 





Fig. 122. Coprinus micaceus. A, cross section through a few of the gills, X 10; B, enlarged 
portion of same, showing four basidia arising from surface of gill, each with four stalked 
basidiospores, X750. 



Agaricus and Other Mushrooms. The common field mushroom 
{Agaricus canipestris) grows in lawns, fields, and along roadsides. It is 
the principal species used for food and practically the only one that is 
cultivated. The mycelium lives on organic matter in the soil. The 
fleshy basidiocarp arises just below the surface as a "button" composed 
of interwoven hyphae. Soon a stalk or stipe and a cap-like pileus become 
differentiated. The gills, which develop on the lower side of the pileus, 
are covered by a membrane called the velum. This extends from the 
margin of the pileus to the stipe, becoming ruptured as the pileus expands. 
In Agaricus and many other mushrooms a portion of the velum remains 
attached to the stipe, forming an annulus around it (Fig. 121). In 
Amanita, a genus of poisonous mushrooms, the young basidiocarp is 
completely enclosed by an outer membrane that ruptures as the stipe 
elongates, forming a cup or sheath, called the volva, at the base of the stipe. 

The hymenium of the Agaricaceae, covering the surface of the gills, 
consists of innumerable basidia, each of which typically bears four basidio- 
spores on slender sterigmata (Fig. 122). The cells of the vegetative 
mycelium are typically binucleate and there are two nuclei in the young 



150 



PLANT MORPHOLOGY 



basidium. These fuse, two successive divisions take place, and the four 
resulting nuclei pass through the sterigmata into the basidiospores (Fig. 
V2iiE-G). The reduction of chromosomes occurs when the fusion nucleus 
divides. The cultivated variety of Agaricus cmnpestris is exceptional in 
that only two basidiospores are borne on a basidium, each of which 











U.'ry. 



'■■■&X-* 



Fig. 123. Clamp formation and development of the basidium in Armillaria mucida. 
A, beginning of damp formation in binucleate terminal cell; B, one nucleus passing into the 
clamp; C, conjugate division of the two nuclei; D, appearance of walls cutting off uni- 
nucleate clamp and basal cells from young binucleate basidium; E, fusion of clamp and 
basal cells, the latter sending out another branch; F, basidium with diploid fusion nucleus; 
G, basidium with four haploid nuclei and the developing sterigmata. {Ajter Kniep.) 

receives two of the four haploid nuclei. The mycelium of both the wild 
and cultivated form is multinucleate and probably unisexual (homo- 
thallic). 

In most mushrooms the basidiospores, upon germination, give rise to 
mycelia of two different sexes. These have uninucleate cells. When two 
mycelia of opposite sex come together, fusions take place between vegeta- 
tive cells, resulting in the formation of a binucleate mycelium. Upon 
this the basidiocarps are produced. 



THALLOPHYTA: FUNGI 151 

Following the formation of a binnoleate cell by the fusion of two uni- 
nucleate cells, a short branch arises into which the two nuclei pass. A 
hook-hke lateral outgrowth, pointing toward the base of the cell, then 
appears at a point directly opposite the two nuclei (Fig. 123.4, B). After 
both of these divide, one of the daughter nuclei passes into the hook and a 
cross wall forms at its base, another wall continuing across the branch 
(Fig. 123C, D). Thus two nuclei of opposite sex are in the terminal cell, 
one nucleus being in the lower cell and one in the hook. The tip of the 
hook now fuses with the lower cell to form a "clamp connection." The 
nucleus in the hook passes into the lower cell, which thereby becomes 
binucleate (Fig. 123£'). The terminal cell continues to grow and, at each 
cell division, a new clamp connection is formed. 

A mycelium with clamp connections is characteristic of many Basidio- 
mycetes, occurring in at least some members of all the orders except the 
Uredinales. Clamp formation in the Basidiomycetes is thought to corre- 
spond to hook formation in the Ascomycetes where, however, it is limited 
to the ascogenous hyphae. It must be remembered that in both groups 
there are many members without any such formations, the ascus or 
basidium developing directly from the terminal cell of a hypha. Clamp 
connections are not present on the mycelium of Agaricus campestris or its 
cultivated variety. 

7. Gasteromycetales 

Like the Hymenomycetales, the Gasteromycetales are often broken 
up into several smaller orders. Nearly all its members are saprophytic 
on humus, but a few grow on decaying wood. There are about 1,000 
species. The very complex basidiocarp entirely encloses the hymenium, 
remaining closed or opening only after the spores are mature. The 
basidiocarp is composed of an outer peridium and a central gleha, the 
latter generally containing many chambers. In the lower forms the 
chambers are filled with hyphae bearing terminal basidia; in the higher 
forms the chambers are lined with a definite hymenium. The basidia are 
one-celled and bear four terminal basidiospores, each at the end of a 
sterigma. 

Families. The principal families of Gasteromycetales, distinguished 
from one another by the character of the peridium and gleba, are as 

follows: 

1. Hymenogastraceae. This family is intermediate between the Hyme- 
nomycetales and the Gasteromycetales. The peridium is simple, being 
one-layered and rupturing irregularly. The glebal chambers are Uned 
with basidia borne at the ends of lateral branches of the glebal hyphae. 
Because the basidiocarps are subterranean, these forms are not commonly 
seen. The chief genera are Hymenogaster and Rhizopogon. 



152 



PLANT MORPHOLOGY 



2. Sclerodermaceae. In this family the basidiocarp is nearly spherical, 
with a thick, leathery, one-layered peridium that ruptures at the apex. 
The gleba is indistinctly chambered. The basidia are borne on lateral 
branches of the glebal hyphae. There are no sterigmata, the basidio- 

spores being sessile. The representa- 
tive genus is Scleroderma. 

3. Lycoperdaceae. These are the 
familiar puffballs. The globular basid- 
iocarps are usually less than 8 cm. in 
diameter but sometimes reach 50 cm. 
or more. The peridium is two-layered 
and has no definite dehiscence. In 
Lycoperdon the outer layer flakes off, the 
inner one bursting at the apex to liber- 
ate the spores. In Geaster the outer 
layer splits into stellate segments that 
spread out on the ground, the inner 
one dehiscing by a terminal pore. In 
this family the gleba is distinctly cham- 
bered. It is lined with a hymenium 
and contains a capillitium consisting of 
fibrous interwoven hyphae that aid in 
spore dispersal. 

4. Nididariaceae. The bird's-nest 
fungi resemble the puffballs in their 
younger stages, but at maturity the 
peridium opens and becomes cup- 
shaped. The separate glebal chambers, 
with much-thickened walls, lie at the 
bottom of the cup like eggs in a nest. 
The two chief genera are Nidularia and 
Cyathus. 

5. Phallaceae. The stinkhorn fungi 
are the highest of the Basidiomy- 

cetes. Their basidiocarps are extremely complex (Fig. 124). At first 
they are white and egg-shaped. The peridium is two-layered but the 
tissue within is differentiated into a hollow sterile axis and an investing, 
dome-like, chambered gleba. When the basidiocarp is mature, these 
become the stipe and pileus, respectively. The gleba becomes mucilagi- 
nous and foul-smelling, attracting carrion flies that distribute the spores. 
The principal genera are Phallus, Mutinus, and Dictyophora. In Dicty- 
ophora there is a conspicuous net-like veil that hangs down beneath the 
pileus and spreads out around the stipe like a skirt. 




Fig. 124. A stinkhorn fungus, P/iaiius 
impudicus, natural size. 



THALLOPHYTA: FUNGI 153 



FUNGI IMPERFECTI 



The Fungi Imperfect! constitute a large assemblage of forms that, 
because of an incomplete knowledge of their life histories, cannot be 
assigned to any of the three natural classes of true fungi: the Phycomy- 
cetes, Ascomycetes, and Basidiomycetes. Generally the only known 
method of reproduction is by conidia. Zygotes, ascospores, or basidio- 
spores are unknown. In many cases the unknown stage has apparently 
been lost from the life history. When a member of this artificial group is 
found to possess any reproductive stage previously not reported, it is 
transferred to its proper genus, family, order, and class. Meanwhile it is 
placed in a "form genus." Many of the imperfect fungi cause important 
plant diseases, such as potato scab, early blight of potato, flax wilt, and 
various anthracnose and leaf-spot diseases. Practically all the fungi that 
cause such human diseases as ringworm and athlete's foot are imperfect 
fungi. 

LICHENES 

A lichen is a plant consisting of a unicellular alga and a fungus living 
together in symbiotic relationship. This association, resulting in a body 
having a distinctive form and structure, suggests a single plant rather than 
a composite one. Lichens are commonly regarded as constituting an 
autonomous group of thallophytes, the Lichenes, which are either made 
coordinate with the Algae and Fungi, or included with the latter as a 
distinct class. By those who consider lichens to be merely fungi parasitic 
upon algae, they are sometimes broken up and distributed among the 
fungous groups that they most closely resemble. 

Lichens are commonly seen growing on rocks, tree trunks, dead wood, 
and on the ground. They are a widely distributed group of which about 
400 genera and 15,000 species are known. A few of the largest genera are 
Lecidia, Buellia, Lecanora, Parmelia, Physcia, Collema, Stida, Cladonia, 
Ramalina, and Usnea. Lichens are mostly gray or grayish green, but 
some are more conspicuously colored. Based on their external form, 
three general types are recognized: (1) crustose Hchens, w^hich occur as 
incrustations on rocks and bark; (2) foliose lichens, which are flat, 
leaf -like, and only partially attached to the substratum; and (3) fridicose 
hchens, branching forms that hang from trees or grow either erect or 
prostrate on the ground (Fig. 125). 

The greater part of a Hchen is composed of a compact mass of tangled 
fungous hyphae, among which are numerous algal cells, either scattered 
irregularly or in a definite layer (Fig. 126). The body is usually differ- 
entiated into a compact cortical region and a lower region of looser tex- 
ture, in either of which the algal cells may occur. In some lichens the 



154 



PLANT MORPHOLOGY 



algae live on the surface of the mycelium, closely covering it. With only 
a few rare exceptions, lichen-forming fungi are ascomycetes belonging 
either to the Pezizales or to the Pyrenomycetales. In three genera of 
lichens the fungus is a basidiomycete, the best-known species being Cora 
pavonia, which is widely distributed throughout Central and South 
America. The lichen-forming algae are members either of the Cyanophy- 
ceae or Chlorophyceae, most of the latter belonging to the Chlorococcales. 



^'Ns 



•"Sr 







Fig. 125. Group of common lithens, natural size. A, a crustose form {Placodium) grow- 
ing on rock; B, a foliose form {Parmelia) growing on bark; C, a fruticose lichen (Cladonia) 
which grows erect on the ground; D, a branching form {Usiiea) that hangs from the limbs 
of trees. 



Lichens were once regarded as single plants. In 1868, their dual nature 
was demonstrated. In 1889, lichens were first synthesized by sowing 
spores from the fungous element of a lichen among appropriate free-living 
algae. The developing mycelium was seen to enclose the algae and 
develop into a lichen. Although the algal symbionts are forms that may 
exist independently, the fungi are known only as constituents of lichens. 
Vegetative reproduction takes place mainly by soredia, globular or 
scale-like bodies composed of a few hyphae closely investing one or more 
algal cells. They arise as buds on the upper surface of the thallus, become 
detached, and are scattered by the wind. The algal components multiply 



THALLOPHYTA: FUNGI 



155 



by fission within the lichen body. The fungous components produce 
ascocarps, generally in abundance (Fig. 127). These are either apothecia 
or perithecia. Sex organs have been observed in many lichens. The 
ascogonium is a spirally coiled multicellular filament commonly terminat- 
ing in a trichogyne. The male cells, or spermatia, are borne on branching 
hyphae arising within a flask-like chamber, or spermogonium. After 




Fig. 126. Cross section through the body of a lichen (Physcia), showing cells of the alga 
(shaded) surrounded by a mass of interlacing fungous hyphae, X 500. 

fertilization, which may not always take place, the ascogonium gives rise 
to many ascogenous hyphae and paraphyses (Fig. 127 B). At the tips of 
the ascogenous hyphae typical asci with eight ascospores are formed. An 
ascospore, in germination, produces hyphae that die unless they come in 
contact with a suitable alga. 

The relation of the two lichen components to each other is important to 
understand. The fungus lives on the alga as a parasite but does not kill 
it. In fact, the alga seems to be only slightly injured, merely sacrificing 
some of the food that it makes. At the same time, however, the alga is 
benefited in that the fungous body readily absorbs and retains moisture, 



156 



PL A N T M ORPHOLOG Y 



without which the alga could not live. The fungus derives food from the 
alga while the alga obtains moisture from the fungus. This reciprocal 
relation makes it possible for many lichens to live in dry exposed situa- 
tions where neither the alga nor the fungus could live alone. Thus the 
relation between the two lichen components is one of mutual advantage. 



w^ 



V0K 



■'?,7'..:^'T 



vi^ 



;;i''l.- 



^•- ^-i-.-j^-' \;'V/( ,•''-- " -v-'.< .'.ii^ 



>f 







Fig. 127. .4, longitudinal section through an apothecium of Physcia, showing hymenium 
and embedded algal cells, X60; B, enlarged view of hymeniuni, showing asci and para- 
physes, X500. 

COMPARISON OF THE CLASSES OF FUNGI 

The chief distinguishing characters of the five classes of fungi are as 
follows : 

Schizomycetes. Plant body unicellular, solitary or colonial, ciliated 
or nonciliated. Cells without a definite nucleus. Cell walls usually 
forming mucilage. Reproduction by fission. 

Myxomycetes. Plant body a naked amoeboid mass of multinucleate 
protoplasm (a plasmodium). Asexual reproduction by small uninucleate 
spores, each with a cell wall and usually borne within sporangia of definite 
form. Sexual reproduction by amoeboid isogametes. 



THALLOPHYTA: FUNGI 157 

Phycomycetes. Plant body typically a nonseptate multinucleate 
mycelium. Asexual reproduction by spores formed by cleavage and 
borne in indefinite numbers in sporangia. Lower members with zoo- 
spores, higher members with aerial spores. Sexual reproduction isoga- 
mous or heterogamous. Heterogamous forms with well-developed sex 
organs. 

Ascomycetes. Plant body typically a septate mycelium. Spores 
borne usually in groups of eight in a sac-like structure, the ascus, their 
nuclei arising by three successive divisions of a fusion nucleus. Zoospores 
wanting. Sex organs reduced, obscure, or entirely absent. 

Basidiomycetes. Plant body a septate mycelium. Spores borne 
usually in groups of four on a club-like structure, the basidium, their 
nuclei arising b}^ two successive divisions of a fusion nucleus. Zoospores 
wanting. Sex organs not present. 

GENERAL CONCLUSIONS 

The fungi are a heterogeneous assemblage of thallophytes of diverse 
origin held together by a physiological character — the absence of chloro- 
phyll. Two classes, the Schizomycetes and Myxomycetes, stand apart 
from each other and from the three classes of "true fungi" (Eumycetes). 
In their unicellular organization, cell structure, and reproduction the 
Schizomycetes resemble the Cyanophyceae much more closely than they 
resemble any of the other fungi. The Myxomycetes, with their naked 
Plasmodia, highly developed sporangia, and amoeboid isogametes, exhibit 
similarities to some of the Protozoa, on the one hand, and to some of the 
lower Phycomycetes (Plasmodiophorales) on the other. 

Some botanists believe that the "true fungi" are a monophyletic group 
that have arisen from colorless flagellates and have subsequently differen- 
tiated into the three existing classes of Phycomycetes, Ascomycetes, and 
Basidiomycetes. According to this theory, no direct relationship exists 
between the algae and fungi, their resemblances being a result of parallel 
evolution along two independent lines. Other botanists believe that the 
"true fungi" have been derived from the algae through loss of chlorophyll, 
their origin having been either monophyletic or polyphyletic. According 
to this theory, the Phycomycetes have evolved from the Chlorophyceae, 
the Ascomycetes from either the Phycomycetes or the Rhodophyceae, 
and the Basidiomycetes from the Ascomycetes. 

Vegetative Body. The characteristic plant body of the fungi' is a 
mycelium, made up of branching hyphae that may be either nonseptate 
and coenocytic (Phycomycetes) or septate (Ascomycetes and Basidiomy- 
cetes) . Only a few forms are unicellular. The hyphae elongate by apical 

1 In the following discussion the term fungi will be limited to the three classes of 
"true fungi." 



158 PLANT MORPHOLOGY 

growth. They may be either loosely or compactly arranged. Some- 
times they are aggregated to form root-like strands or a compact resting 
body (sclerotium) . In the development of fruit bodies in the higher fungi 
— ascocarps and basidiocarps— masses of hyphae become interwoven to 
form a pseudoparenchymatous structure, but no tissue is formed by cells 
dividing in three planes. In the lower Phycomycetes the cell wall con- 
sists largely of cellulose, but in the other fungi its composition is altered 
by the presence of chitin and other substances, such as fatty acids. 
Within the cells of the mycelium are one, two, or many nuclei embedded 
in the cytoplasm. Sugars and glycogen represent the reserve carbo- 
hydrates, no starch being present. Varying amount of fats may also 

occur. 

Spore Reproduction. The Phycomycetes produce spores in sporangia, 
either zoospores in the lower orders or aerial spores in the higher orders. 
The spores are formed in indefinite numbers by cleavage. After escaping, 
they germinate into a mycelium. The entire sporangium may be persist- 
ent, as in the Saprolegniales and Mucorales, or detachable, as in most of 
the Peronosporales. In many of the Ascomycetes and Basidiomycetes 
the detachable sporangia are replaced by conidia, which function as 
spores and produce a new mycelium directly. Many conidia, as well as 
certain other spores, multiply by budding, like the vegetative cells of the 

yeasts. 

Many fungi produce resting spores that are thick-walled and resistant 
to adverse conditions. Often the same species has two or more different 
kinds of spores, as in the rusts. Ascospores, which are characteristic of 
the Ascomycetes, arise by free-cell formation. They are borne internally 
in an ascus, usually in groups of eight, while basidiospores, characteristic 
of the Basidiomycetes, are produced externally on a basidium, usually in 
fours. The formation of ascospores and basidiospores is related to the 
sexual process. 

Gametic Reproduction. In the Phycomycetes sexual reproduction is 
alga-like. The Chytridiales and Plasmodiophorales produce free- 
swimming isogametes that fuse in pairs to produce a zygote. Among 
the heterogamous Phycomycetes (Monoblepharidales, Saprolegniales, and 
Peronosporales), all of which have well-developed antheridia and oogonia, 
only the Monoblepharidales have swimming sperms; in the two other 
orders a male nucleus reaches the egg by passing through a fertihzation 
tube. The gametes are nearly always formed within special cells, the 
gametangia or sex organs. In the higher Phycomycetes (Mucorales and 
Entomophthorales) the gametangia are not differentiated as antheridia 
and oogonia, but the entire contents of two gametangia conjugate to form 
a zygote. 

The Ascomycetes show^ various stages in the degeneration of the sex 



THALLOPHYTA: FUNGI 159 

organs. Where these are well developed, the oogonium (ascogonium) 
often resembles that of the red algae. The zygote may develop directly 
into an ascus or, more commonly, may give rise to many ascogenous 
hyphae that, in turn, produce the asci. The Basidiomycetes have no 
sex organs (unless the spermogonia of the rusts are so regarded), but 
fusions between vegetative cells are common. In the Ascomycetes and 
Basidiomycetes the sexual nuclei come together without immediately 
fusing. The nuclear fusion, which takes place in the ascus or basidium, 
is followed at once by the production of ascospores or basidiospores, 
respectively. 



CHAPTER V 
BRYOPHYTA 

The bryoph>'tes. numbering about 20,000 species, form a well-defined 
division comprising the two classes Hepaticae (liverworts) and :Musci 
(mosses). They are small, rather inconspicuous, green plants nearly all 
of which five on land in moist, shaded places. The bryophj^es doubtless 
have been derived from aquatic ancestors, probably from some group of 
green algae, but it is uncertain whether they have given rise to any of the 
higher plant groups. Nevertheless, the bryophnes represent a general 
condition of structural organization through which the higher plants may 
have passed in the course of their evolution. Although abundant mois- 
ture is necessary for vigorous vegetative gro^^'th, some forms live in dry 
situations and endure considerable desiccation during long rainless 
periods. A few hver worts and mosses live in fresh water, but the aquatic 
habit in the bry ophites, as in the higher groups, has undoubtedly been 
secondarily acquired. 

A well-defined alternation of generations is an established feature of all 
bryophj^es, the gametophj-te and sporoph>i:e always being morphologi- 
cally dissimilar. The gametoph^-te, arising from the spore, is the haploid 
generation, producing sperms and eggs. The sporophyte, arising from 
the zygote, is the diploid generation. It produces spores, the reduction 
in chromosome number taking place in connection ^^-ith their formation, 
as in all the higher plants. S\^-imming spores are entirely ehminated. 
In the green algae the zygote is liberated into the water and is nearly 
always a resting cell, while in the bryophytes and all higher groups it 
germinates at once, without escaping, to produce an embryo sporophyte. 
In the bryoph>i:es the gametophyte, or haploid generation, is always an 
independent indi\'idual, while the sporophyte. or diploid generation, is 
entirely or largely dependent on it for its nutrition. Although the 
garnet oph^-te is thalloid in some of the liverworts, in most bryophytes it 
is dift'erentiated into stem and leaves. Growth takes place through the 
activity of an apical cell. The sex organs, antheridia and archegonia, 
are always multicellular and provided with an outer sterile jacket. 
Throughout the algae the gametangia are prevailingly unicellular but, 
where multicellular, all their cells produce gametes (except in the Charo- 
phyceae). The antheridium is a stalked, spherical or club-shaped organ 
consisting of a mass of spermatogenous tissue enclosed by a jacket of 

160 



BRYOPHYTA 161 

sterile cells. It gives rise to numerous small biciliate sperms, two of 
which arise from each sperm mother cell. The presence of swimming 
sperms, universal among bryophytes and pteridophytes, represents the 
retention of a primitive algal character. 

The archegonium is a very characteristic organ of bryophytes and 
pteridophytes. Although corresponding to the oogonium of the algae, 
it is much more highly developed. The archegonium is usually stalked 
and flask-shaped. It is composed of an axial row of cells surrounded 
by a sterile jacket. The axial row consists of an egg — the basal and 
largest cell of the series — and a variable number of canal cells, which 
disorganize and become mucilaginous prior to fertilization. The fertilized 
egg gives rise to an embryo that develops within the archegonium, the 
basal portion of which enlarges to form a protective covering, the calyptra. 

In all bryophytes the sporophyte is without differentiation into stem 

and leaves and is w^ithout a direct connection with the soil. In nearly 

all bryophj^tes the sporophyte consists of a ba.sal absorbing organ {foot), 

a stalk (seta), and a terminal spore-producing portion (capsule). The 

capsule is a sporangium. All bryophytes are homosporous, the spores of a 

given species being alike in size and form. On germination, the spore 

produces either the main gametophyte directly or, more commonly, a 

filamentous protonema from which the main gametophyte sooner or later 

arises. 

1. HEPATICAE 

The liverworts are primitive land plants, most of them growing in the 
presence of abundant moisture on soil, rocks, and tree trunks. With 
very few exceptions, the gametophyte is dorsiventral. It may be thalloid, 
but more commonly is leafy, the leaves being nearly always without a 
midrib. Unicellular unbranched rhizoids maintain a connection with the 
substratum. The Hepaticae are widely distributed but are more numer- 
ous in the tropics than elsewhere. A few fossil forms are known from the 
Upper Carboniferous of England. There are about 6,000 species of 
liverworts, nearly all being included in four principal orders, the r^Iarchan- 
tiales, Sphaerocarpales, Jungermanniales, and Anthocerotales. 

1. Marchantiales 

The Marchantiales are a well-defined order of about 30 genera and 
400 species. They range from arctic to tropical regions and are well 
represented in the Temperate Zones. In the tropics they occur chiefly 
between altitudes of 900 and 1,500 m. Nearly all of them are terrestrial, 
growing mainly on damp soil or rocks. Some common genera of Mar- 
chantiales, all of widespread distribution, are Riccia, RehouUa, Asterella, 
Conocephalum, and Marchantia. The largest genus is Riccia, wth over 
100 species. 



162 



PLANT MORPHOLOGY 



Gametophyte. The Marchantiales are characterized by a flat, dorsi- 
ventral, thalloid gametophyte — with few exceptions ribbon-hke and 
nearly always rather fleshy. It branches either dichotomously from the 
apex or, less commonly, by means of adventitious outgrowths arising 
apically or ventrally. In Riccia the thallus is small and, as a result of 
repeated dichotomy, often grows in the form of a fan or rosette (Fig. 128). 
In all the Marchantiales growth takes place by means of an apical cell 
situated in an apical notch. It is of the cuneate (wedge-shaped) type, 




Fig. 128. Dorsal view of the gametophyte of Riccia nutans, showing sporophytes in the 
grooves and scales arising from the ventral surface, X 3. 




Fig. 129. Reboulia hemisphaerica. A, longitudinal section of portion of growing region 
of thallus with apical cell and developing air chambers, X 160; B, portion of upper region of 
thallus, showing air pore and air chambers, X85. 

cutting off segments on four sides — above and below as well as left and 
right (Fig. 129A). 

The gametophyte is of simple external form but exhibits a high degree 
of internal differentiation, nearly always consisting of (1) an upper 
epidermal layer; (2) a loose, green, dorsal region having one or more 
layers of air chambers; (3) a compact, colorless, ventral region. The 
epidermis, usually colorless or pale green and often with slightly thickened 
walls, nearly always contains numerous air pores that communicate with 
the air chambers. Air pores and air chambers are not developed in a 
few genera {e.g., Dumortiera and Monoclea), their absence being a result of 
reduction. 



BRYOPHYTA 



163 



In most species of Riccia the dorsal region is composed of erect rows of 
cells separated by very narrow, vertical air chambers, but in some species 
it is spongy, consisting of a loose network of large irregular air chambers 
separated by one-layered partitions that extend in all directions (Fig. 
137A). The uppermost cells form a rather ill-defined epidermis. In 
Riccia nutans simple air pores are present, but in the other species air 
pores are either rudimentary or wanting. Simple air pores consist of a 
single tier of cells surrounding a small central opening, the cells being in 




Fig. 130. Section through the thallus of Marchantia polymorpha, showing epidermis with 
an air pore that leads to an air chamber with green filaments, X200. Rhizoids are shown 
below. 



several concentric circles (only one circle in Riccia natans). Simple air 
pores occur on the thallus of most of the genera of Marchantiales (Fig. 
1295). In Marchantia and a few related forms, however, the thallus 
bears compound air pores. These are barrel-shaped, consisting of four 
or five superimposed layers of cells and having both an upper and a lower 
opening (Fig. 130). Conocephalum, Marchantia, and many other genera 
have a single layer of air chambers from the floor of which special chloro- 
phyllose filaments arise. Rehoulia and Asterella have several layers of air 
chambers without green filaments (Fig. 1295). In many forms the limits 
of the air chambers are plainly visible on the dorsal surface of the thallus 
as polygonal areas, an air pore occurring in the center of each. 

In practically all the Marchantiales the lower surface of the thallus bears 
numerous rhizoids and scales. The rhizoids are of two kinds, smooth and 
tuberculate. The former have smooth walls, the latter peg-like thicken- 



164 



PLANT MORPHOLOGY 



ings that project into the lumen. In Riccia rhizoids are usually abun- 
dant, but frequently ventral scales are rudimentary or absent. ^ In nearly 
all the Marchantiales the ventral scales are arranged in two longitudinal 
rows; in Marchantia they are in four or more rows. 

Throughout the order decay of the older parts of the thallus results in 
the isolation of branches, each of which forms a new plant. In some 
species vegetative propagation occurs by the formation of adventitious 
branches that become detached. In two genera, Lunularia and Marchan- 
tia, multicellular gemmae are produced. These are flat, stalked, discoid 
bodies that arise in groups on the dorsal side of the thallus inside cupules. 





Fig. 131. Male (A) and female (B) plants of Marchantia polymorpha, natural size. 

In Lunularia the cupules are crescentic, while in Marchantia they are 
cup-shaped. The gemmae arise from the floor of the cupule. Each 
gemma has two notches, one on either side, and in each notch is an apical 
cell. Upon separation from the cupule, a single gemma gives rise to two 
new thalli. 

Sex Organs. The sex organs of the Marchantiales are invariably 
dorsal in origin, arising either directly on the thallus itself or on a more or 
less specialized receptacle. Both kinds of sex organs arise in acropetal 
succession from segments of an apical cell. According to the species, the 
antheridia and archegonia occur on the same plant or on separate plants. 
In Riccia each branch of the thallus has a median dorsal groove extending 
backward from the growing apex; in this groove the sex organs are borne. 
Although generally scattered irregularly, they sometimes tend to be 
segregated into separate groups. The sex organs arise singly just behind 

' When Riccia nutans floats on the surface of quiet water, it has numerous large 
scales and few or no rhizoids. When it grows on muddy banks and flats, it has many 
rhizoids and few scales. 



BRYOPHYTA 



165 



the apical cell and soon become sunken in the thallus by upgrowth of the 
surrounding tissues, each coming to lie in an individual pit. 

In the other Marchantiales the antheridia are similarly sunken in pits 
but the archegonia are not. The antheridia may be borne in irregular 
median groups on the dorsal side of the thallus, as in some species of 
Asterella, but more commonly they occur on a definite receptacle. This 
may be cushion-like and sessile, asmReboidiasindConocephalum, or raised 
above the thallus on a stalk, as in Marchantia. The antheridial recepta- 
cle of Marchantia has a number of marginal growing points, from each 




Fig. 132. Male structures of Marchantia polymorpha. A, longitudinal section through 
young male receptacle, showing embedded antheridia, X40; S, nearly mature antheridium, 
X200; C, a single sperm, more highly magnified. 

of which an acropetal series of antheridia extends toward the center, the 
antheridia being sunken in the upper surface of the receptacle (Figs. 
13L4 and 132A). 

In Rehoulia, Asterella, Conocephalum, and many other genera the 
archegonia are borne on a stalked receptacle that, with few exceptions, 
is terminal in position and represents a specialized upright branch of the 
thallus. Unlike the antheridia, the archegonia are not embedded in pits. 
The female receptacle is commonly hemispherical or conical and more or 
less lobed. Each lobe represents a separate growing point back of 
which either one or several archegonia arise. As the receptacle grows, 
the archegonia are carried to a position on its lower side close to the stalk. 
In Marchantia the archegonial receptacle reaches its greatest degree of 
specialization. It does not have lobes, but consists of a number of rays 
alternating with groups of archegonia (Figs. 13 IB and 133.4). The 
archegonia hang with the necks downward. 



166 



PLANT MORPHOLOGY 



Air chiimbers and air pores are developed on both the male and female 
receptacles. In Reboulia and Asterella the air pores on the female recep- 
tacle are compound, while those occurring elsewhere on the plant are 
simple. In Conocephalum the air pores are compound on both the male 




Fig. 133. Female structures of Marchantia polymorpha. A, longitudinal section through 
young female receptacle, showing a row of archegonia, X40; B, mature archegonium with 
egg ready for fertilization, X300; C, young embryo lying within the archegonium, X300. 



and female receptacles, but are simple on the thallus. In Marchantia 
the air pores are everywhere compound. 

The mature antheridia of the Marchantiales are club-shaped structures 
with a short stalk (Fig. 1325). The chamber in which each lies com- 
municates with the surface of the thallus or receptacle by a pore through 
which the sperms escape. The antheridium arises from a single super- 
ficial initial cell that becomes papillate and then divides transversely 
(Fig. 134^). The outer segment undergoes several additional transverse 



BRYOPHYTA 



167 



divisions, resulting in the formation of about four superimposed cells 
(Fig. 1345-Z)). In each of these vertical walls appear at right angles to 
each other and later, by the formation of periclinal walls in the upper 
part of the antheridium, an outer layer of sterile cells is cut off from a cen- 
tral group of spermatogenous cells (Fig. 134£'~//). The lower portion of 




Fig. 134. Early stages in the development of the antheridium of Marchantia polymorpha, 
X750. A, division of initial into an inner and outer cell; B, C, D, formation of a filament 
of four cells from the outer cell; E and F, appearance of vertical walls; G, appearance of 
periclinal walls; H, later stage, showing sterile jacket surrounding spermatogenous cells, 
with stalk below. 

the antheridium forms the stalk. By continued division, the spermatog- 
enous cells give rise to many small, cubical, sperm mother cells, each of 
which produces two biciliate sperms (Fig. 132B, C). 

The archegonium also arises from a single superficial initial that 
becomes papillate and divides transversely (Fig. ISoA). Three vertical 
walls now appear in the outer segment, these being arranged in such a 
way that a middle cell and three peripheral cells are formed (Fig. 1355, H). 
The middle cell is the primary axial cell, the peripheral ones the primary 
wall cells. The primary axial cell, by a transverse division, gives rise to a 
cover cell and a central cell (Fig. 135C). The archegonium now grows in 



1G8 



PLANT MORPHOLOGY 



\e\wt\\ the central cell dividing to form a primary neck canal cell and a 
primary ventral cell (Fig. 135D). As a result of additional transverse 
divisions, the primary neck canal cell gives rise to a vertical row of neck 
canal cells, most commonly either four or eight in number, while the 
primary ventral cell divides transversely to form the ventral canal cell 

and egg (Fig. ISSS-C). , « , u a 

By this time the archegonium has become distmctly flask-shaped, 
the slender neck being sharply marked off from the bulbous venter. In 




E ^-"T^ F ^-1 G 

Fig 135. Development of the archegonium of Marchaidia polymorpha, X600. A, 
division of initial into an inner and outer cell; B, appearance of three vertical walls in the 
outer cell; C, formation of cover cell and central cell from the primary axial cell; D, forma- 
tion of primary neck canal cell and primary ventral cell from the central cell; E and F, 
later stages, with two and four neck canal cells; G, nearly mature archegonium, with egg 
and ventral'canal cell derived from the primary ventral cell; H, cross section of very young 
archegonium, showing primary axial cell surrounded by primary wall cells ; I, later stage, 
.showing six neck cells surrounding a neck canal cell. 

all the Marchantiales the neck consists of six vertical rows of jacket cells 
surrounding the canal (Fig. 135/). The canal cells disorganize, forming 
a mass of mucilage through which the sperms can swim (Fig. 133B). 
The egg is fertilized within the venter of the archegonium, which enlarges 
to form the calyptra, the embryo developing within (Fig. 133C). In all 
the Marchantiales except Riccia, an involucre arises around the arche- 
gonia. In Asterella, Marchantia, and several other genera an additional 
envelope, the pseudoperianth, arises after fertilization and generally 
becomes very conspicuous (Fig. 139). 

Sporophyte. Riccia displays the simplest sporophyte among the 
Bryophyta. In its development, the fertilized egg divides by a trans- 
verse wall, resulting in two cells approximately equal in size (Fig. 136). 



BRYOPHYTA 



169 



Each of these now divides by a vertical wall, followed by another at right 
angles to it. Additional divisions, without definite sequence, take place 
in all three planes. Then periclinal walls cut off an outer layer, the 
amphithecium, from a central group of cells, the endothecium. As the 
embryo continues to grow, the entire central group becomes sporogenous, 
while the outer layer remains sterile. After the sporogenous cells have 
divided for the last time, they separate and round off to become spore 
mother cells (Fig. 137). Each of these then enlarges and undergoes two 





ABC 

Fig. 136. Development of the embryo of Riccia natans, X400. .*i, two-celled stage; B, 
four-celled stage; C, later stage, showing differentiation into amphithecium and endo- 
thecium. 

consecutive divisions during which the number of chromosomes is reduced 
one-half, and a tetrad of cells is formed. The walls thicken and the four 
members of the tetrad separate as mature spores. During the early 
development of the sporophyte, the venter of the archegonium becomes 
two-layered and forms the calyptra (Fig. 136). The sterile jacket of 
the sporophyte and the inner layer of the calyptra break down before 
the spores have ripened, leaving them enclosed within the outer laj^er 
of the calyptra. Riccia has no spore-dispersing mechanism, the spores 
being liberated by progressive decay of the thallus. 

In practically all the other Marchantiales the sporophyte consists of a 
foot, seta, and a capsule containing both spores and elaters. As in Riccia, 
the first division of the fertilized egg is transverse. In some genera, such 
as Reboulia, Asterella, Conocephalum, and others, the next two divisions 
are also transverse, resulting in a filament of four superimposed cells 
(Fig. 138). Then vertical walls come in and, with the formation of 



170 



PLANT MORPHOLOGY 





A D 

Fig. 137. Sporophyte of Riccia natatis. A, longitudinal section of nearly mature sporo- 
phyte embedded in the gametophyte, showing spore tetrads enclosed within the calyptra; 
B, spore mother cell; C, tetrad; D, mature spore; A, X 100; B, C, D, X500. 




C D E 

Fig. 138. Development of the embryo of Cryptomitrium tenerum, X400. A, two-celled 
stage; B, division of lower cell; C, four-celled stage, the two lower cells giving rise to the 
foot and seta, the two upper cells to the capsule; D, eight-celled stage; E, older stage, 
showing differentiation of sporogenous tissue in the capsule. {After Haupt.) 



BRYOPHYTA 



171 



additional walls in the upper part of the embryo, the foot, seta, and cap- 
sule are differentiated. In other genera, such as Marchantia, the first 
division of the fertilized egg is followed by the appearance of two vertical 
walls at right angles to each other in both the upper and lower segments, 
thus forming octants, as in Riccia. Additional walls in all three planes 
produce a globular embryo rather than an elongated one (F'ig. 139), 

An early formation of periclinal walls in the capsular region cuts off the 
amphithecium from the endothecium, the former forming the capsule 




B C 

Fig. 139. Development of the embryo of Marchantia polymorpha. A, four-celled stage, 
X320; B, slightly later stage, X320; C, older embryo, showing the foot (/), seta (s), capsule 
(c) with sporogenous tissue differentiated, pseudoperianth (p), and calyptra (a), X200. 

wall and the latter the sporogenous tissue (Fig. 138£'). In Riccia the 
sporogenous tissue is derived equally from both halves of the embryo. In 
practically all the other genera, whether the embryo is of the filamentous 
or of the octant type, apparently only the upper half contributes to the 
sporogenous tissue, the lower half giving rise to the foot and seta. The 
foot anchors the sporophyte and absorbs nourishment. The seta elon- 
gates, especially after the spores ripen, pushing the capsule through the 
calyptra. 

In most genera, except Riccia, some of the potentially sporogenous cells 
of the young capsule give rise to elaters, while the others directly become 
spore mother cells. In Marchantia, however, the sporogenous ceils 
greatly elongate, some remaining undivided to form elaters, the others 
dividing transversely a number of times to form vertical rows of spore 



172 



PLANT MORPHOLOGY 



mother cells (Fig. 140). As in Riccia, tetrads are formed and the walls 
of the spores thicken. The elaters are long, slender cells, pointed at 
each end, their walls developing spiral thickenings as the protoplasm 
disappears (Fig. 140D). Elaters are hygroscopic and perform squirming 
movements that assist in the liberation of the spores. The capsule wall 
■ one layer of cells thick except in the apical region. In RebouUa, 



IS 







M /"^Z^ 





Fig. 140. Sporophyte of Marchantia polymorpha. A, longitudinal section of nearly 
mature sporophyte, showing the foot (/), seta (s), capsule (c), and ruptured calyptra (a); 
B, two rows of spore mother cells and portion of an undeveloped elater; C, a row of spore 
tetrads; D, three mature spores and the end of an elater; A, XQO; B, C, D, X600. 



Asterella, and related forms local thickenings are not formed on the cells 
of the capsule wall and dehiscence takes place by means of an apical lid. 
In nearly all the other genera, however, the cells of the capsule wall bear 
annular thickenings, dehiscence occurring by irregular clefts. 

Summary. The Marchantiales are a group in which the gametophyte, 
while remaining simple in form, has achieved a high degree of structural 
complexity. In all members of the order the gametophyte is thalloid and 
grows by means of a cuneate apical cell. It is nearly always differentiated 
into an upper epidermis with air pores, a dorsal photosynthetic region 



BRYOPHYTA 173 

with air chambers, and a compact, colorless ventral region. The gameto- 
phyte reaches an extreme of complexity in forms with compound air 
pores and air chambers having green filaments. It bears both smooth 
and tuberculate rhizoids. In the lower members the sex organs are borne 
directly on the thallus, sunken in the dorsal surface, but throughout the 
group there is a marked tendency to restrict and specialize the regions 
producing sex organs, resulting in the development of complex receptacles. 
The female receptacles, and sometimes the male as well, are stalked. 
The antheridia, when mature, are elongated organs lying in a deep 
chamber. Their early development is characterized by a series of trans- 
verse divisions. The neck of the archegonium shows six cells in cross 
section. 

In the lower members the sporophyte is a spherical, undifferentiated 
spore case, all the inner cells forming spores. In the higher members the 
sporophyte is elongated and differentiated into a foot, seta, and capsule. 
The capsule contains both spores and sterile cells, the latter practically 
always developed as elaters. Thus, throughout the group, there is a 
marked tendency to divert potentially sporogenous tissue to functions 
other than spore production. The seta is comparatively short. The cap- 
sule is spherical or nearly so, its wall being composed of a single layer of 
cells (the apex usually thicker). Dehiscence, lacking in Riccia, nearly 
always occurs by irregular clefts or an apical lid. The Marchantiales are 
a group in which a complex gametophyte is combined with a relatively 
simple sporophyte. 

2. Sphaerocarpales 

The Sphaerocarpales comprise a small order of 3 genera and 25 species. 
Sphaerocarpus is a widely distributed genus but Geothallus, represented 
by a single species, has been found only near San Diego, California. Both 
of these forms grow on moist earth. Riella is an aquatic form occurring 
in Europe, Africa, California, and western Texas. 

Gametophyte. The gametophyte of the Sphaerocarpales displays none 
of the internal differentiation seen in the Marchantiales. It consists of 
a simple plate-like thallus that differs somewhat among the three genera. 
In Sphaerocarpus the thallus is small, flat, and often orbicular, with an 
entire or more or less lobed margin. It has a broad indistinct midrib, 
several layers of cells in thickness, that merges gradually into the one- 
layered wings (Fig. 141). In Geothallus the thallus is larger and consists 
of an elongated thickened axis giving rise to crowded leaf-like outgrowths 
on either side, these mostly one layer of cells thick. A large portion of the 
axis becomes converted into a fleshy tuber that lives over into the next 
growing season. In both Sphaerocarpus and Geothallus the thallus may 
be either simple or dichotomously branched. The lower surface lacks 





174 PLANT MORPHOLOGY 

scales but bears numerous colorless rhizoids of the smooth-walled type. 
Growth of the thallus results from the activity of a cuueate apical cell. 

In general appearance Riella is unlike any other liverwort. It is a sub- 
merged aquatic, usually growing erect in standing water. It has a stem- 
Uke axis that bears a dorsal leaf-like wing or, in the Algerian Riella bialata, 
two wings. The wing is mostly one layer of cells thick. It is frequently 
undulate and sometimes spirally twisted. The axis is commonly several 

times dichotomous. It produces 
rhizoids near the base. 

Sex Organs. The antheridia and 
archegonia are borne directly on the 
thallus, each enclosed in a special 
involucre that is open above. They 
arise in acropetal succession from 
dorsal segments of the apical cell. 
V^^^^ In Riella both kinds of sex organs 

''^ B may occur on the same plant, al- 

FiG. 141. Female (A) and male (B) though generally, as in ^Sp/merocarpMS 
gametophytes of Sphaerocarpus caii- ^ Qeothallus, they are borne on 

fornicus, X6. {From Gilbert M . Smith.) > '' 

separate plants. 
In Sphaerocarpus the sex organs are closely crowded on the dorsal sur- 
face of the thallus. The male plants, often purplish, are minute and 
much smaller than the female plants (Fig. 141). The antheridial involu- 
cres are flask-shaped and each contains an ovoid short-stalked antherid- 
ium. In development, two transverse walls appear in the outer cell 
arising from a transverse division of the papillate initial (Fig. 142 A, B). 
In the upper two segments vertical walls are formed at right angles to 
each other and then periclinal walls cut off a layer of outer sterile cells 
from a central group of spermatogenous cells (Fig. 142(7-^). Further 
development takes place as in the Marchantiales. The archegonial invo- 
lucres, each enclosing an archegonium, are tubular or nearly spherical. 
The archegonium develops as in the Marchantiales (Fig. 143). It has 
two to four neck canal cells, its neck showing six cells in cross section. 
Following fertilization, the calyptra becomes two-layered. It is soon 
ruptured by the sporophyte. 

In Geothallus the sex organs are borne and develop as in Sphaerocarpus, 
but are much less numerous and the male plants are only slightly smaller 
than the female. In Riella the antheridia occur in a series along the mar- 
gin of the wing, each enclosed in a pocket. The archegonia are arranged 
serially on the axis, each surrounded by a flask-shaped involucre. 

Sporophyte. The sporophyte of the Sphaerocarpales is more advanced 
than that of Riccia but simpler than that of nearly all the other Marchan- 
tiales. It has a foot, capsule, and very short seta (Fig. 144.4). In 



BRYOPHYTA 



175 



Sphaerocarpus the first division of the fertilized egg is transverse, the 
capsule arising from the outer segment and the foot and seta from the 
inner segment, as in the Marchantiales. Each segment again divides 
transversely at least once before vertical walls come in. The foot 
becomes bulbous and the capsule spherical. In all genera some of the 
sporogenous cells develop into spore mother cells and others into small 




Fig. 142. Early stages in the development of the antheridium of Sphaerocarpus texanus, 
X500. A, division of initial into inner and outer cell; B, first di\'ision of outer cell; C, 
formation of vertical walls; D, formation of perielinal walls; E, later stage, showing sterile 
jacket surrounding spermatogenous cells, with stalk below. 



sterile cells that do not become elaters (Fig. 1445). Instead, they func- 
tion as nutritive cells, being finally absorbed by the spores. As in the 
Marchantiales, the spore mother cells are not lobed. The capsule has no 
regular dehiscence. Its wall consists of a single layer of cells without 
local thickenings. 

Sex Determination. The mechanism of sex determination in Sphaero- 
carpus is of particular interest because here the occurrence of sex chromo- 
somes in plants was first observed. In addition to seven ordinary 
chromosomes, the cells of the female gametophyte have a very large 
X chromosome, while those of the male gametophyte have a very small 



176 



PLANT MORPHOLOGY 



Y chromosome. Consequently all the eggs carry an X chromosome, all 
the sperms a Y chromosome, and the zygote is always XY. This devel- 
ops into a sporophyte with eight pairs of chromosomes, seven ordinary 
pairs and the XY pair. When meiosis occurs at sporogenesis, two of the 
spores in each tetrad have an X chromosome and two have a Y chromo- 
some. The spores with an X chromosome always develop into female 
gametophytes and those with a Y chromosome into male plants. 




B ^ D E 

Fig. 143. Development of the archegonium of Sp/iaerorarpws iea;a«i/s, X500. .4, division 
of initial into inner and outer cell; B, appearance of three vertical walls in the outer cell; C, 
formation of cover cell and central cell ; D, formation of primary neck canal cell and primary 
ventral cell; E, nearly mature archegonium with egg, ventral canal cell, and two neck 
canal cells; F, cross section of neck. 



Summary. The Sphaerocarpales are an aberrant group of liverworts 
showing a distinct relationship to the Marchantiales on the one hand and 
to the Jungermanniales on the other. They resemble the Jungerman- 
niales in the form and structure of the gametophyte, but are like the 
Marchantiales in the structure and development of the sex organs and in 
the structure of the sporophyte. As in the Marchantiales, the apical cell 
is cuneate, the neck of the archegonium has six cells in cross section, and 
the spore mother cells are not four-lobed. The development of the 
antheridium shows a closer resemblance to the Marchantiales than to the 
Jungermanniales. The sporophyte consists essentially of a foot and cap- 
sule, the latter indehiscent and with a wall composed of a single layer of 
cells. In addition to the spores, the capsule contains sterile nutritive cells 
but no elaters. The most distinctive feature of the Sphaerocarpales is 
the presence of a special involucre around each antheridium and each 



BRYOPHYTA 



177 



archegonium. On the whole, they are a primitive group, with a simple 
gametophyte and a sporophyte only slightly more advanced than that of 
Riccia. 




Fig. 144. Sporophyte of Sphaerocarpus texanus, X250. A, early stage, showing foot, 
seta, and capsule with sporogenous tissue; B, spore mother cells and smaller, starch-filled 
nutritive cells. 

3. Jungermanniales 

The Jungermanniales are by far the largest order of Hepaticae, embrac- 
ing 150 genera and approximately 5,500 species. They are most abun- 
dant in tropical regions, where they grow on the ground, on decaying logs, 
and as epiphytes on the stems and leaves of trees. They require abun- 
dant moisture and good drainage. Although much less numerous than 
in the tropics, the group is well represented in temperate regions also. 
The Jungermanniales comprise two great series, the Anacrogynae and the 
Acrogynae. These will be considered separately. 



1. Anacrogynae 



In the "anacrogynous" Jungermanniales all the archegonia originate 
behind the apical cell, none ever arising from the apical cell itself (Fig. 
145yl). Thus the archegonia and sporophytes are always dorsal. Most 



178 



PLANT MORPHOLOGY 



of the Anacrogynae are thallose, but some are leafy. They include 20 
genera and about 500 species. The principal genera, all widely distrib- 
uted, are Riccardia, Metzgeria, Pallavicinia, Symphyogyna, Pellia, and 
Fossombrotna. The largest genus, Riccardia, has over 100 species. 

Gametophyte. The Anacrogynae have a dorsiventral gametophyte 
that is thalloid in most forms but more or less leafy in others (Fig. 146). 
While tending toward a diversity of form, the gametophyte has remained 
structurally simple, displaying little or no internal differentiation. In 
fact, it is composed of compact, almost uniform tissue. The gameto- 
phyte is sometimes unbranched, but generally branches dichotomously 




Fig. 145. Fossombronia cristida. A, longitudinal section through apical region of thallus, 
showing the apical cell, a young archegonium, and a mucilage hair, X500; B, horizontal 
longitudinal section of apical cell, X600; C, vertical longitudinal section of apical cell, 
X600. {After Haupt.) 

or, in some forms, by means of ventral adventitious shoots. Growth 
takes place through the activity of an apical cell that is prevailingly 
dolabrate (hatchet-shaped), cutting off segments on two sides, alternately 
left and right (Fig. 145). Rhizoids, usually formed in abundance, are all 
of the smooth- walled type. Ventral scales are rarely present. 

The gametophyte of Pellia is one of the simplest in the Bryophyta. It 
consists of a thin, flat thallus, wavy along the margin, and with a broad 
indistinct midrib gradually passing into lateral wings composed of a single 
layer of cells (Fig. 146A). Some of the species of Riccardia have a similar 
thallus, while others have one like that of Metzgeria, with a narrow dis- 
tinct midrib sharply marked off from the wings. A distinct midrib is also 
present in Pallavicinia and Symphyogyna, but in both of these genera some 
species have an entire or wavy margin, others a margin that is deeply 
lobed (Fig. 146jB, C). Fossombronia represents an advanced condition, 
the wings of the thallus being dissected into two lateral rows of leaf-like 
segments, the midrib forming a stem. This series leads directly into the 
Acrogynae, where differentiation of the plant body into a leafy axis 
reaches its highest expression. 



BRYOPHYTA 



179 



Vegetative propagation in the Anacrogynae occurs by death of the 
older parts of the thalkis, resulting in the isolation of branches. Also, 
gemmae are produced in certain genera, such as Riccardia and Metzgeria, 
while others produce tubers that live over from one growing season to the 
next. 

Sex Organs. The antheridia and archegonia are borne either on the 
same gametophyte or on separate gametophytes, according to the species. 
They are always dorsal in position, generally occurring singly or in groups 






Fig. 146. Thallus of Pellia epiphylla (A) with a mature sporophyte, of Pallavicinia lyellii 



oi i-euia epipnyua (A) wiiu a mature ot^v^n^i^.»j vy^, v^^ j. ^^v,a,u^„.v^^ „yi^i^„ 
(B) with two groups of archegonia, and of Syrnphyogyna brongniartii {C) with three groups 
of archegonia, twice natural size. 

on the main thallus or, less frequently, on special short branches that in 
Riccardia are lateral and in Metzgeria are ventral in origin. The sex 
organs, unlike those of the Marchantiales, are never borne on stalked 
receptacles. The archegonia are usually protected by an involucre. 
The antheridia are protected in various ways: in Metzgeria, by incurving 
of the thallus; in Riccardia and Pellia, by upgrowth of adjacent tissues; 
in Pallavicinia and Syrnphyogyna, by a special involucre. 

In the Jungermanniales the development of the antheridium is char- 
acteristic, differing considerably from that seen in the Marchantiales and 
Sphaerocarpales. The antheridium arises as a papillate initial that 
undergoes a transverse division (Fig. 147 A). Another transverse wall 
usually appears in the outer cell, but the third wall is a median vertical 
one (Fig. 1475, C). In each of the two terminal segments thus formed a 
periclinal division takes place. Two additional periclinal walls then come 
in at right angles to the first ones, intersecting both these and the median 
wall. As a result, four primary wall cells are cut off from two central 



180 



PLANT MORPHOLOGY 



spermatogenous cells (Fig. 147D, E). Further development of the 
antheridium corresponds to that of the Marchantiales. The mature 
antheridium is generally spherical and either long-stalked or short-stalked. 
In all the Anacrogynae, as previously stated, the formation of archego- 
nia never involves the apical cell, all of them arising from its segments 
(Fig. 145.4). The development of the archegonium is essentially like 








D " t ' F 

Fig. 147. Early stages in the development of the antheridium of Pellia epiphylla, X400. 
A, division of initial into an inner and outer cell; B, division of outer cell into a stalk cell 
and primary antheridial cell; C, vertical division of antheridial cell; D, appearance of peri- 
clinal walls; E, cross section of same; F, later stage, showing two primary spermatogenous 
cells surrounded by sterile jacket, with stalk below. 

that of the Marchantiales and Sphaerocarpales, but the venter is usually 
more slender and the neck shows but five cells in cross section (Fig. 148). 
The number of neck canal cells is variable, but is commonly 6 or 8. In 
such forms as Pallavicinia and Pellia, however, this number may reach 18. 
The calyptra, developed from the venter of the archegonium, may become 
massive, as in Riccardia, Metzgeria, and Symphyogyna. In addition to 
the involucre, a pseudoperianth is formed in Pallavicinia, becoming con- 
spicuous after fertilization. 

Sporophyte. The sporophyte of the Jungermanniales is more advanced 
than that of the Marchantiales and Sphaerocarpales in that a greater 
amount of sterile tissue is formed. Following the first division of the fer- 
tilized egg, which is transverse, the lower cell often does not contribute to 
the embryo proper, but forms an appendage to it. This may become 
haustorial, as in Riccardia. The upper cell undergoes several transverse 



BRYOPHYTA 



181 



divisions before vertical walls appear, so that the embryo becomes elon- 
gated (Fig. 149A-C). A foot, seta, and capsule, always present, are 
differentiated early. The formation of periclinal walls in the upper part 
of the embryo dehmits the amphithecium from the endothecium, the 




D E F 

Fig. 148. Development of the archegonium of Pellia epiphylla, X400. A, division of 
initial; B, appearance of vertical walls in outer cell; C, formation of cover cell and central 
cell; D, formation of primary neck canal cell and primary ventral cell; E, formation of four 
neck canal cells and division of primary ventral cell; F, nearly mature archegonium with 
egg, ventral canal cell, and six neck canal cells; G, cross section of neck. 

former giving rise to the capsule wall, the latter to the sporogenous tissue 
(Fig. U9D). 

The seta undergoes considerable elongation, especially upon the ripen- 
ing of the spores. The capsule is highly developed, producing both 
spores and elaters. It may be spherical, as in Pellia, or more or less elon- 
gated, as in Pallavicinia. In some genera an elaterophore is developed 
inside the capsule. It may be either apical, as in Riccardia and Metzgeria, 
or basal, as in Pellia (Fig. 150). The elaterophore consists of a group of 
sterile cells to which some of the elaters are attached. The fixed elaters 
generally are shorter than the free elaters and often have a greater number 
of spiral bands. The spore mother cells, unlike those of the Marchan- 



182 



PLANT MORPHOLOGY 




Fig. 149. Development of the embryo of Fossomhronia cristula. A, two-celled embryo 
within calyptra, X350; B and C, later stages, X350; D, older embryo within caljptra, 
showing the foot, seta, and capsule with sporogenous tissue differentiated, X250; E, two 
spore mother cells, X500; F, a spore tetrad, X500. (After Haupt.) 





A B 

Fig. 150. Longitudinal section of the capsule of Riccardia {A), with apical elaterophore, 
and oiPellia {B), with basal elaterophore; A, X40; B, X 28. 



BRYOPHYTA 183 

tiales and Sphaerocarpales, become four-lobed just previous to the forma- 
tion of tetrads (Fig. 149^, F). The capsule wall is two or more layers of 
cells in thickness, or only one layer by resorption of the inner layer at 
maturity. Annular thickenings are generally present on one or both 
layers, but are constantly absent in a relatively few forms, such as Pal- 
lavicinia and Symphyogyna. Dehiscence of the capsule is nearly always 
effected by splitting into four valves. 

2. ACROGYNAE 

In the "acrogynous" Jungermanniales archegonia may arise from seg- 
ments of the apical cell, but sooner or later the apical cell itself becomes 
an archegonium. The terminal position of the archegonia and sporo- 
phytes is in marked contrast to their dorsal position in the Anacrogynae. 
Practically all members of the group are leafy. The Acrogynae are a 
well-defined assemblage, comprising 130 genera and about 5,000 species. 
Notwithstanding its size, the group is fairly uniform in regard to general 
morphological features. Some of the largest genera are Nardia, Plagio- 
chila, Lophocolea, Radula, Porella, Frullania, Cephalozia, Scapania, and 
Lejeunia. 

Gametophyte. The gametophyte of the Acrogynae is a dorsiventral, 
branching, leafy axis. Only a few genera are thalloid and even these 
produce leafy fertile shoots. Herherta and a few other genera have an 
erect stem bearing three rows of similar, radially arranged leaves. The 
other Acrogynae have a prostrate stem bearing two rows of dorsal leaves 
and generally a row of ventral leaves (Fig. 151A, B). The ventral leaves, 
which are reduced, are known as amphigastria. In a number of genera 
amphigastria are not present. The dorsal leaves overlap and are gen- 
erally bilobed, the lobes being unequal in size or, in a few cases (e.g., 
Lophocolea), equal. The leaves nearly always consist of a single plate of 
cells and, with rare exceptions, are without a midrib. The stems are 
composed of essentially uniform tissue. Rhizoids are usually abundant 
on the lower side of the stem. They are chiefly anchoring in function, as 
much water absorption takes place directly through the leaves. 

In practically all the Acrogynae the gametophyte grows by means of a 
tetrahedral apical cell (Fig. 15 IC). This has the form of a triangular 
pyramid, cutting off segments on three sides. The two rows of dorsal 
segments give rise, in part, to the dorsal leaves and the single row of ven- 
tral segments, in part, to the amphigastria. Branching in the Acrogynae 
is varied but most commonly is monopodial, with a main axis and lateral 
branches. Vegetative propagation is well developed. Often branches 
break off and give rise to new plants. One-celled or two-celled gemmae 
are frequently borne on the margins or at the apices of leaves, while 
multicellular discoid gemmae are produced in some forms. 



184 



PLANT MORPHOLOGY 




Fig. 151. Porella bolanderi. A, dorsal view of gametophyte with nearly mature sporo- 
phyte, X30; B, ventral view of same, showing row of reduced ventral leaves and dorsal 
leaves with small ventral lobes; C, horizontal longitudinal section of tip of gametophyte, 
showing apical cell and developing leaves, X400; D, longitudinal section of antheridium, 
X250; E, longitudinal section of nearly mature archegonium, showing egg, ventral canal 
cell, and eight neck canal cells, X400. 

Sex Organs. The antheridia and archegonia may occur on the same 
plant or on separate plants, depending on the species. They are never 
sunken in the tissues of the gametophyte. The antheridia arise in the 
leaf axils, very commonly in groups of two to four; but sometimes they 
are solitary, as in Porella, or in groups of more than four. Often the 
antheridia are accompanied by paraphyses. In Lophozia, Nardia, and 



BRYOPHYTA 185 

other simple forms the antheridia are borne on unmodified shoots, but in 
Porella and most members of the group they occur on special, short, lat- 
eral branches. The antheridia are globular and mostly long-stalked, 
developing as in the Anacrogynae except that the wall usually becomes 
several layers of cells in thickness (Fig. 151Z)). 

The archegonia are generally borne on short lateral branches and are 
always terminal. They occur singly or in a small group. They are com- 
monly intermixed with paraphyses, usually surrounded by a perianth and 




Fig. 152. Longitudinal section of sporophyte of Porella, showing capsule with spore 
mother cells and young slaters, X40. 

often also by an involucre lying outside the perianth. Both of these 
envelopes are formed of united leaves. The archegonia develop as in the 
Anacrogynae. In Porella six to eight neck canal cells are formed (Fig. 

Sporophyte. The sporophyte of the acrogynous Jungermanniales is 
similar to that of the anacrogynous forms in that the seta is considerably 



186 PLANT MORPHOLOGY 

elongated and relatively little of the embryonic tissue becomes sporoge- 
nouH (Fig. 152). The embryogeny is known only in a few species. The 
lower cell arising from the first transverse division of the fertilized egg 
may become an appendage to the foot in some forms but not in others. 
Elaters are always present, but there is no elaterophore, except in Gott- 
schca, which has a basal one. The spore mother cells are conspicuously 
f our-lobed. The capsule wall is usually two layers thick, sometimes more, 
the inner layer generally having spiral thickenings. Dehiscence occurs 
by means of four valves, as in the Anacrogynae. 

Summary. In contrast to the Marchantiales, the Jungermanniales 
have a gametophyte that, while remaining simple in structure, is more or 
less differentiated in form. In the lower members the gametophyte is a 
simple thallus, becoming in the higher members a leafy stem. The apical 
cell is dolabrate in most of the Anacrogynae, tetrahedral in the Acrogynae. 
In the Anacrogynae the sex organs are nearly always borne singly or in 
groups on the dorsal surface of the gametophyte, sometimes on special 
branches, but never on stalked receptacles. In the Acrogynae the anther- 
idia are axillary, the archegonia terminal. The early development of the 
antheridium is characteristic, the formation of two transverse walls in the 
initial cell being followed by a median vertical wall in the terminal seg- 
ment. The neck of the archegonium shows five cells in cross section. 

The sporophyte always consists of a foot, seta, and capsule. There is 
much sterilization of potentially sporogenous tissue, the development of 
the seta being especially marked. The capsule, varying in form from 
spherical to cylindrical, produces both spores and elaters. It nearly 
always dehisces by means of four valves. The capsule wall consists of 
two or more layers of cells. The spore mother cells are deeply f our-lobed. 
In the Jungermanniales a relatively simple gametophyte is combined with 
a complex sporophyte. 

4. Anthocerotales 

The Anthocerotales constitute an isolated order of 4 genera and over 
100 species. They are so distinct that they are often set apart from the 
other liverworts as a distinct class of bryophytes. Anthoceros, with 60 
species, and Notothylas are widely distributed in both temperate and 
tropical regions, while Megaceros and Dendroceros are chiefly confined to 
the tropics. Dendroceros is epiphytic on tree trunks, stems, and leaves, 
while the other members grow mainly on damp earth. 

Gametophyte. The gametophyte of the Anthocerotales is a dorsi- 
ventral plate-like thallus often growing in an irregularly dichotomous 
manner (Fig. 153). It is frequently more or less lobed, but does not have 
any leaves. In Dendroceros the thallus is narrow, consisting of a thick- 
ened midrib and lateral wings composed of a single layer of cells. In the 



BRYOPHYTA 



187 



other genera the thallus is several layers of cells thick and without a mid- 
rib. There is no internal differentiation of tissues. The cells are peculiar 
in having, as a rule, a single large chloroplast with a conspicuous pyrenoid, 
a feature of most green algae but not of any other bryophyte. In some 
members of the group two or more chloroplasts are present. 

The thallus grows by means of a cuneate (wedge-shaped) apical cell, 
as in the Marchantiales. There are no air 
chambers or air pores, but some species of 
Anthoceros and Dendroceros have intercellular 
mucilage cavities that open by clefts to the 
ventral surface, and in these cavities colonies of 
Nostoc may live. Smooth rhizoids are present, 
but there are no ventral scales. In some species 
of Anthoceros vegetative propagation is accom- 
phshed by the isolation of branches, in other 
species {e.g., Anthoceros hallii) by the formation 
of small tubers that rest in the soil until the 
next growing season. 

Sex Organs. In most of the Anthocerotales 
the antheridia and archegonia are borne on the 
same plant but in separate groups, the antheridia 
appearing first. Both kinds of sex organs are 
embedded in the dorsal surface of the thallus and 
develop endogenously. 

The antheridium initial is a superficial cell 
arising close to the growing apex. It does not 
become papillate, as in the other Hepaticae, 
but divides transversely, the inner cell giving 
rise to the antheridium. Between the two 
cells a mucilage-filled cleft appears that later 
becomes the antheridial chamber, the roof of 
which is formed by the derivatives of the outer 
cell. In the inner cell two vertical walls at right angles to each other 
now appear, followed by two transverse walls (Fig. 154A). As a result, 
three tiers are formed with four cells in each tier. The stalk is 
derived from the lowest tier. Periclinal walls in the two upper tiers sepa- 
rate the outer sterile jacket from the inner spermatogenous cells (Fig. 
1MB, C). Further development follows the usual liverwort pattern. 

The mature antheridia are spherical or nearly so, generally long- 
stalked, and often bright orange-yellow. When the sperms are ripe, the 
roof of the antheridial chamber bursts. In Notothylas and most species 
of Anthoceros two or four antheridia develop in the same chamber, all 
coming from the inner segment of the same initial cell. This divides by a 




Fig. 153. Anthoceros fusi- 
formis, with three sporo- 
phytes arising from the 
gametophyte, X3. 



188 



PLANT MORPHOLOGY 



vertical wall, or by two vertical walls at right angles to each other, each 
segment giving rise to an antheridium. Frequently additional antheridia 
are budded off from the base of the others. 

The archegonium initial is a superficial cell that, like the antheridium 




Fig. 154. Sex organs of Anthoceros, X 500. A, B, C, stages in development of the anther- 
idium; D, young archegonium showing transverse division of axial cell; E, formation of 
cover cell, primary neck canal cell, and primary ventral cell; F, division of primary neck 
canal cell and formation of ventral canal cell and egg; G, later stage with four neck canal 
cells; H, mature archegonium with egg ready for fertilization. (A to C, Anthoceros pulcher- 
rimus; D to H, Anthoceros fusiformis.) 



initial, does not become papillate. The usual three vertical walls appear, 
cutting off the wall cells from the primary axial cell. The latter divides 
transversely (Fig. 154/)), while a second transverse division occurs in the 
outer cell, resulting in a row of three cells (Fig. 154£'). These are the 



BRYOPHYTA 



189 



cover cell, primary neck canal cell, and primary ventral cell. The cover 
cell later divides by one or two vertical walls; the primary neck canal cell 
gives rise to four or sometimes six neck canal cells; and the primary ven- 
tral cell produces the ventral canal cell and the egg (Fig. 154F, G). Just 
previous to fertilization the cover cells and canal cells break down, leaving 
the egg in a cavity below the surface of the thallus (Fig. 15-iH). 




Fig. 155. Embryos of Anthocerotales, X300. A, yonng emhryo oi Anthoceros fusiformis; 
B, slightly older enibrjo of Anthoceros piinctatus, showing differentiation of amphithecium 
and endothecium; C, embryo of Megaceros, showing two sporogenous cells cut off from 
amphithecium; D, later stage, showing further development of sporogenous tissue. 

Sporophyte. In Anthoceros the first division of the fertilized egg is 
vertical, the second transverse, and the third vertical at right angles to 
the plane of the first division (Fig. 155A). A fourth division occurs 
transversely in the upper part of the embryo, resulting in three tiers of 
four cells each. The upper tier produces the capsular region, the middle 
tier an intermediate zone, while the lower tier forms the foot. In the 
development of the capsular region, which occurs very early, the amphi- 
thecium is cut off from the endothecium by periclinal walls (Fig. 1555). 
The latter forms the columella, an axis of sterile tissue. The amphithe- 
cium soon becomes two-layered, the inner layer giving rise to the spo- 
rogenous tissue and the outer layer to the sterile wall (Fig. 155C, D). The 
derivation of the sporogenous tissue from the amphithecium rather than 
from the endothecium is very characteristic and stands in marked contrast 
to the condition in the other Hepaticae. In some species of Notothijlas, 
however, the endothecium does not produce a sterile columella but, 
instead, gives rise to the sporogenous tissue. 



190 PLANT MORPHOLOGY 

The foot becomes bulbous and in many species penetrates the thallus 
by means of rhizoid-like papillae (Fig. 15G). The intermediate zone is 
meristematic. It contributes somewhat to the development of the foot, 
but is chiefly concerned with the elongation of the capsular region. There 
is no seta. Notothylas has a short capsular region, the other genera a long 
one. The young sporophyte is protected by the surrounding tissue of the 
thallus, which grows upward with it to form a massive involucre. This 
is later ruptiu'ed by the elongation of the sporophyte, forming a basal 
sheath. 

The columella consists of elongated cells. It may be regarded as repre- 
senting the beginning of a conducting system. In Anthoccros the colu- 
mella shows about 16 cells in cross section. In the young sporophyte the 
sporogenous tissue caps the columella in a dome-like manner. It soon 
becomes two-layered above and then gives rise to spore mother cells. 
Meanwhile new sporogenous tissue continues to be differentiated in the 
meristematic region lying just above the foot (Fig. 1565). Although, in 
Anthoccros, the sporogenous tissue generally becomes two-layered, it may 
remain one-layered, as in Anthoccros hawaiiensis, or may become three or 
four layers thick, as in Anthoccros hallii and Mcgaceros. In Notothylas 
the amount of sporogenous tissue is greatly increased; in some species a 
definite columella is not formed and the sporogenous tissue arises from 
both the amphithecium and endothecium, or from the endothecium alone. 

The wall of the capsule becomes four to eight layers of cells thick. In 
Anthoccros, but not in the other genera, the outer layer, constituting the 
epidermis, develops stomata. These are not like the air pores seen in the 
gametophyte of the Marchantiales, but resemble the stomata of the higher 
plants. The wall layers beneath the epidermis develop chloroplasts and 
intercellular spaces, thus becoming a photosynthetic region. 

The intermediate zone elongates constantly, adding to the capsular 
region from below. Thus spores continue to be produced over a long 
period. It is noteworthy that the Anthocerotales are the only bryophytes 
whose sporophyte displays indeterminate growth. Some of the sporog- 
enous cells become sterile, small groups of these alternating with groups 
of spores and so tending to break up the spore mass into separate units. 
As a rule, these sterile cells give rise to peculiar short elaters that are often 
branched. In Anthoccros the elaters, where present, are smooth- walled ; 
in Notothylas they have short, curved, thickened bands on their walls; in 
Mcgaceros and Dcndroccros the elaters have spiral thickenings like those of 
other liverworts. The capsule dehisces by splitting into two valves. 

Summary. The Anthocerotales are of phylogenetic interest in that 
they may represent a stage of progress through which the higher plants 
have passed in the course of their evokition. As in the Jungermanniales, 
the gametophyte is simple and the sporophyte complex, but the complex- 



BRYOPHYTA 



191 




m 



H 





Hi 





Fig. 156. Longitudinal sections of the sporophyte of Anthoceros laevis. A, entire sporo- 
phyte with foot embedded in the gametophyte, X20; B, basal region, showing origin of 
sporogenous tissue, X 100; C, higher level, showing spore mother cells, X 100; D, level where 
spore tetrads are forming, X 100. 



192 PLANT MORPHOLOGY 

ity is of an entirely different kind. The gametophyte is a flat thallus 
without structural differentiation. It grows by means of a cuneate apical 
cell. The sex organs are endogenous. The antheridium is formed from 
the inner half of the initial cell and the sequence of early wall formation 
is distinctive. The archegonium represents a new departure in that the 
primary neck canal cell is cut off from the outer segment arising from a 
transverse division of the initial. 

The sporophyte consists of a foot and a cylindrical capsule. It dis- 
plays a great development of sterile tissue. The presence of green tissue, 
stomata, and rhizoid-like processes suggests that the sporophyte is becom- 
ing independent. The breaking up of the sporogenous tissue into smaller 
units may represent an initial stage in the formation of sporangia. The 
establishment of a sterile axis by the transfer of sporogenous tissue from 
the endothecium to the amphithecium may represent the beginning of a 
conducting system. A meristematic region in the sporophyte results in 
its continued growth. Dehiscence of the capsule is accomplished by 
means of two valves. 

2. MUSCI 

The mosses constitute the larger and more highly developed class of 
bryophytes, numbering about 14,000 species. They are widely distrib- 
uted and, although abundant in arctic and alpine regions, are represented 
in nearly all habitats except the ocean. The fossil history of the group is 
very fragmentary, there being few reliable evidences of its existence 
earlier than the Tertiary. The gametophyte is leafy and, in contrast to 
that of the liverworts, is typically radial rather than dorsiventral. The 
leaves generally have a midrib. The rhizoids are septate and usually 
branched. The Musci comprise three orders, the Sphagnales, Andreaea- 
les, and Bryales. 

1. Sphagnales 

The Sphagnales, or bog mosses, are a group of about 350 species, all 
belonging to the genus Sphagnum. They are relatively large, pale mosses 
generally living in bogs at high altitudes and high latitudes. Because 
their accumulated remains form peat, they are often called peat mosses. 

Gametophyte. Upon germination, the spore produces a short filament 
that, in turn, gives rise to a flat green thallus (Fig. 157 A). This consists 
of a single layer of cells bearing numerous septate rhizoids. The thallus 
gives rise to an erect leafy branch and then disappears (Fig. 1575). The 
erect shoot develops rhizoids below and becomes the mature gametophyte. 
The rhizoids soon die but the shoot continues to grow from year to year 
(Fig. 158A). Branching in Sphagnum is very profuse, there being 
branches of limited growth crowded near the apex of the main stem and 
others occurring in tufts farther down. The stem grows by means of a 



BRYOPHYTA 



193 



tetrahedral apical cell, and from each of its three rows of segments a row 
of leaves arises, these being spirally arranged. 

The leaves are composed of a single layer of cells and lack a midrib. 
They are peculiar in structure, some cells being enlarged, rhomboidal, and 
hyaline, while others are small, narrow, and green (Fig. 158£'). The 
green cells form the meshes of a network enclosing the hyaline cells. The 
latter are dead cells filled with water; their walls bear large circular pores 





B 



Fig. 157. Sphagnum. A, young gametophyte, showing filament arising from the spore, 
a rhizoid, and the thallus beginning to develop by an apical cell; B, mature thallus, with 
rhizoids, producing a leafy shoot; C, an antheridium arising between two leaves, X250. 
(A arid B, after Schimper.) 

and usually spiral thickenings as well. The leaves have an extraordinary 
power of absorbing and retaining water. At first the leaf cells are 
uniform, but later from each a narrow cell is cut off on two sides, as 
represented by Fig. 1585-Z). 

The stem of Sphagnum is differentiated into three regions: (1) a cortex 
of dead hyaline cells that absorb and store water; (2) a cylinder of small 
elongated cells with thick walls; and (3) a pith-like axis. Vegetative 
propagation occurs by branching and death of the older parts of the plant. 
This is the principal method of reproduction. 

Sex Organs. Depending on the species, Sphagnum is either monoe- 
cious or dioecious. The antheridia appear on special, short, lateral 
branches that arise near the apex of the main shoot. They are solitary in 
the leaf axils, unaccompanied by paraphyses, and arise in acropetal sue- 



194 



PLANT MORPHOLOGY 



cession. The initial is a superficial cell that undergoes several transverse 
divisions, resulting in a short filament. Then the terminal cell functions 
as a dolabrate apical cell (one with two cutting faces), the lower segments 
forming the stalk and the upper ones the rest of the antheridium. The 
spermatogenous tissue is differentiated from the jacket cells by the forma- 
tion of periclinal walls. The mature antheridia are long-stalked and 




C ' D E ^mmm^ F 

Fig. 158. Sphagnum. A, leafy stem with terminal cluster of sporophytes, natural size; 
B, surface view of portion of very young leaf, X260; C, diagram showing how the leaf 
cells divide, cutting off cells marked 1, and then cells marked 2; D, appearance of leaf after 
these cells have been cut off; E, surface view of portion of mature leaf, showing the narrow 
elongated cells with chloroplasts and the larger hyaline cells with pores and slender bands 
of thickening, X300; F, longitudinal section of nearly mature sporophyte, showing the 
capsule, neck-like seta, and the foot, X24. {After Chamberlain.) 

nearly spherical, opening irregularly to discharge their sperms (Fig. 
157C). 

The archegonia appear at the apex of short branches that, like the 
antheridial branches, arise at the upper end of the main shoot. They are 
borne in groups of one to five, without paraphyses, and are stalked and 
free. An archegonium arises directly from the apical cell, as in the 
acrogynous Jungermanniales, and then several others may arise from the 
last-formed segments of the apical cell. After a short filament has been 
produced by the formation of walls that may be either transverse or 
obliciue, the usual three vertical walls appear in the terminal cell, cutting 



BRYOPHYTA 



195 



off three primary wall cells from the primary axial cell (Fig. 159A). The 
axial cell, by a transverse division, gives rise to the cover cell and central 
cell (Fig. 159B). The development of the axial row is similar to that of 
the Jiingermanniales. The central cell divides to form the primary neck 
canal cell and primary ventral cell (Fig. 159C). The neck canal cells, 




B 






F 

Fig. 159. Development of the archegonium of Sphagnum subsecundum. A, formation of 
primary axial cell; B, fornnation of cover cell and central cell; C, formation of primary neck 
canal cell and primary ventral cell, the cover cell divided vertically; D, archegonium with 
primary ventral cell and two neck canal cells; E, later stage with four neck canal cells; F, 
archegonium with egg, ventral canal cell, and nine neck canal cells; A to E, X525; F, 
X300. {After Bryan.) 

numbering eight or nine, all arise directly from the primary neck canal 
cell, while the ventral canal cell and egg, approximately equal in size, are 
produced by a transverse division of the primary ventral cell (Fig. 
159D-F). The mature archegonium has a long stalk, a massive venter, 
and a long twisted neck. 

Sporophyte. The fertilized egg of Sphagnum undergoes a series of 
transverse divisions that result in the formation of a short filament of six 
or seven cells (Fig. 160). Vertical walls then appear and the embryo 
becomes cylindrical. It next becomes differentiated into an upper fertile 



196 



PLANT MORPHOLOGY 



region (the capsule), a middle region (the neck), and a basal portion (the 
foot). As in Anthoceros, a columella is formed from the endothecium, the 
sporogenous tissue being cut off from the amphithecium and capping the 
columella like a dome. The sporogenous tissue becomes two to four 
layers of cells in thickness, while the outer portion of the amphithecium 
forms the capsule wall, eventually composed of five to seven layers of 
cells. This becomes a region of green tissue with intercellular spaces, the 
outer layer developing rudimentary stomata. The sporogenous tissue is 
surrounded by a definite nutritive layer, the tapetum. As in the other 








n 



ABC D 

Fig. 160. Early stages in the development of the embryo of Sphagnum sub secundum, 
X200. A, two-celled stage; B, four-celled stage; C, seven-celled stage; D, appearance of 
vertical walls. {After Bryan.) 

Musci, no elaters are formed, all the sporogenous cells becoming spore 
mother cells. 

The foot becomes large and bulbous, but the seta does not develop 
beyond the neck-Hke stage (Fig. 158F). The seta is replaced functionally 
by the pseudopodium, a leafless stalk developed from the stem of the 
gametophyte, in the tip of which the foot of the sporophyte is embedded. 
The pseudopodium elongates rapidly after the spores have ripened, carry- 
ing the capsule upward. When mature, the capsule is globular and dark 
brown or black. It dehisces by means of a lid or operculum, as in the 
Bryales. The spores are discharged wdth force. As in the Hepaticae, the 
sporophyte remains enclosed by the calyptra until the spores are ripe. 

Summary. The Sphagnales are a synthetic group, combining char- 
acters found both in liverworts and mosses. A thallus like that of the 
anacrogynous Jungermanniales gives rise to an erect leafy shoot that 
becomes the mature gametophyte. This shows some internal differentia- 
tion, both in the leaves and stem. The antheridia are spherical and axil- 



BRYOPHYTA 197 

lary, as in the acrogynous Jungermanniales, but in development resemble 
the Bryales. In position, origin, and development the archegonia show a 
resemblance to those of the acrogynous Jungermanniales, except that, 
when mature, the venter is massive, as in the Bryales. The general 
organization of the sporophyte is like that of the Anthocerotales, the 
sporogenous tissue being dome-shaped and derived from the amphithe- 
cium; but there is no meristematic region. The seta is replaced func- 
tionally by a pseudopodium. The capsule contains green tissue and 
rudimentary stomata. It dehisces by an operculum, as in the Bryales. 

2. Andreaeales 

This order comprises a single genus, Andreaea, of about 125 species. 
They are small, tufted, dark-colored mosses growing on rocks in dry situa- 
tions, especially in cold regions. In warmer regions they are restricted to 
high mountains. 

Gametophyte. In the germination of the spore, its protoplast pro- 
duces inside the spore wall a mass of cells called the primary tubercle. 
After rupturing the wall, one or more of its superficial cells give rise to 
branching filaments. These correspond to the protonema of the Bryales. 
Some of the filaments turn brown and function as rhizoids, while others 
may give rise either to flat thalh or cylindrical masses. A leafy shoot is 
then organized (Fig. 161). It may arise from the flat thallus, from 
the cylinder, or directly from the protonema. The stem, which is pros- 
trate, exhibits sympodial branching (like dichotomy, but with unequal 
branches). It produces many rhizoids. The stem is without a central 
strand, consisting of uniform, thick-walled cells. It grows by means of a 
tetrahedral apical cell. The leaves, formed in three rows, are generally 
without a midrib, being composed usually of thick-walled cells. 

Sex Organs. Andreaea is generally monoecious, the antheridia and 
archegonia occurring in terminal groups on separate branches. The 
apical cell is involved in the formation of the sex organs. In develop- 
ment, the antheridium corresponds very closely to that of Sphagnum and 
similarly, when mature, is long-stalked and nearly globular. The arche- 
gonia develop as in the Bryales, the cover cell contributing to the row 
of neck canal cells. 

Sporophyte. The first division of the fertilized egg is transverse, the 
inner segment forming the foot and the outer segment the rest of the 
sporophyte. A dolabrate apical cell is organized in the outer segment and 
about a dozen cells are formed before vertical walls come in. The spo- 
rogenous tissue is cut off from the endothecium as the outermost layer of 
cells and caps the columella as a dome. It eventually becomes two- 
layered. As in Sphagnmn, a pseudopodium is formed, the seta remaining 
undeveloped. The calyptra encloses the sporophyte until it is nearly 



198 



PLANT MORPHOLOGY 



mature. The capsule is without an operculum, dehiscence taking place 
by means of four longitudinal slits, but these usually do not extend to the 
apex of the capsule (Fig. 161). 

Summary. Like the Sphagnales, the Andreaeales are a synthetic 
group. The gametophyte begins either as a thallus, as in the Sphag- 




FiG. 161. Gametophyte of Andreaea petrophila with mature and ininiature sporophytes, 
X3. {F rom Gilbert M. Smith.) 

nales, as a filamentous protonema, as in the Bryales, or as a cylindrical 
body. The antheridia are terminal, as in the Bryales, but are long- 
stalked and globular, as in the Sphagnales. The archegonia develop like 
those of the Bryales. The sporogenous tissue is derived from the endo- 
thecium, a feature of the Bryales, but caps the columella in a dome-like 
manner, a feature of Sphagnum. Similarly, as in Sphagnum., a pseudo- 
podium is developed. The capsule dehisces by means of four valves, as 
in the Jungermanniales. 

3. Bryales 

The Bryales, or true mosses, are the culminating order of bryophytes. 
They constitute a highly specialized, as well as a very distinct group, dis- 



BRYOPHYTA I99 

playing remarkable uniformity with respect to basic morphological fea- 
tures. The Bryales are by far the largest group of bryophytes, number- 
ing about 13,500 species included in 80 families. Although world-wide 
in distribution, they are particularly abundant in moist northern regions. 
They grow on rocks, tree trunks, fallen logs, and on the ground, often 
forming extensive mats. Some grow in dry situations, while a few are 
aquatic. Some of the largest genera are Fissidens, Lencobryum, Barbula, 
Tortula, Grimmia, Funaria, Bryum, Mnium, Bartramia, Hypnum, Polyt- 
richum, and Pogonatum. 

Gametophyte. In nearly all the Bryales the spore produces a proto- 
nema~a green, branching, septate filament (Fig. 162). Some of the 




Fig. 162. Moss protonema arising from a spore and bearing a bud from wliich an erect 
leafy shoot will arise, X 100. 

branches penetrate the soil, turn brown, and become rhizoids. The pro- 
tonema, which is the morphological equivalent of the thallus of Sphag- 
num, gives rise to an erect leafy stem, the gametophore. This arises as a 
bud on the protonema and, in most genera, grows by means of a tetrahe- 
dral apical cell. After formation of the leafy shoot or, more commonly, 
of several or many leafy shoots, the protonema usually disappears; but it 
may persist, turn browai, and contribute to the mass of rhizoids that arise 
from the lower end of the stem. Branching of the stem, where it occurs, 
is nearly always monopodial (with a true main axis). Generally the 
leaves are spirally arranged and borne in three vertical rows (Fig. 163). 
Usually they consist of a single layer of cells, except for a slightly thick- 
ened midrib, which is nearly always present. The stems of such mosses 
as Mnium and Polytrichum contain a strand of elongated thick-walled 
cells, but the stem tissue of most mosses is uniformly thin-w^alled. 

Vegetative propagation in the Bryales is highly developed and varied. 
It may occur by isolation of branches following death of the older parts 
of the plant, by small multicellular gemmae, by resting buds (bulbils) on 
the protonema, or by the formation of a protonema from almost any part 
of the plant. 



200 



PLANT MORPHOLOGY 



Sex Organs. In many mosses sex organs are rarely produced, repro- 
duction taking place chiefly by vegetative means. The sex organs occur 
in terminal groups (Fig. 163). In the "acrocarpous" forms they are 
borne at the apex of the main stem or its principal branches; in the 
"pleurocarpous" forms they occur at the apices of short lateral branches. 
The sex organs are usually surrounded by a sheath or rosette of modified 
leaves forming the perichaetium. According to the species, mosses may be 




Fig. 163. Leafy shoots of Funaria hygrometrica, X 3. A, male plant with terminal cluster 
of antheridia; B, female plant with an archegonium in which an embryo sporophyte has 
started to develop; C, older stage, the sporophyte elongating and carrying the calyptra 
upward. 



either monoecious or dioecious. If monoecious, the antheridia and arche- 
gonia usually occur in the same cluster. Multicellular paraphyses are 
commonly present. Both kinds of sex organs arise from segments of the 
apical cell, in many cases from the apical cell itself. The formation of 
sex organs limits growth of the vegetative axis. 

The antheridium arises from a superficial cell that becomes papillate. 
Several transverse divisions may take place and then the terminal cell 
becomes a dolabrate apical cell (one with two cutting faces), cutting off 
a series of segments (Fig. 164A). Periclinal walls in the younger seg- 
ments delimit the jacket cells from the primary spermatogenous cells 
(Fig. 1645, C). As additional divisions occur, the antheridium becomes 
club-shaped, with a stalk of variable length (Fig. 164D). A large num- 
ber of sperm mother cells are formed, each giving rise to two sperms. 



BRYOPHYTA 



201 



When mature, the antheridium ruptures at the apex, the sperms being 
discharged in a mass. 

The archegonium also develops by means of a dolabrate apical cell, but 
only a comparatively few segments are cut off (Fig. 165.4). In the ter- 
minal cell then appear the three characteristic walls that differentiate the 
primary wall cells from the primary axial cell (Fig. IGoB). A transverse 






-a) 



L^ 



^©' 



Fig. 164. Development of the antheridium of Mnium affine, X400. A, young stage; B, 
beginning of differentiation of primary spermatogenous cells; C, slightly older stage; D, 
antheridium showing subdivision of spermatogenous tissue. 

division of the axial cell results in the formation of the cover cell and cen- 
tral cell, the latter soon giving rise to the primary neck canal cell and 
primary ventral cell (Fig. 165C, D). Later development is characteristic 
in that the cover cell cuts off lateral segments that add to the neck cells 
and inner segments that contribute to the neck canal cells. Thus the 
upper neck canal cells are derived from the cover cell, the lower ones from 
the primary neck canal cell, while the ventral canal cell and egg, as 
usual, arise from the primary ventral cell (Fig. \65E-H). The mature 
archegonium has a long stalk, a massive venter, and many neck canal 
cells — sometimes up to 50 or 60. 

Sporophyte. In the Bryales the sporophyte reaches a high degree of 
specialization. During its early development a large calyptra is formed 



202 



PLANT MORPHOLOGY 



from the stalk and venter of the archegonium. This is soon ruptured and 
carried upward on the top of the sporophyte as a conspicuous hood, which 
may remain until the sporophyte is mature (Fig. 163C). 

The first division of the fertilized egg is transverse or oblique. Each 






B 







Fig. 165. Development of the archegonium of Milium affine, X400. A, young stage; B, 
formation of primary axial cell; C, formation of cover cell and central cell, the former 
divided by a radial wall; D, formation of primary neck canal cell and primary ventral cell; 
E, archegonium with primary ventral cell and three neck canal cells; F, later stage with 
seven neck canal cells; G, archegonium with egg, ventral canal cell, and eight neck canal 
cells; H, later stage. 

segment becomes an apical cell through the activity of which a slender 
elongated embryo is developed (Fig. 166). Apical growth continues for a 
long time. Finally, at the upper end of the embryo, after the appearance 
of two sets of four vertical walls, periclinal divisions cut off the amphithe- 
cium from the endothecium. The amphithecium gives rise to several 



BRYOPHYTA 



203 



layers of cells. The sporogenous tissue is cut off from the endothecium as 
the outermost layer of cells, the remainder form- 
ing a sterile columella. The sporogenous tissue is 
not continuous over the columella as a dome, but 
has the form of a hollow cylinder. It may extend 
to the base of the capsule, as in Polyirichuni, or may 
not. The sporogenous tissue usually becomes two- 
layered, all its cells giving rise to spore mother cells. 

As seen in longitudinal section, the mature cap- 
sule is very complex, being made up of an epider- 
mis with stomata, several wall layers of colorless 
cells, an air-chamber region consisting of green 
tissue, an outer tapetum, the sporogenous tissue, 
an inner tapetum, an inner air-chamber region 
(present only in highly specialized forms, such as 
Polyt7'ichum.) , and a. central columella (Fig. 167). 

The operculum, which forms the upper part of 
the capsule, is complex in its development (Fig. 168). 




Fig. 166. Embryo of 
Funaria hygrometrica 
enclosed by the calyp- 
tra, X500. {After 
Sachs.) 



It is often dif- 




-- Peristome 
— Operculum 

— Columella 

Spore-bearing 
Tissue 



"WaW 

Air Chamber 



Fig. 167. Longitudinal section of the capsule of Funaria hygrometrica. (From Sinnott.) 



204 PLANT MORPHOLOGY 

ferentiatctl into an upper annulus, consisting of several series of large 
thin-walled cells, and a lower rim. When the capsule is mature, the 
annulus collapses and the operculum comes off, exposing the peristome. 
This consists of one or two rings of tooth-like projections that are 
anchored to the rim. The usual number of teeth is 16, but they may 
occur in some other multiple of 4. The teeth, which are hygroscopic, 
assist in spore dispersal, bending inward and outward. In a few genera. 




Fig. 168. Longitudinal section of the upper part of the capsule of Mnium, showing the 
operculum and two teeth of the peristome, X 90. (After Chamberlain.) 

said to be "cleistocarpous," an operculum and a peristome are lacking, 
the capsule wall rupturing irregularly in dehiscence. In the other genera, 
which are "stegocarpous," an operculum and a peristome are present. 

Often the lower portion of the capsule does not produce spores, but 
forms a chlorophyll-bearing region, called the apophysis, in which stomata 
are present (Fig. 167). In many mosses the apophysis is ring-like. The 
seta of mosses is nearly always very long. It has a central strand of 
elongated cells, but these are not conductive in function. The foot is 
simple. 

Summary. The Bryales are the most highly developed group of bryo- 
phytes. The gametophyte consists of a protonema giving rise to a leafy 
shoot, the latter being differentiated in form and somewhat in structure. 
The antheridia are terminal and club-shaped, developing by means of a 



BRYOPHYTA 



205 



dolabrate apical cell, as in all the Musci. The archegonia have a long 
stalk, a massive venter, and a long twisted neck. The number of neck 
canal cells is greater than in any other group of plants. The sporophyte 
displays the greatest amount of sterilization of potentially sporogenous 
tissue seen in the Bryophyta. The sporogenous tissue arises from the 
outer part of the endothecium and has the form of a hollow cylinder. 
The seta is elongated, no pseudopodium being formed. The capsule 
shows an extraordinary degree of specialization, both in the organization 
of an operculum, peristome, and apophysis, and in its internal differentia- 
tion of tissues. It contains both green tissue and stomata. 

COMPARISON OF LIVERWORTS AND MOSSES 

The most important distinguishing characters of the Hepaticae (liver- 
worts) and Musci (mosses) are as follows: 



Hepaticae 
Gametophyte almost invariably dorsiven- 

tral; thalloid or leafy 
Leaves, where present, without a midrib 

Rhizoids unicellular and mostly 
unbranched 

Protonema, where present, small and tran- 
sitory 

Sporophyte remaining enclosed by the 
calyptra, or breaking through only 
when spores are ripe 

Elaters usually present 



Musci 
Gametophyte typically radial ; leafy 

Leaves generally with a midrib (except in 

Sphagnum and Andreaea) 
Rhizoids septate, mostly branched 

Protonema usually conspicuous, rela- 
tively persistent 

Sporophyte breaking through the calyp- 
tra at an early stage of development 
(except in Sphagnum) 

Elaters absent 



GENERAL CONCLUSIONS 

The bryophytes, undoubtedly derived from aquatic ancestors, are the 
simplest group of green plants growing on land. Their position in the 
plant kingdom is an expression of the degree of evolutionary progress they 
have made, but is not necessarily an indication of their phylogenetic rela- 
tionship to the higher plants. One theory holds that the bryophytes are 
an ancient stock from which the pteridophytes have been derived. 
Another theory contends that the pteridophytes have originated directly 
and independently from the algae and that, therefore, the bryophytes 
represent a blindly ending line of descent. But even if the latter view 
should prove to be the correct one, the first land plants may have passed 
through a general stage of development similar to that reached by the 
bryophytes of the present. 

The bryophytes differ from the algae in the possession of archegonia, 
multicellular antheridia, and an established heteromorphic alternation of 
generations. Because the liverworts are simpler than the mosses, it 
seems very probable that they are more primitive and thus more ancient 



20G PLANT MORPHOLOGY 

than the mosses. Both groups may have had a common origin, but it is 
more Hkely that the mosses were derived directly from the liverworts. 
Each has subseciuently pursued its own course of evolution, the mosses 
having advanced considerably beyond the liverworts. 

The Land Habit. The establishment of the land habit was one of the 
most important events in the history of plant life, for it made possible all 
subsequent progress. Its advantage lies in the greater opportunity for 
photosynthesis in the presence of more light. Its disadvantage lies in the 
danger of excessive transpiration. The first land plants may have arisen 
from some green alga consisting of a simple plate of cells, perhaps from a 
form somewhat like Coleochaete. Adjustment to the land environment 
must have involved structural changes facilitating the absorption of water 
from the soil and the retention of water by parts exposed to the air. This 
adjustment on the part of bryophytes is manifested chiefly by the devel- 
opment of compact bodies, absorptive filaments (rhizoids), jacketed sex 
organs, heavy-walled aerial spores, and, in some cases, by an epidermis 
with air pores, in others, by primitive conducting cells. 

Because the simplest liverworts are thalloid, it is reasonable to suppose 
that they have given rise to the leafy forms. An alternative view is that 
the leafy body is primitive, the thalloid type having been derived from it 
by reduction. Since an erect leafy body permits the exposure of a greater 
photosynthetic surface to the light than is afforded by a flat thallus, it 
would seem to represent a more advanced state of adaptation to the land 
habit. 

The Gametophyte. The bryophytes are a group in which the gameto- 
phyte is the dominant generation. It is always a green, independent 
plant body. In its simplest form it is a flat thallus, one to several layers 
of cells thick, and without any internal differentiation of tissues. Such a 
gametophyte is seen in Sphaerocarpus, P cilia, Notothylas, and a number of 
other Hepaticae. It may be regarded as having given rise to two diver- 
gent lines of descent: (1) a line in which the gametophyte, remaining 
thalloid, has undergone differentiation in structure; (2) a line in which the 
gametophyte has remained simple structurally but has become differen- 
tiated in form, finally becoming a leafy shoot. The first line of evolution 
has been followed by the Marchantiales, the second by the Jungerman- 
niales. In the Musci the gametophyte reaches its highest development, 
the erect leafy shoot of the higher mosses, with its radial symmetry, show- 
ing differentiation in both form and structure. 

The Sporophyte. The simplest sporophyte among the bryophytes is 
that of Riccia, where, except for the single layer of wall cells, all the cells 
are sporogenous. If this sporophyte be regarded as primitive rather than 
reduced, all subsequent progress has resulted from sterilization of poten- 
tially sporogenous tissue and its diversion to other functions. This is 



BRYOPHYTA 207 

seen in the development of a foot and seta, as well as in the formation of 
elaters. The foot absorbs food from the gametophyte. The seta places 
the capsule in a favorable position with reference to spore dispersal. 
Except in Riccia, the Sphaerocarpales, and a few other forms, a seta is 
found in all bryophytes. Its absence in the Anthocerotales is correlated 
with the indeterminate growth of the elongated capsule and in the Sphag- 
nales and Andreaeales with the presence of a pseudopodium, the func- 
tional equivalent of a seta. The development of a definite means of 
dehiscence, seen in nearly all bryophytes except Riccia and the Sphaero- 
carpales, represents an advanced feature. Elaters are present in almost 
all liverworts but not in any of the mosses, where spore dispersal is aided 
by other means, as by a peristome. 

Further progress of the sporophyte has come about through additional 
sterilization and earlier diversion of potentially sporogenous tissue. Thus 
an elaterophore is developed in some of the Jungermanniales. The for- 
mation of a columella in the center of the capsule and of additional sterile 
tissue in its outer portion is an advanced feature of the Anthocerotales 
and Musci. The failure of a columella to develop in certain members of 
each group indicates that the entire central region of the capsule was 
originally sporogenous. The sporophyte reaches its highest development 
in the Bryales, where the most extensive amount of sterilization seen in 
all bryophytes has resulted in a greatly elongated seta and a capsule of 
extreme complexity. 

The bryophytes show two well-marked lines of evolution with respect 
to the sporophyte. The one, emphasizing spore dispersal, ends blindly 
with the mosses. The other, in which the sporophyte attains par- 
tial independence and exhibits indeterminate growth, culminates in 
Anthoceros. 

Plan of the Mosses. In the mosses photosynthesis and fertilization 
are functions of the gametophyte, the sporophyte being concerned mainly 
with the production and dispersal of spores. An erect leafy gametophyte, 
best developed in this group, favors photosynthesis but at the same time 
hinders fertilization, since the sex organs are carried upward where it is 
difficult for swimming sperms to function. The moss sporophyte is 
highly specialized for spore dispersal, but without fertilization there can 
be no sporophyte. It is evident, therefore, that combining photosyn- 
thesis with fertilization as functions of the gametophyte is an unprogres- 
sive tendency, because these two functions have different rerjuirements. 
It follows that the mosses must be regarded as a blindly ending evolution- 
ary line. 

Plan of Anthoceros. In the Anthocerotales the sex organs are borne 
on a flat thallus, and so fertilization is easily accomplished. Although 
still largely dependent upon the gametophj^te, the sporophyte develops 



208 PLANT MORPHOLOGY 



much green tissue and so exhibits a marked tendency toward independ- 
ence The combining of photosynthesis with spore dispersal as functions 
of the sporophyte permits of further progress, since both, favored by a tall 
body have the same requirements. The plan of Anthoceros is the one car- 
ried forward by the higher plants, where the sporophyte is dependent 
upon the gametophyte only during early life. Thus A7ithoceros displays a 
strong tendency that, if present in the ancestors of the ptendophytes, may 
have led to their origin. 



CHAPTER VI 
PTERIDOPHYTA 

The pteridophytes are a comparatively small group of plants today, 
but in past geologic times they were much more numerous. They are 
represented by over 9,000 living species, very unequally divided among 
the four classes Psilophytinae, Lycopodiinae, Equisetinae, and Filicinae. 
Most pteridophytes are terrestrial plants, but some are epiphytic and a 
few are secondarily aquatic. They grow in a wide variety of habitats. 

Pteridophytes may be characterized as vascular plants without seeds. 
Like the bryophytes, they display a distinct alternation of generations. 
Their great advance lies in the development of an independent sporophyte 
with complex roots, stems, and leaves, and one in which a prominent 
vascular system is present. The sporophyte has now become the dom- 
inant generation, the gametophyte always being small and inconspicuous. 

The sporophyte presents a great range in size and habit, although one 
not so extreme as in the spermatophytes. Nearly all existing pterido- 
phytes are herbaceous or somewhat woody, the tree ferns being a notable 
exception. Branching of the stem, where present, is dichotomous in 
some members, monopodial in others. Elongation of the root and stem 
generally occurs through the activity of an apical cell; in some forms this 
is replaced by a meristem. The spores are produced in sporangia, which 
are usually borne in connection with the leaves. Most living pterido- 
phytes are homosporous, all the spores being alike. Some are heterospo- 
rous, with spores of two different kinds, these always being produced in 
separate sporangia. As in all bryophytes and spermatophytes, the reduc- 
tion in chromosome number occurs in connection with the formation of 
spores. Consequently the sporophyte is the diploid generation, while the 
gametophyte, produced by a spore, is the haploid generation. 

In the homosporous pteridophytes the gametophyte is either a simple 
green thallus or a tuberous body that is subterranean, colorless, and 
saprophytic. In the heterosporous forms the gametophytes are sexually 
differentiated and greatly reduced in structure, developing entirely or 
largely within the spore wall. The sex organs of pteridophytes are essen- 
tially similar to those of bryophytes, but are simpler. Generally both the 
antheridia and archegonia are embedded structures. Swimming sperms 
are universally present. Following fertilization, the embryo develops 
within the venter of the archegonium, which forms the calyptra. The 

209 



210 PLANT MORPHOLOGY 

young sporoiiliytc lives on the gametophyte until able to maintain itself 
as an independent plant. 

THE VASCULAR SYSTEM 

The vascular system of pteridophytes and spermatophytes is made up 
mainly of two kinds of complex tissues: xijlem and 'phloem. Each of these 
consists of several different kinds of elements. Xylem conducts water 
and dissolved substances absorbed from the soil, while phloem carries food 
away from the leaves and other organs where it is synthesized. The unit 
of tiie xylem is the tracheid—a. slender, elongated, thick-walled cell gen- 
erally pointed at each end and without living contents when mature. 
The most important element of the phloem is the sieve tube — an elongated, 
thin-walled, living cell whose end walls, and often side walls as well, have 
many fine pores (Fig. 314A, B). 

The cell walls of tracheids are thickened with lignin, which is deposited 
on the inner surface to form a spiral, rings, parallel bars, or an irregular 
network, in accordance with which spiral, annular, scalariform, and retic- 
ulate types are recognized. Most commonly the lignin is so abundant 
that the walls are pitted, the pits being unthickened areas. Vessels 
resemble tracheids except that each represents a longitudinal row of cells 
whose end walls break down. Vessels are of rare occurrence in pterido- 
phytes but are the chief xylem elements of angiosperms (Fig. 314C--E'). 

In addition to tracheids and/or vessels, xylem may contain paren- 
chyma. Phloem may contain parenchyma in addition to sieve tubes. 
Fibers, which are elongated, thick-walled, nonconducting cells, may also 
form part of the xylem or phloem (Fig. 314F). 

Development of Xylem. A short distance behind the apex of the root 
and shoot, which is composed of embryonic tissue, the first xylem is differ- 
entiated. This is known as protoxijlem. The next xylem to lignify is 
called metaxylem. The position of the metaxylem with reference to the 
protoxylem is of considerable importance. There are three conditions, 
as follows: 

(1) If the lignification begins at the outside (periphery) of the root or 
stem and proceeds toward the center, in a centripetal direction, the devel- 
opment is exarch. This type is characteristic of all roots and of the stems 
of lycopods (Figs. 176, 188, 220, 227, and 311). (2) If the lignification 
spreads out in all directions, so that the metaxylem surrounds the proto- 
xylem, the development is mesarch. This type is characteristic of the 
stems of ferns (Fig. 239). (3) If the lignification begins near the center of 
the stem and proceeds outward, in a centrifugal direction, the develop- 
ment is endarch. Only a few pteridophytes have reached this condition, 
but it is almost universal in the stems of spermatophytes. 

Protoxylem consists mainly of spiral and annular tracheids, while meta- 



PTERIDOPHYTA 211 

xylem is generally made up of scalariform or of pitted tracheids. Scalari- 
form tracheids, in which the bands of thickening reseml)le the rungs of a 
ladder, are most common in pteridophytes and pitted tracheids in 
spermatophytes. Primary xylem, consisting of all wood differentiated 
directly from embryonic cells of a terminal meristem, includes both proto- 
xylem and metaxylem. Secondary xylem is wood that arises through the 
activity of a cambium. It occurs in only a few living pteridophytes, but 
is characteristic of nearly all spermatophytes except the monocotyledon- 
ous angiosperms. 

Types of Steles. The vascular tissues lie within the stele,^ which in 
roots and stems generally forms a central core. This is surrounded by a 
cylindrical region, the cortex, outside of which lies a layer of cells constitut- 
ing the epidermis. The innermost layer of the cortex is the endodermis. 
Four main types of steles occur in vascular plants and all of them are 
found in pteridophytes. These are as follows: 

(1) The protostele is the simplest and most primitive type. Here the 
xylem forms a solid central strand surrounded by phloem, no pith being 
present (Figs. 176, 188, 200, 235, and 311). (2) The amphiphloic siphono- 
stele has the xylem in the form of a hollow cylinder enclosing pith, with 
phloem both inside and outside the xylem (Figs. 236 and 249). (3) The 
ectophloic siphonostele also has the xylem surrounding pith, but there is no 
internal phloem (Figs. 221, 237, 269, 285, 294, and 315). (4) The dictyo- 
stele is the most advanced type. Here the vascular cylinder is broken up 
into a network of separate strands that, as seen in cross section, may be 
either arranged in a circle or scattered (Figs. 238 and 316). 

The arrangement of xylem and phloem with reference to each other fol- 
lows four general types, three of which occur in pteridophytes. (1) In 
the radial arrangement, which is most primitive, the xylem and phloem 
are in separate strands and occupy alternating radii (Figs. 171, 176, 220, 
227, and 311). The xylem may or may not meet in the center. (2) In 
the amphicribral arrangement the phloem completely surrounds the xylem 
(Figs. 188 and 239). (3) In the collateral arrangement the xylem and 
phloem lie side by side on the same radius, with the phloem external to the 
xylem (Figs. 221, 294, and 315). Where the phloem occurs both outside 
and inside the xylem, the arrangement is bicollateral. (4) In the amphiva- 
sal arrangement the xylem surrounds the phloem. This is an advanced 
type occurring only among the monocotyledonous angiosperms (Fig. 317). 

In practically all vascular plants the root is an exarch radial protostele. 
Stems display a great variety of vascular structure. Those of lycopods 
are much like roots and so are very primitive. The stems of ferns display 
all four stelar types, the amphiphloic siphonostele and the dictyostele 

* Practically all stems have a single stele, and thus are sometimes designated as 
monostelic. Polystelic stems, containing more than one stele, are very rare (see p. 231). 



212 PLANT MORPHOLOGY 

being most common. Most fern stems have mesarch amphicribral bun- 
dles. The characteristic stem of seed plants is an ectophloic sipho- 
nostele with endarch collateral bundles, a type that is uncommon in 
pteridophytes. 

Traces and Gaps. In all vascular plants the conducting sj^stem is 
essentially continuous throughout the plant body. A strand of conduct- 
ing tissue extending from the stele of the stem through the cortex to a leaf 
is called a leaf trace. A strand connecting the vascular tissue of the stem 
with that of a branch is a branch trace, while a root trace occurs where a 
root arises from a stem or from another root. A gap is a break or inter- 
ruption in a siphonostele caused by the departure of a leaf trace or a 
branch trace (Fig. 221). It consists of parenchyma. Gaps are not 
formed by root traces. Branch gaps are present in all siphonostelic 
stems, but leaf traces occur only in ferns and seed plants. 

1. PSILOPHYTINAEi 

The Psilophytinae comprise a group of primitive pteridophytes that 
were abundant and widespread during the Devonian, but are represented 
today by only two genera of somewhat restricted distribution (Fig. 258). 
They are all rootless plants and either leafless or provided with small, 
simple, spirally arranged leaves. The sporangia are solitary and terminal 
on branches that are elongated or, in modern forms, greatly reduced. 
The Psilophytinae comprise two orders, the extinct Psilophytales and the 
existing Psilotales. 

1. Psilophytales 

The Psilophytales are the oldest and most primitive of all known vas- 
cular plants. They appeared in the late Silurian and flourished during 
the early and middle Devonian. Their remains have been found in many 
parts of the world, but the best-preserved material has come from Scot- 
land. The chief genera are Rhynia, Hornea, Psilophyton, and Asteroxylon. 

The Psilophytales were small herbaceous plants that lived on land. 
Few exceeded 60 cm. in height. The sporophyte consisted of a rhizome 
bearing slender, erect, dichotomously branched shoots (Fig. 169). The 
stems of Asteroxylon were covered with small simple leaves, but the three 
other genera were leafless. In some species of Psilophyton the stems were 
spiny. Apparently true roots were not present, but in some genera the 
rhizome bore numerous rhizoids. In Psilophyton the tips of the branches 
were circinately coiled, as in the young leaves of ferns. In all members 
of the group the stem was a protostele, a narrow zone of phloem enclosing 
a central mass of xylem. Around the stele was a wide cortex surrounded 
by a cutinized epidermis with typical stomata. The presence of stomata 

1 Also called Psilopsida. 



PTERIDOPHYTA 



213 



on the erect stems demonstrates that they were aerial and green. In 
Asteroxylon the xylem was deeply lobed. The conducting tissues were 
very simple. There was no secondary thickening. The leaves, where 
present, were without veins, although the base of each leaf was con- 




FiG. 169. Psilophytales from the Devonian of Scotland. A, Asteroxylon mackiei; B, 
Rhynia major. {After Kidston and Lang.) 



nected with the stele by a strand of vascular tissue constituting a leaf 
trace. 

The sporangia were borne singly at the ends of the branches and not in 
association with the leaves. They were relatively large (in some species 
up to 12 mm. long, but usually smaller), cylindrical, and homosporous. 
The sporangium wall was several layers of cells in thickness. As in other 
pteridophytes, the spores were formed in tetrads. In Hornea the sporog- 



214 



PLANT MORPHOLOGY 



enous tissue was dome-shaped, capping a sterile columella, as in Anthoc- 
eros and Sphagnum. Nothing is known of the garaetophyte generation. 
Some botanists see in the simpler Psilophytales a resemblance to the 
liverwort Anthoceros and regard this group as a connecting link between 
the bryophytes and pteridophytes. Others feel that a closer relationship 
exists between the Psilophytales and the algae and that the Psilophytales 
were derived directly from alga-like ancestors. But, regardless of their 
origin, there is general belief that the group may have been ancestral to 
the other great pteridophyte lines — the lycopods, horsetails, and ferns — 
all of which are represented in the later Devonian deposits (Fig. 258). 

2. Psilotales 

The Psilotales are a modern order including only two genera. Psilo- 
tum, with two species, occurs in tropical and subtropical regions in both 




D CAB 

Fig. 170. Psilotales. A, upper portion of shoot of Psilotum nudum with sporangia, 
slightly reduced; B, closed sporangium of same, enlarged; C, open sporangium; D, cross 
section through an unripe sporangium, showing three locules containing spore mother 
cells; E, shoot of Tmesipteris tannensis, slightly reduced; F, a sporophyll of same, enlarged. 
{After Wetistein.) 

the Eastern and Western Hemispheres. Tmesipteris, with a single spe- 
cies, is confined to Australia, New Zealand, the Philippine Islands, and 
parts of Polynesia. The Psilotales were formerly classified with the 
Lycopodiinae, but they are now generally regarded as being more closely 
related to the extinct Psilophytales. 



PTERIDOPH YTA 215 

Sporophyte. The sporophyte consists of a rhizome bearing rhizoids 
and giving rise to slender, green, aerial stems (Fig. 170^, E). A peculiar 
feature is the absence of roots. The aerial stems of Psilotum branch 
dichotomously and generally grow erect upon the ground, but are some- 
times epiphytic and drooping. They reach a length of 20 to 100 cm. 
The leaves are few, scale-like, and without veins. Tmesiptcris may grow 
as an erect terrestrial plant but is generally epiphytic and pendulous, 




Fig. 171. Cross section of the central portion of the aerial stem of Psilotum nudum, X 100. 
The protoxylem lies at the tips of the xylem rays, the phloem between them. The center 
of the stele is occupied by a group of fibers. 

especially on tree ferns. The stem is unbranched, rarely showing a single 
dichotomy. It reaches a length of 5 to 25 cm. The leaves are narrow, 
12 to 18 mm. long, and have a single vein. In both genera the leaves are 
more or less spirally arranged. The rhizome and aerial stem grow by 
means of a tetrahedral apical cell (having the form of a triangular 
pyramid). 

Vascular Anatomy. In both genera the rhizome is a protostele. In 
the aerial stem the xylem forms a star-shaped mass enclosing a pith that, 
in Psilotum, is occupied by a group of fibers (Fig. 171). The rhizome is 
exarch in both genera, the first-formed xylem (protoxylem) lying outside 
the later-formed xylem (metaxylem) . The aerial stem is exarch in Psilo- 
tum but mesarch in Tmesipteris, in the latter the metaxylem surrounding 
the protoxylem. The phloem, which lies outside the xylem, is poorly 



216 



PLANT MORPHOLOGY 



developed. There is no cambium. An endophytic fungus is present in 
the outer cortical region of the rhizome. 

Sporangium. The sporangia are borne singly in the axils of the upper 
leaves, which are bifurcated in both genera, a sporangium arising at the 
point of forking (Fig. 170). Each sporangium is situated at the end of a 
short stalk. In Tmesipteris the large sporangium is divided transversely 




Fig. 172. Prothallium and sex organs of Psilotum nudum. A, surface view of an entire 
prothallium of rather small size, bearing rhizoids, antheridia, and art-hegonia, X28; B, 
cross section of prothallium, shomng two antheridia (one discharged), two archegonia, and 
endophytic fungus, X38; C, a nearly mature antheridium, X 145; D, several sperms, X450; 
E, a nearly mature archegonium, X 145. {After Lawson.) 

by a sterile plate into two separate chambers, so that two sporangia seem 
to be present. In Psilotum the sporangium is three-chambered. The 
wall consists of several layers of cells. No tapetum is present, but among 
the spore mother cells are numerous sterile cells that are resorbed by the 
developing spores. Both genera of the Psilotales are homosporous. 
Dehiscence of the sporangium occurs by means of a longitudinal slit. 

The nature of the sporangium-bearing structures of the Psilotales is not 
clearly understood. One interpretation is that the upper, forked leaves 
are bifid sporophylls bearing solitary adaxial sporangia that are bilocular 
in Tmesipteris and trilocular in Psilotum. Another view is that the whole 
structure is a sporangiophore bearing two leaves and a terminal sporan- 



PTERIDOPHYTA 217 

gium ; that the sporangiophore is not a lateral branch but a reduced mem- 
ber of a dichotomous branch of the main stem. The existing evidence 
favors this second interpretation. It is also not clear whether the sporan- 
gium should be regarded as a single sporangium or a synangium, which is a 
group of united sporangia. In early developmental stages each mass of 
sporogenous tissue has an independent origin, but it is luicertain whether 
or not the partitions represent sterile sporogenous tissue. 

Gametophyte. The prothallia of both Psilotum and Tmesipferis are 
tuberous, subterranean, saprophytic bodies without any chlorophyll 
(Fig. 172 A, B). They are light brown, cylindrical, simple at first but 
later branched, and up to 20 mm. in length. Long rhizoids uniformly 
cover the surface. An endophytic fungus is present. In Psilotum, but 
not in Tmesipteris, the largest prothallia sometimes possess a strand of 
conducting tissue consisting of a few tracheids. Both kinds of sex organs 
occur in abundance on the same prothallium. The antheridia are glob- 
ular and projecting, each producing many multiciliate sperms (Fig. 172C, 
D). The antheridium initial, which is superficial, undergoes a periclinal 
division, the outer cell giving rise to the sterile jacket and the inner cell to 
the spermatogenous tissue. The archegonia are sunken in the prothal- 
lium, but the neck protrudes (Fig. 172E). Apparently two neck canal 
cells are present. 

Embryo. The embryos of Psilotum and Tmesipteris are very similar. 
They are peculiar in that no suspensor, root, or leaf is present. The fer- 
tilized egg divides transversely, the outer cell giving rise to the stem and 
the inner cell to the foot. In Tmesipteris a second stem tip often appears 
near the base of the first one; later both may grow erect and produce 
leaves. 

Summary. The stem of Psilotum is elongated and branched, the 
leaves scale-like and relatively few. The stem of Tmesipteris is also elon- 
gated but generally unbranched; the leaves are small and numerous. 
Roots are absent in both genera. The stem is differentiated into an 
underground and an aerial portion. The rhizome is an exarch protostele. 
The aerial stem, which is medullated, is exarch in Psilotum and mesarch in 
Tmesipteris. The arrangement of xylem and phloem is amphicribral. 
There is no cambial activity. The sporangia are borne singly in the axils 
of the upper leaves, each at the end of a short sporangiophore. There is 
no definitely organized strobilus. The sporangium is either bilocular 
(Tmesipteris) or triloeular (Psilotum), homosporous, and longitudinally 
dehiscent. A tapetum is not organized. The prothallia are subterra- 
nean and tuberous. The antheridia produce many multiciliate sperms. 
The archegonia have two neck canal cells. The embryo is without a sus- 
pensor. The Psilotales form an isolated order not closely related to other 
living pteridophytes. 



218 



PLANT MORPHOLOGY 



2. LYCOPODIINAE' 

The lycopods, numbering nearly 950 species, are an ancient group 
represented in our modern flora by only four surviving genera. They 
were abundant in the Devonian but reached their greatest display during 
the Carboniferous (Fig. 258), when some were trees 30 m. tall. Today 
all lycopods are herbaceous, generally growing close to the ground. They 
are characterized by leaves that are mostly small, simple, and spirally 
arranged and by sporangia that are always solitary, adaxial, and uniloc- 
ular. There are four orders, the Lycopodiales, Selaginellales, Lepidoden- 
drales, and Isoetales. Of these, the third is extinct. 




Fig. 173. Two tropical species of Lycopodium from Costa Rica, about one-third natural 
size. A, Lycopodium cernuum, a terrestrial species with upright stems and nodding cones; 
B, Lycopodium tubulosurn, a pendent epiphytic species with loosely organized cones at the 
ends of the branches. 

1. Lycopodiales 

Only two living genera belong to this order. Lycopodium, with 180 
species, is widely distributed throughout the world but is most abundant 
• Also called Lycopsida. 



PTERIDOPHYTA 219 

in the tropics. Phylloglossum has a single species confined to Aiistraha, 
Tasmania, and New Zealand. Fossil forms known as Lycopodites have 
been found in Carboniferous and later formations. 

Sporophyte. The sporophyte of Lycopodium consists of a slender stem, 
generally branched, and bearing roots and numerous small leaves. Most 
of the species are terrestrial plants either with erect stems or, more com- 
monly, with elongated, trailing or subterranean stems giving rise to 




Fig. 174. Median longitudinal section through the stem tip of Lycopodium reflexum, show- 
ing the apical meristem and developing leaves, X 100. 

upright branches (Figs. 173A, 179, and 180). Many of the tropical spe- 
cies are epiphytes with erect or pendent stems (Fig. 1735). The terres- 
trial species grow close to the ground, few exceeding 30 cm. in height. 
Numerous roots penetrate the soil. Generally both the roots and stems 
branch dichotomously, but in some species the branching of the stem is 
apparently monopodial, the branches arising laterally from a true main 
axis. The leaves are small and often scale-like, simple, entire, and 
densely cover the branches. They are generally borne in spiral arrange- 
ment. Growth of the root and stem takes place by means of an apical 
meristem, no apical cell being present (Fig. 174). 

Although Phylloglossum is much simpler than Lycopodium, it is gen- 
erally regarded, not as a primitive form, but as one reduced from more 
highly developed ancestors. The sporophyte is only 3 to 5 cm. high and 



220 



PLANT MORPHOLOGY 



consists of a short tuberous stem bearing a few small fleshy leaves that 
form a cluster around the stem apex (Fig. 175). As a rule, only a single 
root is present, but sometimes there are two or three roots. In some spe- 
cies of Lycopodium the sporophyte begins its development as a small 
tuberous body like that of Phylloglossum. This disappears after giving 
rise to the ordinary type of leafy stem wdth roots. In other species no 

embryonic body of this kind occurs in the life 
history. 

Vascular Anatomy. Anatomically the roots 
and leaves of Lycopodium are essentially like those 
of other vascular plants. The leaves are only a 
few layers of cells in thickness and consist of uni- 
form mesophyll enclosed above and below by an 
epidermis with stomata. Each leaf has a single 
median vein. 

The stem structure of Lycopodium is very 
primitive. A cross section shows an outer cortex 
and a central cylindrical stele. Because of the 
absence of a pith, this type of vascular system is 
a protostele. In some species the xylem forms a 
star-shaped mass between the rays of which lies 
the phloem (Fig. 176). In other species, although 
fundamentally radial, a modification is seen in that 
the xylem and phloem are somewhat intermixed 
(Fig. 177), while in still others the two kinds of 
conducting tissues occur in alternating, transverse, 
parallel bands (Fig. 178). A protostele that is 
star-shaped in outline is often designated as an 
actinostele; one that is circular in outline is called 
a haplostele; while the type with separate, par- 
allel plates of xylem is termed a plectostele. The radial type is most 
primitive and the banded type most advanced. This is shown by the 
development of the vascular system in a young plant, where, if the banded 
condition appears, it is always preceded by the radial condition. Also, in 
many species, the growing stem is radial at the tip and gradually becomes 
banded farther back. 

All the vascular tissues are primary, there being no cambial activity. 
The smaller cells at the tips of the radiating arms of xylem are the first 
elements to lignify, subsequent lignification proceeding toward the center 
of the stem. Thus the development of the xylem is exarch, the proto- 
xylem lying external to the metaxylem (Figs. 176, 177, 178). The proto- 
xylem is composed of narrow^ spiral, and annular tracheids, the metaxylem 
of larger, scalariform tracheids. 




Fig. 175. Fertile plant 
of Phylloglossum drum- 
mondii, twice natural 
size; sir, strobilus; /, 
leaves; r, root; tl, pri- 
mary tuber; t2, secondary 
tuber. 



PTERIDOPHYTA 



221 



per 




Fig. 176. Cross section of the central portion of the stem of Lycopodium liicidulum, show- 
ing the "radial" type of stele, X250; end, endodermis; per, pericycle; px, protoxylem; mx, 
metaxylem; ph, phloem. 




Fig. 177. Cross section of the central portion of the stem of Lycopodium cernuum, showing 
the "mixed" type of stele, X70. 



222 PLANT MORPHOLOGY 

The stele is enclosed by a parenchymatous pericycle, one or more layers 
of cells in thickness, outside of which is an ill-defined endodermis, consist- 
ing of a layer of cells with cutinized walls (Fig. 176). As in all vascular 
plants, the conducting system is essentially continuous throughout the 
plant body. From the stele of the stem a strand of conducting tissue, 
called a leaf trace, extends through the cortex to enter each leaf and 




Fig. 178. Cross section of the central portion of the stem of Lycopudium compla/iattirn, 
showing the "parallel-banded" type of stele, X 130. 

become its vein. Similarly each branch is connected with the stele of the 
main stem by a branch trace and each root by a root trace. These traces 
are present in all vascular plants. 

Like the stem, the root of Lycopodinm is an exarch protostele, but 
nearly always shows the radial arrangement of xylem and phloem, even 
where the stem is of the mixed or parallel-banded type. The striking 
similarity between the root and stem of Lijcopodium bespeaks a very 
ancient origin for the genus, for in the higher groups of vascular plants the 
organization of the stem becomes increasingly more advanced, while that 
of the root remains unchanged. 

The vascular system of Phylloglossum is poorly developed and shows 
evidences of reduction. The tuber consists mainly of compact storage 



PTERIDOPHYTA 



223 



parenchyma with a small amount of xylem in its upper portion. Appar- 
ently no phloem is present. In sterile plants the vascular system is a 
protostele, but in fertile plants the .xylem surrounds a central mass of 
parenchyma and is thus a siphonostele. The development of the xylem 




Fig. 179. Lycopodium rejlcxum, an upright tropical .species in which most of the leaves are 
sporophylis; about natural size. 

is mesarch, the metaxylem arising both inside and outside the protoxylem. 
This is a more advanced condition than that seen in Lycopodium. 

Sporangium. The sporangia of Lycopodium are borne singly in the leaf 
axils. Each sporangium is large and more or less kidney-shaped, with a 
very short stalk, a wall several layers of cells in thickness, and a central 
mass of small, yellow, thick-walled spores. When mature, it dehisces by 
means of a transverse slit. A leaf that bears a sporangium is termed a 
sporophyll. 

In the simplest species of Lycopodium every leaf on the plant is a sporo- 



224 



PLANT MORPHOLOGY 



phyll, or at least potentially so, but in most species just the upper leaves 
bear sporangia, the lower leaves being sterile and functioning merely as 
foliage leaves (Figs. 179 and 180). An aggregation of sporophylls is 
called a cone or strohilus. The sporophylls may be loosely arranged but, 
more commonly, are compactly organized. They may be similar to the 




Fig. 180. Lycopodiiim obscurum, a conimou ipecacs occurring throughout the northeastern 
United States, about one-half natural size. Upright branches bearing terminal cones arise 
from long trailing stems. 



foliage leaves but generally are smaller, of a different form, and without 
chlorophyll. 

Because of this variation among the species of Lycopodium, it is not 
difficult to arrange a series representing progressive stages in the differen- 
tiation of the sporophylls and organization of a strobilus. It seems rea- 
sonable to suppose that such a series represents the course of evolution 
that the more complex species have followed. This is confirmed by other 
characters. Thus most of the species without a definite strobilus have 



PTERIDOPHYTA 



225 




Fig. 181. Early development of the sporangium oi Lye o podium selago. A, radial section 
of base of young sporophyll (l) arising from stem (st), showing initial cell (one of a transverse 
row); B, slightly later stage; C, division of initial into primary wall cell and primary 
sporogenous cell (latter shaded); D, tangential section of same; E, further development of 
sporogenous tissue; F, radial section of same; G, later stage, showing development of 
tapetum; H, sporangium showing stalk, wall, tapetum, and spore mother cells. {After 
Bower.) 



dichotomously branched stems showing the radial type of stele, while 
many of the cone-bearing species have stems with monopodial branching 
and most of them show the parallel-banded type of stele. 

The large solitary sporangium is always adaxial in its relation to the 
sporophyll and is also unilocular. The sporangium arises from a super- 
ficial group of initials consisting of a transverse row of 6 to 12 cells (Fig. 
181 A, B). In some species there may be two or three such rows. Each 



226 



PLANT MORPHOLOGY 



initial divides by a periclinal wall to form an outer and an inner row, the 
former constituting the primary wall cells and the latter the primary 
sporogenous cells (Fig. \8W, D). The sporangium wall becomes at least 
three layers of cells thick, the inner layer forming the tapetum (Fig. 

\^\E-G). This is a nutritive layer 
that, instead of disorganizing, as in 
the horsetails and ferns, remains 
intact for a long time. After the 
sporogenous cells have increased 
in number, spore mother cells are 
organized and from each a tetrad of 
spores arises (Fig. 181//). The 
development of the sporangium of 
Lycopodium takes place according 
to the eusporangiate method. This 
means that the sporogenous tissue 
is derived from the inner segment 
following the first periclinal division 
of the initial. All vascular plants, 
except the higher ferns, are euspo- 
rangiate. 

In Phylloglossum the apex of the 
tuber gives rise to an erect naked 
stalk bearing a small terminal strob- 
ilus (Fig. 175). If no strobilus is 
formed, the stem tip produces a 
new tuber at the close of the grow- 
ing season, but otherwise a second- 
ary tuber appears adventitiously 
at the end of a short stalk. The 
strobilus consists of a few spirally 
arranged sporophylls, each bearing 
a solitary, unilocular, adaxial spo- 
rangium. The sporangium is short- 
stalked and kidney-shaped. It 
consists of a central mass of sporog- 
enous tissue surrounded by a wall three layers of cells in thickness, the 
inner layer forming the tapetum. As in Lycopodium, dehiscence occurs 
by means of a transverse slit. 

Gametophyte. The spores of Lycopodium are remarkably long-lived 
and often do not germinate for a number of years. Eventually they give 
rise to gametophytes, or prothallia as they are usually called. These are 
small tuberous bodies that vary widely in form, depending on the species, 




Fig. 182. Gametophyte of Lycopodium 
complanatum. A, the entire gametophyte, 
twice natural size; B, longitudinal section 
with anthericUa to the left, some of which 
have shed their sperms, and with archegonia 
to the right, in one of which an embryo has 
developed, X 25. The shaded cells in the 
lower portion contain a fungus. {After 
Bruchmann.) 



PTERIDOPHYTA 



227 



being turnip-shaped, cylindrical, flat, or irregularly bulbcnis. In Lycopo- 
dium cernuum, a widespread tropical species, the prothallium is an erect 
cylindrical body only 2 or 3 mm. long. It grows at the surface of the 
ground and consists of a colorless basal portion buried in the soil and a 
conspicuously lobed aerial crown that is green and bears the sex organs. 
The lower portion produces rhizoids and contains an endophytic fungus. 
The spores germinate promptly and the prothallium reaches maturity in a 




Fig. 183. Gametophyte of Lycopodiitm davatum. A, the entire gametophyte, tvnce 
natural .size; 5, longitudinal .section with antheridia in center and archegonia to the left 
and right, X 20. In the embryo, shown in outline, a young .shoot has developed above, a 
young root to the right, and a large foot below and to the left. The shaded cells in the 
lower portion of the prothallium contain a fungus. {After Bruchmann.) 

single season. Several other species of Lycopodimn, as well as Phylloglos- 
sum, have a similar type of prothallium but some lack the fungus. 

In most other species, including nearly all those of the North Temperate 
Zone, the prothallium is larger, commonly 12 to 18 mm. long, and entirely 
subterranean, colorless, and saprophytic (Figs. 182 and 183). An endo- 
phytic fungus, restricted to the lower portion of the prothallium, is 
always present and seems to play an essential part in its nutrition. The 
spores do not germinate for 3 to 8 years and the prothallium may not 
reach maturity for an equally long period, growth being extremely slow. 
The production of sex organs may continue for a number of years. The 
gametophyte may be erect and somewhat turnip-shaped, as in Lycopo- 
dium complanatum, with the sex organs borne on an irregularly lobed 
crown that is not as well developed as in Lycopodium cernuum (Fig. 182). 
In certain other species, such as Lycopodium davatum, the crown is 
reduced even more, the prothallium being flat and irregularly cup-shaped, 
with a depressed center surrounded by a broad rim (Fig. 183). The sex 
organs are borne in the center. This type of prothallium grows farthest 



228 



PLANT MORPHOLOGY 



below the surface of the ground and requires the greatest number of years 
to reach maturit^^ 

It is an interesting fact that only species with a green aerial prothal- 
lium have a sporophyte that passes through a Phylloglossum-\ike stage in 
its early development. The prothalliura of such species doubtless repre- 
sents a primitive type from which the colorless subterranean prothallia 
have been derived. Their development may have been caused by 
delayed germination of the spores, resulting in their burial in the soil. 






Fig. 184. Antheridial development in Lycopodiiim clavatum, X 150. A, to the right, a 
young antheridium after first division of initial cell; to the left, a much older stage; B, 
vertical division of primary wall cell and primary spermatogenous cell; C, further division 
of spermatogenous cells; D, nearly mature antheridium, showing wall and spermatogenous 
tissue; E, two sperms, X625. (After Bruchmann.) 



In fact, in some species, a spore will produce a green prothallium if it 
germinates on the surface of the ground and a colorless one if it germinates 
below the surface. 

Both kinds of sex organs are borne in rather large numbers on the same 
gametophyte. The antheridia are either completely embedded or 
slightly projecting. They are globular, have a sterile jacket consisting of 
a single layer of cells, and produce many small, slightly curved, biciliate 
sperms (Fig. 184). In development, a superficial initial divides by a 
periclinal wall to form an outer 'primary wall cell and an inner primary 
spermatogenous cell. The former gives rise to the sterile jacket, the latter 
to the mass of spermatogenous tissue. 

The archegonia are also embedded in the prothallium, only the neck 
protruding (Fig. 185). The initial is superficial and gives rise, by a peri- 
clinal division, to an outer primary neck cell and an inner cell that again 
divides to form the central cell and hasal cell (Fig. 185A, B). The central 
cell gives rise to two cells, the outer one being the primary neck canal cell 



PTERIDOPHYTA 



229 



and the inner one the 'primary ventral cell (Fig. 18oC). The primary neck 
canal cell, by additional transverse divisions, gives rise to a variable num- 
ber of neck canal cells, while the primary ventral cell, by a single transverse 
division, produces the ventral canal cell and egg (Fig. \8oD-F). In some 
species there are 4 to 6 neck canal cells. In Lycopodium cernuum the 
number has been reduced to 1, while in Lycopodium complanatum as many 
as 16 have been counted. The presence of numerous neck canal cells is a 
primitive feature not seen in other living pteridophytes. 




Fig. 185. Archegonial development in Lycopodium clavatum, X 150. A, young arche- 
gonium after first division of initial cell; B, vertical division of primary neck cell and trans- 
verse division of inner cell to form basal cell and central cell (both shaded) ; C, division of 
central cell to form primary neck canal cell and primary ventral cell; D, later stage, showing 
basal cell, primary ventral cell, and four neck canal cells; E, nearly mature archegonium 
with egg, ventral canal cell, and six neck canal cells; F, older stage, the canal cells breaking 
down. (After Bruchmann.) 

Embryo. The fertilized egg divides transversely to form an outer sus- 
pensor cell and an inner embryonal cell (Fig. 186A). The suspensor cell 
may or may not divide again but usually elongates and pushes the embryo 
a short distance into the prothallium. The embryonal cell, by two ver- 
tical divisions at right angles to each other, gives rise to quadrants, each 
of which then divides transversely to form eight cells in two tiers (Fig. 
186B-^). Of these, the tier lying next to the suspensor develops the 
foot, while the lower tier gives rise on one side to the stem and on the other 
side to the leaf (Fig. 186i^-7). The foot is a temporary organ that 
absorbs food from the gametophyte. It persists until the sporophyte has 
become independent. The primary root is formed relatively late and 
arises from the same tissue that produces the leaf. 

Summary. In Lycopodium an elongated, generally branched stem 
bears numerous small leaves. In Phylloglossum a short tuberous stem 
produces a few small leaves in a cluster. The vascular system of the stem 
of Lycopodium is a protostele, fundamentally radial in organization, with 
exarch development of xylem. Secondary thickening is absent. In the 



230 



PLANT MORPHOLOGY 



simplest species of Lycopodium all the leaves are sporophylls, but in the 
more advanced species a differentiation exists between sporophylls and 
foliage leaves, the former forming a more or less distinct strobilus. All 
the Lycopodiales are homosporous. Dehiscence of the sporangium occurs 
by means of a transverse slit. The gametophyte is a subterranean tuber- 
ous body, sometimes with an aerial portion. The antheridia develop 




Fig. 186. Development of the embryo of Lycopoduim clavatum. A, first division of the 
fertilized egg into suspensor and embryonal cells; B, second division of embryonal cell; C, 
second division of suspensor cell; D, cross section, showing third division of embryonal cell; 
E, transverse division (IV) of four cells derived from the embryonal cell; F, G, H, later 
stages; /, young sporophyte, showing foot (/), primary root (r), and stem bearing scale 
leaves. {After Bruchmann.) 

endogenously and produce many small biciliate sperms similar to those 
of bryophytes. The archegonia are primitive, being characterized by a 
large number of neck canal cells. The embryo has a suspensor. On the 
whole, the Lycopodiales are a very primitive group of pteridophytes. 

2. Selaginellales 

The Selaginellales include a single genus, Selaginella, mth nearly 700 
species. The great maj ority of these are tropical or subtropical in distri- 
bution, but a few occur in temperate regions. Most species require 
abundant moisture and shade, while some grow in open, dry situations. 
Selaginellites, a fossil genus, has been recognized in deposits as old as 
the Lower Carboniferous. 



PTERIDOPHYTA 



231 




Sporophyte. In general, the sporophyte of Selaginella has the same 
habit as that of Lycopodium, but is nearly always smaller and more deli- 
cate. The stems are dichotomously branched and usually trailing, but 
are often erect or climbing. Rhizophores, which are special leafless 
branches of the main stem, are found in many species. They produce 
adventitious roots at their tips. The leaves are scale-like and numerous, 
generally occurring in four longitudinal rows. Each leaf bears a ligide, a 
minute flap-like outgrowth arising from the basal portion of the adaxial 
surface. The ligule is prominent only 
during the early development of the 
leaf. 

In some species of Selaginella the 
leaves are all alike and symmetrically 
arranged around the stem, but in most 
species the leaves are spread out hori- 
zontally and usually of two kinds. 
These are regularly arranged with ref- 
erence to each other, there being two 
rows of small dorsal leaves and two 
rows of large ventral ones (Fig. 189^). 
In contrast to Lycopodium, a definite 
apical cell is usually present at the 
tip of the root and stem, but some 
species appear to have an apical meri- 
stem (Fig. 187). 

Vascular Anatomy. The leaves have an epidermis and a loose meso- 
phyll, the stomata usually being confined to the lower surface. The 
chloroplasts are large and few in number, sometimes only one occurring 
in each cell. Each leaf has a single median vein. The adult stem gen- 
erally consists of a single, dorsiventrally flattened protostele with two 
lateral protoxylem groups (Fig. 188). The metaxylem develops toward 
the center, and so the stem is exarch. In some species the stele is cylin- 
drical and in some two, three, or more separate steles are present. One 
species has reached the siphonostelic condition. The conducting tissues 
show an amphicribral arrangement, the xylem being completely sur- 
rounded by the phloem. Outside the pericycle, which is generally one- 
layered, a broad air space occurs. This is traversed by elongated cells 
(trabeculae) consisting of endodermal cells united with one or more cells 
that have a common origin with the endodermis. As in Lycopodium, no 
cambium is present and so there is no secondary thickening. In some 
species the cortical cells are thick-walled. 

Sporangia. In all species of Selaginella definite terminal strobili are 
present. Where the foliage leaves are all alike, the sporophylls and leaves 



Fig. 187. Median longitudinal section 
through the stem tip of Selaginella 
bigelovii, showing the apical cell and its 
derivatives, X 500. 



232 



PLANT MORPHOLOGY 



are either similar or only slightly differentiated. Where the foliage leaves 
are of two kinds, the sporophylls are smaller than the large leaves (Fig. 
189.4). As in Lycopodium, the sporangia are solitary, adaxial, unilocular, 
and eusporangiate in development (Fig. 1895, C). Each is probably 
derived from a transverse row of initials. These generally appear on the 




Fig. 188. Stem structure of Selaginella flabellata. A, diagram of cross section, showing 
central flattened protostele surrounded by cortex, X 10. B, enlarged view of portion of 
stele, showing exarch xylem surrounded by a wide zone of phloem and an air space traversed 
by trabeculae, X 200. 



stem just above the place where the young sporophyll arises from it (Fig. 
190A-C). Although cauline in origin, the young sporangium is soon car- 
ried out on the sporophyll and then looks as if it had originated there. 
The wall of the young sporangium consists of a single layer of cells but it 
soon becomes two-layered. In contrast to the other lycopods, the tape- 
tum is not derived from the wall tissue but from the outermost layer of 
sporogenous tissue. As in Lycopodium, it does not break down until the 
spores are formed. The innermost wall layer also disorganizes at this 
time, and so the mature sporangium has only a single layer of wall cells. 
Each sporangium is borne on a short stalk. Dehiscence takes place by 
means of a vertical slit. 



PTERIDOPHYTA 



233 



SelagineUa is heterosporous, each strobihis usually bearing two kinds of 
sporangia — microsporangia and megasporangia (Fig. 1895, C). The 
microsporangia, which are often reddish, generally occur in the upper part 
of the strobilus, while the megasporangia, which are commonly yellowish, 
are borne below. The megasporangia are usually sUghtly larger than the 
microsporangia and are generally four-lobed. 





B 



Fig. 189. SelagineUa willdenovii. A, branch with leaves and .strobili, twice natural size; 
B, a microsporophyll with a microsporangiiun containing numerous microspores, X25; C, 
a megasporophyll with a megasporangium containing four megaspores, X25; several 
microspores and a megaspore drawn to the same scale are also shown. 

Both kinds of sporangia develop alike as far as the stage in which the 
sporogenous tissue is differentiated. In the microsporangium practically 
all the mother cells divide to produce tetrads, and consequently many 
small spores are formed (Fig. 192A). These are the microspores. In the 
megasporangium, on the other hand, all the mother cells degenerate but 
one, which greatly enlarges and forms a tetrad of thick-walled spores that 
eventually fill the sporangium (Figs. 191 and 192B). These are the mega- 
spores. The sporophylls that produce the microsporangia are micro- 
sporophylls, while those bearing megasporangia are megasporophylls. 
Usually the sporophylls themselves, however, are of approximately the 
same size and form. Like the foliage leaves, each sporophyll bears a 
ligule. It is situated just beyond the sporangium (Figs. 190 and 192). 

Upon germination, the microspores give rise to male gametophytes and 
the megaspores to female gametophytes. Thus heterospory involves not 
only a differentiation of spores but also a differentiation of gametophytes. 



234 



PLANT MORPHOLOGY 



Gametophytes. The male gametophyte of Selaginella is developed 
entirely within the microspore. It is without chlorophyll and greatly 
reduced. Its development is initiated before the spore is shed from the 
sporangium and is completed later. A small prothallial or vegetative cell is 




Fig. 190. Development of the microsporangium of .SeZagweZZa (7oZeo«M, X320. yi, median 
longitudinal section of portion of apex of strobilus, showing early stages; B, slightly later 
stage with ligule to the left; C, young sporangium with sporogenous tissue surrounded by 
tapetum and two wall layers; D, older sporangium, the sporogenous cells beginning to 
round up. 

cut off, the large remaining cell forming a single antheridium (Fig. 193). 
At first this consists of four primary spermatogenous cells surrounded by a 
sterile jacket of eight cells, and usually the male gametophyte is shed from 
the microsporangium in this condition. Later the spermatogenous cells 
increase in number to 128 or 256, each finally giving rise to a sperm. 
Meanwhile the jacket cells disintegrate, leaving the mass of sperms free 
within the microspore wall. The sperms are small, curved, and biciliate. 



PTERIDOPHYTA 



235 




A - y r-< B 

Fig. 191. Longitudinal sections of young megasporangia of Selaginella, X200. A, mega- 
sporangium of Selaginella emmeliana with spore mother cells, the functional one enlarging; 
B, megasporangium of Selaginella apoda, showing three of the four megaspores. 




Fig. 192. Longitudinal sections of sporangia of Selaginella emmeliana, X80. A, a micro- 
sporangium with numerous microspores; B, a megasporangium with three of the four 
developing megaspores. 

The female gametophyte develops within the megaspore but is not so 
greatly reduced as the male gametophyte. The megaspore germinates 
while still within the megasporangium and long before it has reached its 
full size. The protoplast of the young megaspore is apically situated and 
consists of a vesicle with a small nucleus. It has a thick membrane that 
seems to grow more rapidly than itself, leaving a fluid-filled space between 
the protoplast and the membrane. The membrane soon differentiates 



236 



PLANT MORPHOLOGY 



into an outer and inner layer that also separate as a result of a more rapid 
growth of the outer layer, thus forming a second fluid-filled space (Figs. 
\9\B and 1925). The female gametophyte begins to develop by free- 
nuclear division. Its protoplast enlarges until it comes in contact with 
the inner spore coat, which later comes in contact with the outer coat, 
thereby obliterating both cavities. The nuclei in the young gametophyte 




Fig. 193. Sections through the male gametophyte of Selaginella kraussiana in different 
stages of development. A, early stage, consisting of a small prothallial cell and an antherid- 
ial cell; B, later stage with prothallial cell and antheridium consisting of four primary 
spermatogenous cells surrounded by eight jacket cells; C, mature gametophyte with nearly 

ripe sperms lying free inside microspore wall. {After Slagg.) 

» 

lie in a peripheral layer of cytoplasm surrounding a large central vacuole 
(Fig. 194A). 

After the female gametophyte has undergone a series of free-nuclear 
divisions, wall formation begins at the apical (pointed) end. At this 
place the spore wall ruptures and the gametophytic tissue protrudes 
slightly, developing archegonia and in some species rhizoids also, or rhi- 
zoids and chlorophyll. The main portion of the gametophyte, lying 
within the megaspore wall, acts as a large food reservoir. In many spe- 
cies there is a marked differentiation between the deeper nutritive region 
and the exposed portion, and often the former is not divided into cells 
(Fig. 194B). The development of the archegonium is similar to that of 
Lycopodium except that the neck is very short and no basal cell is formed. 
There is usually only one neck canal cell. 

Although, in most species, the early development of the female game- 
tophyte occurs while the megaspore is still within the megasporangium, 



PTERIDOPHYTA 



237 



archegonia generally do not appear until the megaspore is shed. In some 
species the megaspore is retained until fertilization has occurred, or even 
until the embryo has appeared. Here the male gametophytes are carried 
to the megasporophylls by wind or gravity and there they liberate their 
sperms. In such cases it is apparent that a condition closely approaching 
seed formation is reached. 

Embryo. The embryo of SelagineUa resembles that of Lycopodium in a 
general way, but shows certain important differences and some variation 








A 



Fig. 194. Female gametophyte of SelagineUa apoda. A, section of megaspore containing 
young gametophyte in free-nuclear stage; B, section of megaspore with mature gameto- 
phyte, consisting of a large nutritive cell and small-celled tissue in which an archegonium 
has developed. {After Lyon.) 

among the different species (Fig. 195). Commonly the first division of 
the fertilized egg, which is transverse, separates an outer suspensor cell 
from an inner embryonal cell, but the suspensor usually becomes more 
highly developed than in Lycopodium. No quadrant stage is formed. 
Instead the embryonal cell produces three cells — a terminal one, which 
forms the stem, and two lateral ones, each of which gives rise to a leaf. 
One of the leaf segments produces the foot and later the primary root. In 
some species the foot and root, as well as the suspensor, are derived from 
the upper cell that arises from the first division of the fertilized egg. 

Summary. The stem of SelagineUa is elongated and branched, the 
leaves numerous and small. Each leaf bears a ligule. The vascular sys- 
tem of the stem is typically an exarch protostele with amphicribral organ- 
ization ; sometimes more than one stele is present. There is no secondaiy 
thickening. All species have a definite strobilus and are heterosporous. 
The microsporangia produce many microspores, the megasporangia four 
megaspores. Dehiscence takes place by means of a vertical slit. The 
male gametophyte, developed entirely inside the microspore, consists of 



238 



PLANT MORPHOLOGY 



only one prothaUial cell and one antheridium, the latter producing many 
small bic-iliate sperms. The female gametophyte, with relatively exten- 
sive vegetative tissue, develops largely within the megaspore, the protrud- 
ing portion forming several archegonia. These are of an advanced type 




Fig. 195. Development of the embryo of Selaginella martensii. A, first division of ferti- 
lized egg into suspensor and embryonal cells; B, embryonal cell divided into three cells; C, 
differentiation of stem and leaf primordia; D, later stage, the stem and leaf primordia 
developing by an apical cell; E, later stage, showing differentiation of foot and root; F, 
older embryo; sus, suspensor; s, stem tip, If, leaf; I, ligiile; r, root tip; /, foot. (After 
Pfeffer.) 

with only one neck canal cell. The embryo has a suspensor. Selaginella 
is related to Lycopodium, differing from it chiefly in being heterosporous. 

3. Lepidodendrales 

The Lepidodendrales are a Paleozoic group of lycopods. They ranged 
from the Devonian through the Permian but made their greatest display 
during the Upper Carboniferous, when they were one of the dominant 
plant groups. They were chiefly tree-like and often reached a height of 
30 m. or more. The two most prominent genera were Leyidodendron and 



PTERIDOPHYTA 



239 




Fig. 196. Portion of restoration of Carboniferous swamp forest in the Chicago Natural 
History Museum, showing trunks of Sigillaria rugosa (to left) and Sigillaria saitlli (to right), 
and cones of Lepidostrohns ovatifolius (upper left) ; also two seed ferns: Neuropteris decipiens 
(right center) and Neuropteris heterophylla (below). 



240 



PLANT MORPHOLOGY 



Sigillaria^ (Fig. 19G). Both bore numerous narrow simple leaves that, 
upon falling, left characteristic scars on the stem. The stems of Lepido- 
dendron were dichotomously and freely branched and the leaves were 
frequently up to 15 or 20 cm. long. The stems of Sigillaria were slightly 
or not at all branched and the leaves, in some species, reached a length of 
1 m. The leaves of all the Lepidodendrales had a single vascular bundle 
and in all of them a ligule, deeply sunken in a pit, was present. In both 




Fig. 197. Transverse section of stem of Lepidodendron wunschianum, an ectophloic 
siphonostele. The central pith is surrounded, in turn, by a narrow cylinder of primary 
wood, an extensive zone of secondary wood, and an outer layer of bark. 

Lepidodendron and Sigillaria the base of the main stem was attached to 
four descending branches (rhizophores) that spread out horizontally and 
underwent repeated forking. They were covered with roots. 

The stem was either a protostele or, more commonly, an ectophloic 
siphonostele (with the phloem outside the xylem). A primitive feature 
was the presence of exarch primary wood. The stem was characterized 
by marked secondary thickening (Fig. 197). The stem of Sigillaria 
sometimes reached a diameter of 2 m. 

^ Plant fossils usually occur as detached organs or fragments. Only rarely is one 
part of the plant found attached to another part. Until such connections are found, 
detached organs of the same kind are placed in a "form genus." For example, 
Lepidodendron was originally a stem genus. Its leaves were placed in the form genus 
Lepidophylluyn and its cones in Lepidosirobus. Its root-bearing parts, indistinguish- 
able from those of Sigillaria, are included in the form genus Stigtnaria. 



PTERIDOPHYTA 



241 



Definite strobili were present, in Lepidodendron, at the ends of the 
branches, in Sigillaria, in whorls along the stem. The sporophylls and 
foliage leaves were rather similar in form. As in all lycopods, the large 
sporangia were solitary, adaxial, and uni- 
locular. The ligule was situated beyond 
the sporangium. The Lepidodendrales 
were heterosporous, the megasporangia 
generally containing 8 to 16 megaspores. 
In some cases trabeculae, consisting of 
sterile plates forming incomplete parti- 
tions, were present both in the micro- 
sporangia and megasporangia. 

The gametophytes were developed in- 
side the spores. The nature of the sperms 
is unknown. The archegoniawere similar 
to those of Selaginella. The embryo is 
also unknown. In Lepidocarpon, a cone 
genus, the mature megasporangium had 
only one megaspore and, except for a nar- 
row opening at the top, was invested by an 
integument that arose from below. This 
sporangium, although seed-like, was shed 
with the sporophyll before fertilization 
took place. 

4. Isoetales 

Isoetes is the only living genus belonging 
to the Isoetales. It has about 60 species 
widely distributed throughout temperate 
regions but rare in the tropics. It grows 
on muddy flats, in wet meadows, along 
stream and pond margins, or submerged 
in shallow water. A few species grow in 
drier habitats. Fossils resembling Isoetes 
are known from the Cretaceous and Ter- 
tiary periods. 

Sporophyte. Superficially Isoetes is en- 
tirely different in appearance from any 
other living pteridophyte, resembling a 

small rush or tufted grass. Its common name is "quillwort." The stem 
is erect, tuberous, unbranched, and very short (Fig. 198). It gives rise to 
a crowded rosette of linear, spirally arranged leaves that are commonly 
about 5 to 15 cm., rarely 30 cm. or more, in length. The stem is either 




Fig. 198. 
size. 



Isoetes nuttallii, natural 



242 PLANT MORPHOLOGY 

tAvo-lobed or three-lohed, depending on the species. It is covered by over- 
lapping leaf bases. Between its lobes arise numerous dichotomously 
brancired roots. As in Selaginella, each leaf has a ligule, arising at its 
base on the adaxial side. The root and stem grow by means of a meri- 
stem (Fig. 199). The stem has both an apical and a basal meristem. 

Vascular Anatomy. Each leaf has a single vascular bundle and four 
long air passages with numerous transverse partitions. Stomata are pres- 
ent only on leaves exposed to the air. The stem structure is rather com- 
phcated and difficult to interpret. Many botanists regard the upper part 




Fig. 199. Longitudinal section through the stem tip of Isoetes howellii, showing the apica 
meristem and developing sporophylls, each with a prominent ligule, X300. 

of the stem, which bears the leaves, as the stem proper and the lower por- 
tion, which bears the roots, as a rhizophore, although no such differentia- 
tion is evident externally. 

The vascular cylinder, representing a greatly reduced protostele, is 
surrounded by an extensive cortex (Fig. 200). A notable feature of the 
stem of Isoetes is the occurrence of secondary thickening. The primary 
xylem, consisting of extremely short tracheids intermixed with consider- 
able parenchyma, is surrounded by a narrow zone of primary phloem. 
This, in turn, is enclosed by a "prismatic layer," which represents the 
internal product of cambial activity. On the outside the cambium adds 
new tissue to the cortex. This tissue, which is parenchymatous, has the 
position of secondary phloem but not its structure. The tissues forming 
the prismatic layer are not uniform, but are differentiated into alternating 
zones of thin-walled and thick-walled cells. The thin-Avalled cells are 
ordinary parenchyma, while the thick-walled cells are lignified and have 
scalariform and reticulate markings. Thus the prismatic layer has the 
position of secondary xylem but not its typical structure. Whether the 



PTERJDOPHYTA 



243 



prismatic layer contains any secondary phloem, as has been claimed, is a 
matter of considerable uncertainty. 

As cambial activity continues, the outer tissues of the stem are con- 
stantly sloughed ofT, the cortex finally being made up wholly of secondary 
tissue. Numerous leaf traces arise from the stele of the stem, one going 
to each leaf. In the lower part of the stem (rhizophore) root traces sim- 




■ . r. J --' 






>*#V 



■■.'(?:*-■•■■; 







« # . . . 






Fig. 200. Cross section of the central portion of the stem of Isoetcs howdlii, X 100. The 
primary xylem, in the center, is surrounded by a narrow zone of primary phloem enclosed 
by tissues derived from the cambium. 

ilarly pass from the stele to the roots. The root traces contain much more 
xylem and phloem than the leaf traces. 

Sporangia. Isoetes, like Selaginella, is heterosporous, but nearly all the 
leaves are sporophylls. As a rule, the outer leaves bear megasporangia 
and the inner ones microsporangia, while the few central leaves are sterile. 
The sporangia, mostly -4 to 7 mm., but up to 10 mm. in length, are larger 
than those of any other living pteridophyte. They are solitary and adax- 
ial, each one being sunken in a cavity at the base of the sporophyll just 
below the Hgule (Fig. 201 A, B). Each sporangium is partially or com- 
pletely overgrown by a membrane called the velum. The microsporangia 
may produce as many as 300,000 microspores, the megasporangia up to 
300 megaspores. In each of the two kinds of sporangia sterile plates, 



244 



PLANT MORPHOLOGY 



called trabccidae, extend inward to form incomplete partitions. The 
spores are freed by the gradual decay of the sporangium wall. 

The sporangium arises from a transverse row of initials, three or four m 
number, and is eusporangiate in development (Fig. 20 IC). In early 
stages the two kinds of sporangia are indistinguishable from each other. 






Fig. 201. Sporangia of Isoetes. A, cross section of microsporangium of Isoetes mdtallii, 
X15; B, longitudinal section of megasporangium of Isoetes nuttallii, XlO; C, longitudinal 
section of young niicrosporophyll of Isoetes howellii, showing sporogenous tissue (shaded), 
X250; /, ligule; v, velum. 

After a large amount of potentially sporogenous tissue has been differen- 
tiated, some of it forms the sterile trabecular The sporangium wall con- 
sists of four layers of cells, the inner layer forming a tapetum that also 
borders the trabeculae (Fig. 202). The tapetum does not disorganize for 
a long while. In the microsporangium all the cells not taking part in the 
formation of the wall, trabeculae, and tapetum become functional spore 
mother cells. In the megasporangium the trabeculae are fewer and 
thicker, the tapetum comprises several layers, and most of the spore 
mother cells divide and contribute nourishment to a much smaller number 
that enlarge and form tetrads. As in Selaginella, the megaspore wall is 
very thick. 



PTERIDOPHYTA 



245 



Gametophytes. The male gametophyte, like that of Selaginella, is 
developed inside the microspore and similarly consists of a small prothal- 
lial cell and an antheridium (Fig. 203). The latter has a sterile jacket of 
four cells investing four spermatogenous cells, each of which gives rise to a 




Fig. 202. Cross section of small portion of a microsporangium of Isoetes nuttaUii, showing 
the wall, tapetum, a trabecula, and spore mother cells, X400. 




A 



B 




F G H I J 

Fig. 203. Male gametophyte of Isoetes lacustris. A, microspore; B, prothallial cell cut 
ofT; C to F, formation of four jacket cells and primary spermatogenous cell; G and H, divi- 
sion of spermatogenous cell; / and J, formation of four sperms. H and / sectioned at right 
angles to G and I. (After Licbig.) 

single sperm. It is noteworthy that only four sperms are produced, as 
this is the lowest number in pteridophytes. The sperms are large, coiled, 
and multicihate, thus differing from those of the other Lycopodiinae 
(Fig. 2ME). 



246 



PLANT MORPHOLOGY 



The female gametophyte develops inside the megaspore and resembles 
in a general way that of Selaginella (Fig. 204A). The megaspore is 
uninucleate when shed. Free-nuclear division occurs, followed by wall 
formati(Mi in the apical region. Then walls fill in the entire megaspore 




D ^-^ F 

Fig. 204. Female gametophyte and archegonia of Isoetes echinospora {A to D), sperm of 
Isoetes malinveriana (E), and embryo of Isoetes lacustris (F). A, female gametophyte with 
a mature arfhegonium, the megaspore wall removed, X140; B, young archegonium with 
primary ne(-k cell, neck canal cell, and primary ventral cell, X400; C, later stage, showing 
two tiers of ne(;k cells, neck canal cell, ventral canal cell, and egg, X400; D, archegonium 
with mature egg, X4.30; E, sperm; F, embryo;/, foot; r, root; //', first leaf; Ip, second leaf; 
Ig, ligule; Is, sheath of first leaf. (A to D, after Campbell; E, after Belajeff; F, after Liebig.) 



cavity, developing centripetally as in gymnosperms. The female game- 
tophyte does not protrude, as it does in Selaginella, but a triradiate crack 
develops in the megaspore wall along which one or several archegonia and 
numerous rhizoids appear. In some species the rhizoids are few or want- 
ing. The archegonium is completely embedded. It consists of four tiers 
of neck cells, a single binucleate neck canal cell, a ventral canal cell, and 
an egg (Fig. 204B-Z)). No basal cell is formed in development. The 
archegonium of Isoetes, like that of Selaginella, represents an advanced 
type. 



PTERIDOPHYTA 247 

Embryo. The embryo of Isoetes differs from that of other living Lyco- 
podiiiiae in lacking a suspensor. The fertilized egg undergoes a trans- 
verse division, but both segments take part in the formation of the 
embryo proper. A quadrant stage is organized. It seems probable that 
the two outer cells form the foot, one of the two inner cells the root, and 
the other inner cell the leaf. As the embryo develops, a greater growth 
on one side causes it to curve until it finally becomes inverted, the foot 
lying below and the root and leaf above (Fig. 204F). The stem makes its 
appearance later between the root and leaf. It may originate from either. 

Summary. Isoetes has an unbranched tuberous stem bearing a rel- 
atively few large leaves, each of these having a ligule. The vascular sys- 
tem of the stem is a greatly reduced protostele, amphicribral in organ- 
ization, and -^-ith secondary thickening. A definite strobilus is not 
organized, unless the whole plant be considered as one. Isoetes is hetero 
sporous, the megasporangia producing many megaspores, the micro- 
sporangia a much greater number of microspores. Trabeculae are formed 
in both kinds of sporangia. There is no regular dehiscence. The male 
gametophyte, formed inside the microspore, consists of a single prothallial 
cell and a single antheridium, the latter producing four large multiciliate 
sperms. The female gametoph\i:e, developed inside the megaspore, has 
considerable vegetative tissue and one to several archegonia. These have 
only one neck canal cell. The embryo is \\'ithout a suspensor. Although 
having a number of characters in common with the other Uving lycopods, 
Isoetes occupies an isolated position because of its general habit, leaves, 
multiciUate sperms, and absence of a suspensor. 

3. EQUISETINAEi 

The Equisetinae, like the Lycopodiinae, are a group of ancient origin 
and were much more abundant and diversified during the Paleozoic than 
they are today (Fig. 258). They are characterized by jointed, longi- 
tudinally fluted stems bearing mostly small, simple leaves arising in 
whorls and usually united to form a sheath around each node. This 
cycUc arrangement of leaves is in marked contrast to the spiral arrange- 
ment characteristic of other pteridophytes. The sporangia are mostly 
numerous and borne on the underside of stalk-like sporangiophores that 
are nearly always organized to form a compact strobilus. Of the four 
orders — the Hyeniales, Sphenophyllales, Equisetales, and Calamitales — 
only the third has Uving members. 

1. Hyeniales 

The Hyeniales are the oldest and most primitive order of Equisetinae, 
in some respects resembling the Psilophytales. They comprise two 

* Also called Sphenopsida or Articulatae. 



248 



PLANT MORPHOLOGY 



genera, Hyenia and Calamophyton, both of which Hved during the middle 
Devonian. The aerial shoots were slender and dichotomously branched 
and, in one species of Hyenia, are known to have arisen from a stout 

horizontal rhizome. The stems 
were jointed in Calamo'phyton but 
not in Hyenia (Fig. 205). The 
leaves were small, narrow, and 
whorled. In Hyenia they were 
forked several times, in Calamo- 
'phyton forked only once. Little 
is known of the vascular anatomy. 
The stem of Calamo'phyton was 
apparently siphonostelic and is 
thought to have undergone some 
secondary thickening. 

The sporangia of the Hyeniales 
were borne on sporangiophores 
that were grouped to form a loose 
strobilus in which no bracts were 
present. In both genera the spo- 
rangiophores were once forked, the 
tip of each division being recurved 
and bearing two or three pendent 
sporangia in Hyenia, but only one 
sporangium in Calamophyton. 
Presumably the Hyeniales were 
homosporous. 




A 

Fig. 205. Calamophyton primaevum. A, 
reconstruction of aerial shoot; B, sterile 
leaves; C, sporangiophores. (After Krausel 
and Weyland.) 



2. Sphenophyllales 

This is an order of Paleozoic 
plants ranging from the Devonian 
to the Triassic. In many respects 
it is intermediate between the 
Lycopodiinae and Equisetinae; in fact, it is often considered as a separate 
class of pteridophytes. There are three important genera: Spheno- 
phyllum, Cheirostrobus, and Pseudobornia. 

The slender fluted stem bore whorls of leaves separated by elongated 
internodes, with branches arising at the nodes (Fig. 206). The leaves, 
usually six at a node, were mostly simple and wedged-shaped, but were 
often dichotomously divided into narrow lobes, while in Pseudobornia 
fern-like leaflets were present. The stem, m all cases, was a protostele 
with exarch xylem and a considerable amount of secondary thickening. 
The strobili were terminal and composed of whorled sporophylls show- 



PTERIDOPHYTA 



249 



ing little or no resemblance to the foliage leaves. In Sphenophylhim the 
bases of the sporophylls were united to form a cup-like sheath, but the 
tips were free (Fig. 207). The sporangia were borne singly or in pairs on 
long sporangiophores that arose from the adaxial side of the sporophylls, 




Fig. 206. Reconstruction of the shoot of Sphenophyllum cuneifoUum, one-third natural 
(From Gilbert M. Smith.) 



size. 



either singly or several together. The sporangia were pendent from the 
distal end of the sporangiophore, which was often expanded to form a ter- 
minal disk. The strobilus of Cheirostrohus, a form genus of Carboniferous 
age, was more complex than that of Sphenophyllum, each sporophyll con- 



250 



PLANT MORPHOLOGY 



sisting of three lower sterile segments arranged in one plane and three 
upper fertile ones. Each fertile segment was a sporangiophore bearing 
four sporangia. The strobilus of Cheirostrobus was the most complex one 
in all pteridophytes. 

All the Sphenophyllales were homosporous. Nothing is known of the 
gametophyte generation. 





A 1/ B 

Fig. 207. Sphenophyllum dawsoni. A, diagram of longitudinal section of cone, showing 
three whorls of sporophylls and, above, whorl of sporophylls in surface view seen from the 
inside; B, diagram of one-half of a single whorl of sporophylls and sporangiophores. (A, 
after Scott; B, after Hirmer.) 



3. Equisetales 

The Equisetales, often called horsetails, are herbaceous plants compris- 
ing a single surviving genus, Equisetum, with about 25 species cosmopoli- 
tan in distribution. Although related to Paleozoic forms, this order 
became prominent in the Mesozoic. Equisetites, one of the Triassic 
horsetails, had a stem 20 cm. in diameter and in general was built on a 
vastly grander scale than modern forms. The common horsetails of 
temperate regions grow in swamps, meadows, forests, and sandy wastes. 

Sporophyte. The largest living species, Equisetum giganteum, of trop- 
ical America, reaches a height of 12 m. but has a weak stem only about 
2 to 3 cm. in diameter at the base. Most of the other species are less than 



PTERIDOPHYTA 



251 



1 m. tall. The sporophyte of Eqiiisetum has a horizontal branching rhi- 
zome with whorled leaves at the nodes. It gives rise to erect green shoots 
that may be either simple or monopodially branched, the branches, like 
the leaves, arising in whorls (Fig. 208). In some species the shoots 




Fig. 208. Erect shoots of Equisetum hyemale. 
bear whorls of scale-like leaves at the nodes. 



The stems are green, unbranched, and 



branch repeatedly. The stems are longitudinally ridged and grooved, the 
ridges of one internode alternating with those of the internode immedi- 
ately above and below. The stems are more or less impregnated with 
silica, giving them a rough, harsh feel. Roots occur on the rhizome and at 
the base of the erect stems. They arise in whorls at the nodes. 

The nodes are soUd but the internodes have a large central cavity. 
The aerial stems carry on practically all the work of photosynthesis. 
The leaves are scale-like, their tips being free but their bases united to 



252 



PLANT MORPHOLOGY 



form a sheath around the node. The number of leaves at a node corre- 
sponds to the number of ridges on the stem, each leaf standing directly 
above a ridge of the intcrnode directly below it. The stem branches are 
not axillary but arise at the node alternately with the leaf primordia and 
at the same level, later breaking through the united leaf bases. Thus the 
number of branches at a node usually equals the number of leaves. 
Growth of the root and stem takes place by means of a tetrahedral apical 

cell that cuts off three rows of lateral 
segments with striking regularity (Fig. 

209). 

Vascular Anatomy. The stem of 
Equisetum is characterized by a much- 
reduced vascular system. The greatest 
development of xylem occurs at the 
nodes, where it forms a transverse 
band. From here a leaf trace goes to 
each leaf, forming a single median vein. 
A cross section through an internode 
shows an extensive cortex bounded ex- 
ternally by an epidermis, a circle of 
small, isolated, vascular bundles sepa- 
rated from one another by broad bands 
of parenchyma, and a hollow pith (Fig. 
210^1). The cortex is pecuhar in hav- 
ing a ring of large air spaces, called 
vallecular canals, one of which lies 
beneath each furrow present on the outer surface of the stem. 

The epidermis has thick, strongly sihcified cell walls. Underlying it is 
a band of sclerenchyma projecting inward beneath the ridges and some- 
times not continuous across the grooves. Green tissue occupies most of 
the cortical region, occurring largely or entirely beneath the furrows. 
Stomata, communicating with the green tissue, are situated in the 
grooves. Their guard cells are peculiar in that each Ues inside and next 
to a subsidiary cell, so that they seem to be double. 

Internal to the vallecular canals and alternating with them, and so 
lying beneath the ridges, are the smaller carinal canals, one of which 
belongs to each vascular bundle. The carinal canals mark the position 
of the protoxylem, the disorganization of which results in their formation. 
The metaxylem, which is greatly reduced in amount, lies along the sides 
of and external to the carinal canals (Fig. 210B). It develops centrif- 
ugally, a condition designated as endarch. A small group of phloem 
elements, consisting of sieve tubes and parenchyma, are present. The 
phloem has a collateral relation to the xylem. No secondary tissues are 




P"iG. 209. Median longitudinal sec- 
tion through the stem tip of Equisetum 
arvense, showing apical cell and its 
derivatives, X200. 



PTERIDOPHYTA 



253 



formed. Generally a continuous endodermis surrounds the ring of vas- 
cular bundles, but often an endodermis occurs both inside and outside the 
bundles. Sometimes each bundle is enclosed by its own endodermis. 
Although the vascular bundles are small and widely separated, because of 




outer 
endodermis 

carina/ 
canal 



inner 
enaodermis 



mefaxylem 
protoxylem 



Fig. 210. Stem structure of Equisetum hyemale var. intermedium. A, portion of cross 
section of an internode, showing thick-walled epidermis with three stomata, sclerenchyma, 
green tissue, two vallecular canals, and three vascular bundles, X 100; B, a single vascular 
bundle, X250. 

their arrangement and the position of the xylem and phloem, the stem of 
Equisetum is an ectophloic siphonostele. 

Sporangium. The strobili of Equisetum are solitary and terminal on 
the main stem or sometimes on its branches. Generally they are borne on 
ordinary green shoots, but in some species they occur on special shoots. 
In Equisetum arvense, for example, there are two kinds of aerial shoots. 
They arise from the same rhizome but at different times of the year. 



254 



PLANT MORPHOLOGY 




The first shoots to appear above ground in the spring are fertile but are 
unbranched, yellowish brown, and lacking in chlorophyll. They wither 
soon after the spores are shed. Green, branching, sterile shoots then 
appear and persist throughout the summer. 

The strobilus of Equisetum consists of a central axis bearing numerous 
whorled sporangiophores (Fig. 211A). The development of sporophylls 

is entirely suppressed. Each sporan- 
giophore is peltate and bears 5 to 10 
pendent, sac-like sporangia attached 
to the margin of a six-sided disk. This 
is supported by a short stalk that arises 
directly from the axis of the strobilus 
and is perpendicular to it (Fig. 2115, 

C). 

The sporangium, which is eusporan- 
giate in development, arises from a 
single superficial initial and not, as in 
the Lycopodiinae, from a row of initials 
(Fig. 212). However, the sporogenous 
tissue is derived not only from the 
inner segment that results from the first 
periclinal division of the initial but also, 
in part, from the outer segment (Fig. 
212D). The tapetum is derived from 
the wall, which becomes several layers 
thick but, when the sporangium is ma- 
ture, consists of a single layer of cells, 
the inner layers breaking down. In 
contrast to that of the lycopods, the 
tapetum becomes two- or three-layered and soon disorganizes, forming a 
Plasmodium around the spore mother cells. Following the formation of 
tetrads, the tapetal Plasmodium is absorbed by the developing spores. 
Equisetum is homosporous. The spores are unique in containing numer- 
ous chloroplasts when ripe and in having, on the outside, two slender 
bands derived from the outer layer of the spore wall (Fig. 211D). These 
are hygroscopic and assist in spore dispersal. Each band is attached to 
the spore at its middle, the tips being spatulate. The bands uncoil when 
dry and wrap around the spore when moist. When the spores are ripe, 
the axis of the cone elongates slightly, separating the sporangiophores. 
The sporangia dehisce by means of a longitudinal slit that appears on 
their inner side. 

Gametophyte. The spores of Equisetum are short-Uved and germinate 
at once. The gametophyte is usually less than 10 mm. in diameter. It 




wr't 



A C 

Fiu. 211. Spore-beariiig structures 
of Equisetum arvense. A, terminal 
portion of fertile shoot with strobilus, 
natural size. B, a single sporangio- 
phore, X15; C, longitudinal section 
of same, X15; D, two ripe spores, 
X150. 



PTERIDOPHYTA 



255 



consists of a rounded, cushion-like base of colorless tissue that gives rise to 
numerous upright lobes of green tissue (Fig. 213A). These are irregular, 
thin, and plate-like. Rhizoids are abundantly produced on the lower 
surface. The base has a marginal meristem. Both kinds of sex organs 




Fig. 212. Early development of the sporangium of Equisetum arvense. A, longitudinal 
section of young sporangiophore with two sporangium initials, X200; B, longitudinal 
section of young .strobilus, X12; C, a sporangiophore of same, showing first division of 
initials, X200; D, longitudinal section of one-half of sporangiophore, showing early dif- 
ferentiation of sporogenous tissue, X200; E, longitudinal .section of slightly older strobilus 
than shown above, X8; F, a sporangium of same, showing further development of sporog- 
enous tissue and differentiation of tapetum, X 200. 



are borne on the basal portion of the same prothallium but are seldom 
present at the same time. The archegonia usually appear before the 
antheridia. When closely crowded, the prothallia are small and bear 
only antheridia. For this reason, they were once erroneously regarded 
as dioecious. The development of the antheridium resembles that of the 
lycopods (Fig. 21ZB-E). The superficial initial divides by a transverse 
wall, the outer segment producing the sterile jacket and the inner one the 
spermatogenous tissue. The sperms are large, coiled, and multiciliate. 



256 



PLANT MORPHOLOGY 



The archegonium develops as in other pteridophytes, but no basal cell is 
formed (Fig. 213F-H). It has either one neck canal cell or two of them 
separated by a vertical wall. 




D H 

Fig. 213. Equisetum telmateia. A, gametophyte with archegonia occurring beneath the 
upright lobes, X38; B, young antheridium, showing sterile jacket and spermatogenous 
tissue; C, slightly older antheridium; D, mature antheridium with nearly ripe sperms; E, 
sperm of Equisetum arvense; F, young archegonium with primary neck cell and central cell; 
G, slightly older archegonium, showing neck cells, primary neck canal cell, and primary ven- 
tral cell; H, nearly mature archegonium with neck canal cell, ventral canal cell, and egg; 
B, C, D, X210; F,'g, H, X325. {A, after Walker; E, after Sharp; B, C, D and F, G, H, after 
Gilbert M. Smith.) 

Embryo. The embryo does not have a suspensor. The fertihzed egg 
divides transversely, then into quadrants, the inner segments forming the 
foot and root, the outer ones the stem and leaf. The stem and root seg- 
ments soon form an apical cell. 

Summary. The elongated, jointed, longitudinally fluted stem bears 
numerous small, simple, whorled leaves united to form a sheath around 



PTERIDOPHYTA 



257 




Fig. 214. Portion of restoration of Carboniferous swamp lorest m the Chicago Natural 
History Museum, showing Calamites (tree in center) and a fallen trunk of Sigillaria. The 
small plants in the foreground are Sphenophyllum emarginatum. 

each node. Branches, where present, are also whorled and arise alter- 
nately with the leaves. The vascular system of the stem is a much- 
reduced ectophloic siphonostele with widely separated, endarch, collateral 
bundles. There is no secondary thickening. All species are homospo- 
rous and have a definite strobilus composed of whorled, peltate sporangio- 
phores, each bearing 5 to 10 pendent sporangia with longitudinal dehis- 



258 



PLANT MORPHOLOGY 



cence. Sporophylls are absent. The prothallium is aerial and cushion- 
like, with erect, green, ribbon-like lobes. The sperms are numerous, 
large, and multiciUate. The archegonia have one or two neck canal cells. 
The embryo lacks a suspensor. The Equisetales are a distinct order, 
superficially unUke any other group of living pteridophytes. 

4. Calamitales 

This is a Paleozoic order closely related to the Equisetales and often 
combined with it. It ranged from the Devonian to the Triassic. The 





A B 

Fig. 215. Longitudinal sections of cones of Calamitales. A, Palaeostachya, showing 
peltate sporangiophores in axils of bracts, diagrammatic; B, Archaeocalamites, showing 
axis bearing sporangiophores only. (A, after Scott; B, after Renault.) 

principal genus is Catamites (Fig. 214). The Calamitales were tree-like 
forms, some reaching a height of 20 or 30 m. and a diameter of 1 m. The 
hollow stems bore whorled leaves and branches, the leaves being either 
free or united at the base. Although mostly small and narrow, the leaves 
were larger than in modern horsetails, while in Archaeocalamites they were 
large, dichotomously divided into narrow segments, and somewhat fern- 
like in appearance. 

The vascular anatomy was of an advanced type. The stem w^as an 
ectophloic siphonostele, the primary xylem occurring in isolated, collateral 
vascular bundles arranged in a circle around a hollow pith. The bundles 
were prevailingly endarch but were mesarch in Protocalamites. The 
young stem of the Calamitales was essentially similar to an adult stem of 
Equisetum but became different as a result of secondary thickening, a 



PTERIDOPHYTA 259 

feature of the group. Generally the primary tissues were surrounded by 
a continuous cylinder of secondary wood. 

The strobili were made up of whorled, peltate sporangiophores resem- 
bling those of Equisetum, except that each bore only four pendent spo- 
rangia. In Archaeocalamites the cone consisted entirely of sporangio- 
phores, but in most of the other genera bracts were also present, a whorl 
of bracts alternating with a whorl of sporangiophores (Fig. 215). These 
bracts have been interpreted by some botanists as sporophylls. In Cala- 
mostachys the alternating whorls of bracts and sporangiophores were 
equidistant, but in Palaeostachya the sporangiophores were situated just 
above each whorl of bracts, i.e., in their axils. 

The Calamitales were either homosporous or heterosporous, depending 
on the species. Many megaspores were produced in each megasporan- 
gium. The difference between the two kinds of spores was not so pro- 
nounced as in the heterosporous lycopods and ferns. The gametophyte 
generation is unknown. 



CHAPTER VII 
PTERIDOPHYTA (CONTINUED) 

4. FlLICINAEi 

The ferns constitute the largest and most representative group of pter- 
idophytes of the present day, numbering about 7,800 species. They are 
widely distributed over the earth, nearly all growing in moist, shady 
places. Although making their best display in the tropics, both in num- 
ber of species and in luxuriance of growth, they are also well represented 
in temperate regions. The branched or unbranched stem usually bears a 
few large, spirally arranged leaves that are sometimes simple but are gen- 
erally divided into leaflets. There are no strobili, the sporangia being 
very numerous on the margin or abaxial side of the leaves, or borne in 
special structures called sporocarps. The sporangia may be solitary but 
more commonly are borne in groups. 

Like other vascular plants, the Filicinae possess branch, leaf, and root 
traces that arise from the stele of the stem and pass outward through 
the cortex. In all the Filicinae, except those with protostelic stems, the 
departure of a leaf trace causes an interruption in the continuity of the 
stele, forming a leaf gap (Fig. 221). Leaf gaps are present in ferns and 
seed plants but not in the lower pteridophytes. Branch gaps are pres- 
ent, however, in all vascular plants having siphon ostelic stems (except in 
Equisetum) . 

The Filicinae were well represented in the Paleozoic, but did not hold as 
dominant a place in the flora as was once thought (Fig. 258). Most of the 
fossil fern leaves found in Carboniferous deposits belong to the Cycado- 
filicales, an order of primitive gymnosperms. Some of the Paleozoic 
ferns may be referred to two orders with living representatives (Marat- 
tiales and Filicales), but most of them belong to the Coenopteridales, an 
extinct order whose relationships to the others are not clear. Living 
ferns belong to four orders: Ophioglossales, Marattiales, Filicales, and 
Hydropteridales. 

1. Coenopteridales 

The Coenopteridales range from the Devonian to the Permian. They 
are regarded as the most primitive group of Filicinae, in some respects 
resembling the Psilophytales. These ferns were all of small or medium 
size. The stems were erect or horizontal and always protostelic. The 

' Sometimes combined with the Spermatophyta under the name of Pteropsida. 

260 



PTERIDOPHYTA 



261 



stele was either circular in outline (Botryopteridaceae) or more or less 
lobed (Zygopteridaceae). In the most primitive members division of the 
frond was not limited to one plane and the leaf stalk bore a series of 
bifurcating branches (Fig. 216). In some cases the differentiation 
between stem and leaf was imperfect in that no blade was formed. 




Fig. 216. Reconstruction of the leaf of Etapteris lacattei; fertile portion above and sterile 
portion below; one-quarter natural size. (After Hirmer.) 

The Coenopteridales were eusporangiate and homosporous. The spo- 
rangia were large and either terminal or marginal on the ultimate divisions 
of the frond. Sometimes the sporangia were united to form a synangium- 
like structure. The output of spores was large. The sporangium wall 
was more than one layer of cells thick. Dehiscence took place by a ter- 
minal pore or a longitudinal slit; in the latter case a rudimentary annulus 
was present. 

2. Ophioglossales 

The Ophioglossales comprise 3 genera and about 80 species. Ophio- 
glossum and Botrychium, each with about the same number of species, are 



262 



PLANT MORPHOLOGY 



widely distrilnited, while Helminthostachys, with a single species, is con- 
fined to Polynesia and tropical Asia. The Ophioglossales are unknown as 
fossils but constitute the most primitive order of Uving ferns. 

Sporophyte. Most of the Ophioglossales are erect terrestrial plants. 
The stem is a short, upright, unbranched rhizome producing a few large 




Fig. 217. Ophioglossum calif ornicum, about one and one-half times natural size. 

leaves and numerous rather fleshy roots. Usually only one leaf is formed 
each year. The smallest species of Ophioglossum and Botrychium are less 
than 8 cm. tall, but several species of Botrychium may reach a height of 
60 cm. Ophioglossum pendulum, an epiphyte of the Oriental tropics, has 
a creeping stem and pendent leaves that are frequently 1.5 m. long. The 
leaf of Helminthostachys is about 30 cm. in length. 

As a rule, the leaf blade of Ophioglossum is simple, while that of Botrych- 
ium is pinnately divided (Figs. 217 and 218). The leaf blade of Helmin- 



PTERIDOPHYTA 



263 



thostachys is palmately divided. Except in Botrychium virginianum, the 
leaves are fleshy. The blade has many veins that in Ophioglossum are 
reticulate but in the two other genera branch dichotomously and end 
freely. At the apex of the rhizome is a large bud containing the primordia 
of leaves that expand diu'ing the next four or five seasons. In all genera 







Fig. 218. Botrychium dissectum, about one-half natural size. (From Chamberlain.) 

the stem tip and each successive leaf are ensheathed by the base of the next 
older leaf. The vernation of the Ophioglossales is not circinate, as in the 
higher ferns, but erect. Both the root and stem increase in length by 
means of a tetrahedral apical cell. 

Vascular Anatomy. The leaf blade is simple in structure, with an 
epidermis enclosing uniform mesophyll. Stomata may occur on both 



264 



PLANT MORPHOLOGY 




Fig. 219. Cross section of the stelar portion of the root of Ophioglossum calif or nicum, 
showing the thick-walled xylem in contact with the thin-walled phloem, both surrounded by 
the pericycle and endodermis, X250. 




Fig. 220. Cross section of the stelar portion of the root of Botrychium virginianum, show- 
ing four xylem groups, X 150. 



sides of the blade or only on the lower side. In O'phioglossum the root is 
very simple in structure, generally having but one xylem group and either 
one or two phloem groups (Fig. 219). In Botrychium the root has two to 
four xylem groups (Fig. 220), in Helminthostachys four to seven. The 
roots of all genera are exarch and without secondary thickening. 

The vascular anatomy of the stem is of an advanced type. In Botrych- 



PTERIDOPHYTA 



265 



ium and Helminthostachys the stem is an ectophloic siphonostele, while in 
Ophioglossum, because of the presence of very large, overlapping leaf gaps, 
the bundles are widely separated and a dictyostele is formed. In all 
genera the relation of the xylem to the phloem is collateral. The devel- 
opment of the xylem is mesarch in Helminthostachys and endarch in the 
two other genera. A notable feature of Botrychium is the presence 
of a stelar cambium and of marked secondary thickening (Fig. 221). 




-^J 



Fig. 221. Cross section of the central portion of the rhizome of Botrychium virginianum, 
an ectophloic siphonostele, X 72. Most of the xylem is of secondary origin. 

Although a considerable amount of secondary xylem is formed, there is 
httle or no secondary phloem. The vascular rays are one layer of cells in 
width. An advanced feature is the occurrence, in Botrychium and Hel- 
minthostachys, of large tracheids with bordered pits instead of the sca- 
lariform markings found in other ferns. In Ophioglossum the tracheids 
are reticulate. 

Sporangium. All the Ophioglossales are homosporous and eusporan- 
giate. The most distinctive feature of the group is the presence of a 
"fertile spike," a sporangium-bearing stalk that arises from the basal por- 
tion of the leaf as a specialized leaf segment. In Ophioglossum the fertile 
segment is relatively simple, being cylindrical and unbranched, and bear- 
ing two lateral rows of sunken sporangia (Fig. 217). Each sporangium 
does not seem to arise from a single initial cell but from a small group of 
initials. 



266 PLANT MORPHOLOGY 

In Botrychium the fertile segment is more complex than in Ophioglos- 
sum. In nearly all species it is pinnately branched, the narrow divisions 
bearing two rows of spherical sporangia that are not embedded but project 
on a very short stalk (Fig. 218). In development, the sporangia arise 
separately, each from a single initial. In Helminthostachys the fertile seg- 
ment is spike-like and bears two rows of crowded, oval, stalked sporangia, 
a number of which may be borne on a single stalk. The vascular anat- 
omy of the leaf and the occurrence of occasional reversions indicate that 








Fig. 222. Cross section of the petiole of Botrychium virgiiiianum, X 10. 

the fertile segment of Ophioglossiim and Botrychium represents two united 
basal leaflets, while in Hehninthostachys it represents a single leaflet. 

In almost all the Ophioglossales each leaf trace arises from the stele of 
the stem as a single strand, but branches before or as it enters the leaf. A 
cross section of the petiole of Ophioglossum shows a single row of vascular 
bundles arranged in a circle, those on the adaxial side passing into the 
"fertile spike" and the others going into the sterile blade. The petiole 
of Botrychium, just below the fertile segment, usually displays two pairs 
of vascular bundles arranged symmetrically on each side (Fig. 222). 
From the two larger ones, which are crescent-shaped, a pair of smaller 
bundles branch off and pass into the fertile segment, while at higher levels 
two small bundles similarly depart to each pair of sterile leaflets. 

The sporangium wall, in the Ophioglossales, is about five layers of cells 
thick. No annulus is formed. The tapetum is probably derived from 
the innermost layer of wall tissue and may consist of one layer or several 
layers of cells. In all three genera the tapetum is peculiar in that the 



PTERIDOPHYTA 



267 



protoplasts are liberated from their cells before the spores are in the tetrad 
stage (Fig. 223). These protoplasts fuse to form a multinucleate Plas- 
modium that surrounds groups of spore mother cells and contributes 
nourishment to them. These groups break up just prior to the formation 
of tetrads. The Plasmodium disappears as the spores mature. Each 
sporangium produces a large number of spores. Dehiscence occurs by 




Fig. 223. Longitudinal section through portion of young fertile spike of Ophioglossum 
californicum, showing groups of spore mother cells surrounded by multinucleate Plasmo- 
dium derived from the tapetum, X 130. 



means of a longitudinal slit in Helminthostachys and by a transverse slit in 
the two other genera. 

Gametophyte. In all the Ophioglossales the prothallium is subterra- 
nean, saprophytic, and without chlorophyll. An endophytic fungus is 
always present. The prothallium of Ophioglossum is cylindrical and 
either simple or branched (Fig. 22-iC, D). It may reach a length, in 
Ophioglossum vulgatum, of 6 cm. Rhizoids are wanting. The antheridia 
and archegonia are scattered and intermixed. The gametophyte of 
Botrychium is tuberous, dorsi ventral, and flattened (Fig. 224A). It 
reaches a length, in some species, of 18 mm., in others, of only 3 mm. 
The surface may be smooth or covered with rhizoids, according to the 
species. The antheridia are borne on a median dorsal ridge, while the 
archegonia, appearing later, form a row on each side of the ridge (Fig. 
224B). In Helminthostachys the prothallium is somewhat similar to that 
of Botrychium, but is more irregular and has a lobed basal portion that 



268 



PLANT MORPHOLOGY 



gives rise to an upright cylindrical branch bearing sex organs. As in 
Bolnjchium, the antheridia appear before the archegonia. 

The antheridium of the Ophioglossales is large and sunken (Fig. 225 A, 
B). It develops as in the lower pteridophytes, the spermatogenous tissue 




B D 

Fig. 224. Prothallia of Ophioglossales. A, prothallium of Botrychium virginianum with 
dorsal ridge bearing antheridia, X 16; 5, cross section of same, showing antheridia on ridge, 
archegonia on the sides, and fungal zone below, X17; C and D. Ophioglossum vulgatiim; 
C, entire prothallium, about X2; £), one-half of a prothallium with antheridia and arche- 
gonia on surface and, to left, a young sporophyte with first root, X30. (A and B, after 
Jeffrey; C and D, after Bruchmann.) 

arising from the inner segment resulting from a periclinal division of the 
superficial initial. In Ophioglossum the wall remains one-layered, but in 
the two other genera it becomes two-layered. The sperms are numerous, 
large, coiled, and multiciliate (Fig. 225C). The archegonium initial is 
also superficial and divides periclinally, the outer cell forming the neck 
and the inner one giving rise to the central cell and basal cell (Fig. 
225D-G). A basal cell is not present, however, in some species of 
Botrychium. The axial row, derived from the central cell, consists of a 



PTERIDOPHYTA 



269 



single binucleate neck canal cell, a very inconspicuous ventral canal cell, 
and an egg. 

Embryo. The eml^ryo of Botrijchium dissectum and that of Helmintho- 
stachys are uniciue in that a suspensor is developed, but in all the other 




H G 

Fig. 225. Sex organs of Ophioglossaies. A, antheridium of Ophioglossum vulgcdum, 
X150; B, antheridia of Botrychium virginianum, X200; C, sperm of Ophioglossum; D to G, 
development of archegonium of Ophioglossum vulgatum, X225; D, first division of initial; 
E, young archegonium with two neck cells, central cell, and basal cell; F, later stage, show- 
ing division of central cell; G, mature archegonium; H, young embryo of Botrychium vir- 
ginianum, X250. {B and H, after Jeffrey; others after Bruchmann.) 



Ophioglossaies none is present. The first wall in the fertilized egg is 
transverse and usually a quadrant stage is formed, but the subsequent 
divisions are irregular and rather indefinite (Fig. 225//). The embryonic 
organs are differentiated rather late, and so it is not possible to assign 
them to definite quadrants. The entire inner portion of the embryo 
forms the foot, while the outer portion gives rise to the root, stem, and 



270 PLANT MORPHOLOGY 

leaf. Generally the root arises first and grows considerably before the 
other organs are differentiated. The leaf is the last member to appear. 
Summary. The Ophioglossales are homosporous and eusporangiate. 
The sporangia are borne on a characteristic "fertile spike," which prob- 
ably represents a single leaflet in Helminthostachys and two united basal 
leaflets in Ophioglossian and Botrychium. The sporangium wall is several 
layers of cells thick and is without an annulus. The leaves are erect in 
vernation, not circinate. The gametophyte is subterranean, saprophytic, 
and without chlorophyll. It contains an endophytic fungus. The 
antheridium develops as in the other eusporangiate pteridophytes. The 
inner portion of the embryo forms the foot, the outer portion the root, 
stem, and leaf. The Ophioglossales have a number of distinctive fea- 
tures. The vegetative structure of the sporophyte is advanced, but the 
spore-producing structures and the gametophyte are primitive. The 
order may have been derived from the Coenopteridales, but does not seem 
to have given rise to any other modern group. 

3. Marattiales 

The Marattiales are an ancient order of ferns extending back into the 
Paleozoic. In certain respects they are intermediate between the Ophio- 
glossales and Filicales. Although once widespread and abundant, they 
are represented today by only 7 genera and about 55 species almost 
exclusively tropical in distribution. The largest genus is Angiopteris, 
with 25 species. It is found only in the Eastern Hemisphere. Two other 
important genera, each with about 13 species, are Marattia, occurring in 
tropical regions throughout the world, and Danaea, confined to tropical 
America. The four other genera, each with a single species, are confined 
to southern Asia. 

Sporophyte. The Marattiales are mostly large ferns with thick fleshy 
leaves (Fig. 226). In most species of Danaea the stem is creeping and 
occasionally branched, but in nearly all other members of the order it is 
short, stout, erect, and unbranched. The stem is always covered with 
persistent leaf bases. The roots are thick and fleshy and the leaves, 
which in Angiopteris may exceed 5 m. in length, are in nearly all cases 
pinnately divided. A peculiarity of the group is the occurrence of a pair 
of fleshy stipules at the base of each leaf. The venation is dichotomous 
and open except in one genus (Christensenia) , where it is reticulate. The 
vernation is circinate throughout the order. Elongation of the root and 
stem takes place by means of a meristem, an apical cell being present only 
in young plants. In possessing an apical meristem, the Marattiales differ 
from all other ferns. 

Vascular Anatomy. A cross section through a leaf reveals an epidermis 
on both surfaces, with stomata present only below. Palisade tissue is 



PTERIDOPHYTA 



271 




Fig. 226. Angiopteris evecta. (From Wettstein.) 




Fig. 227. Cross section of the stelar portion of the root of Marattia, an exarch radial 
protostele with many protoxylem points, X75. 



272 



PLANT MORPHOLOGY 



developed beneath the upper epidermis, the rest of the mesophyll consist- 
ing of spongy tissue. The roots of the Marattiales are characterized by a 
hirge numbeitof protoxylem points (Fig. 227). The xylem is usually 
lignified to the center. 

The vascular anatomy of the stem is very complex. The cortex con- 
sists of parenchyma containing mucilage canals, but no sclerenchyma is 
present. The vascular cylinder is a dictyostele with large overlapping 
leaf gaps and widely separated bundles. The latter, in all genera, are 









Fig. 228. Portion of leaflet of Angiopteris (A) with sori, leaflet of Marattia (B) with oval 
synangia, and portion of leaflet of Danaea (C) with elongated sunken synangia, X3. 

amphicribral (phloem surrounding the xylem) and either mesarch or 
endarch in development. In Danaea the vascular bundles are seen, in a 
cross-sectional view of the stem, to be arranged in a single circle surround- 
ing the pith. In Marattia the stem is more complex in that two concen- 
tric circles of bundles are present, while in Angiopteris, where the stem 
reaches its greatest degree of complexity, there is a series of four or five 
circles of vascular bundles. In all the Marattiales the stem bundles 
undergo more or less branching and fusion, and commissural strands, con- 
necting certain parts of the vascular system with one another, arise inside 
the dictyostele. Secondary thickening does not occur. As in the higher 
ferns, the tracheids are scalariform. 

Sporangium. The Marattiales resemble the Ophioglossales in being 
homosporous and eusporangiate, in having a sporangium wall consisting 
of several layers of cells, and in lacking an annulus. The sporangia, how- 



PTERIDOPHYTA 



273 



ever, are not borne in a "fertile spike" but on the abaxial side of the 
leaves. Generally the fertile and sterile leaves are alike in form, but in 
Danaea they are different. The sporangia are sessile and borne in distinct 
sori, these being generally in two rows. In Angiopteris the sporangia are 
free (Fig. 228.4), but in Marattia and Danaea they are united to form 
synangia. The synangia of Marattia are superficial, oval, or rounded, and 
borne near the ends of the veins (Fig. 228B) . The synangia of Danaea are 
sunken, linear, and borne along the veins; they cover almost completely 
the backs of the fertile leaflets (Fig. 228C). 




Fig. 229. Section through two sporangia of Angiopteris evecta, showing wall, tapetum, 
and sporogenous tissue, X200. 

All the sporangia in a sorus originate at the same time, each arising 
from a single initial cell. The sporogenous tissue is differentiated early 
and, from the cells immediately surrounding it, the tapetum, consisting 
of one or two layers, is derived (Fig. 229). The tapetum breaks down 
when the spore tetrads are formed, its substance being absorbed by the 
developing spores. It does not form a Plasmodium. The sporangium 
of the Marattiales is a relatively large structure, producing a great many 
spores (1 ,500 to 7,000) . A rudimentary annulus is present in Angiopteris, 
but there is none in Marattia and Danaea. Dehiscence takes place by 
means of a median slit or, in Danaea, by a terminal pore. 

Gametophyte. The Marattiales have a comparatively large gameto- 
phyte, sometimes reaching a length of 3 cm., and consisting of a flat, dark 
green thallus that may be heart-shaped, orbicular, or irregularly^ lobed. 
It resembles the gametophyte of the Filicales except that it is relatively 
thick, long-lived, and, as in the Ophioglossales, provided with an endo- 
phytic fungus. The median portion of the prothallium forms a thick 



274 



PLANT MORPHOLOGY 



cushion. Rhi/oids arise from the ventral surface; in Angiopteris and 
Marattia they are unicelhilar, but in Danaea they are septate. The 
anthoridia occur on both the upper and lower surfaces of the prothallium, 
but the archegonia are confined to the lower side, where they are borne on 
the thickened median portion. Both antheridia and archegonia are 
sunken. They develop as in the Ophioglossales (Figs. 230 and 231). In 
the archegonium a basal cell is usually formed in Marattia but not in 





B 





C D 

Fig. 230. Early stages in the development of the antheridium of Angiopteris evecta, X350. 

A. division of antheridial initial into primary wall cell and primary spermatogenous cell; 

B, anticlinal division of primary wall cell and periclinal division of primary spermatogenous 
cell; C, further divisions of spermatogenous cells; D, slightly later stage, showing completion 
of antheridial wall by cutting off of a layer of cells from adjacent cells of the prothallium. 
(After Haupt.) 

Angiopteris and Danaea. There may be either two neck canal cells 
or a single binucleate one. As in all ferns, the sperms are coiled and 
multiciliate. 

Embryo. The development of the embryo is unlike either that of the 
Ophioglossales or of the higher ferns. The first division of the fertilized 
egg is transverse and a quadrant stage is organized. The two outer seg- 
ments (those next to the neck of the archegonium) give rise to the foot, 
the two inner ones to the stem and leaf, the leaf arising from the segment 
nearer the apical region of the prothallium. The root appears later from 
the inner portion of the embryo. A suspensor has been observed in some 
species of Danaea and exceptionally in Angiopteris. 

Summary. Like the Ophioglossales, the Marattiales are homosporous 
and eusporangiate, but the sporangia are borne on the abaxial side of the 
leaves, usually in synangia. The sporangium wall consists of several 
layers of cells and is without a definite annulus. The vernation is circi- 
nate. The leaves have a pair of fleshy stipules. The gametophyte is 



PTERIDOPHYTA 



275 



flat, green, and aerial. It is relatively thick and contains an endophytic 
fungus. The development of the antheridium is like that of the Ophio- 
glossales. The inner portion of the embryo forms the stem, leaf, and root, 




Fig. 231. Development of the archegonium of Angiopteris evecta, X350. A, division of 
archegonium initial into primary neck cell and central cell; B, anticlinal division of primary 
neck cell; C and D, periclinal division of central cell to form neck canal cell and ventral 
cell; E, formation of two tiers of neck cells; F, division of ventral cell to form ventral canal 
cell and egg; G and H, formation of a single binucleate neck canal cell, of three tiers of neck 
cells, and of the sterile jacket; I, mature archegonium with egg ready for fertilization. 
{After Haupt.) 



the outer portion the foot. The Marattiales have characters in common 
both with the Ophioglossales and the Filicales and, in their degree of com- 
plexity, occupy a position between them. These three orders do not form 
a phylogenetic series. Instead, they seem to have been derived indepen- 
dently from a common ancestry. 



276 



PLANT MORPHOLOGY 



4. Filicales 

The Filicales constitute by far the largest order of modern ferns, includ- 
ing 12 families, about 1 70 genera, and approximately 7,600 species. They 
make their greatest display in the tropics, but are also well represented in 
temperate regions. The fossil record of the two most primitive families 
(Osmundaceae and Schizaeaceae), and possibly of a third one (Gleichen- 
iaceae), extends back into the Paleozoic, but that of the other families 
does not reach beyond the Mesozoic. 




Fig. 232. Gleichenia costaricensis, photographed near the summit of the Pods volcano in 
Costa Rica. 

Families. The Filicales comprise 7 principal families and 5 small ones 
of minor importance. All of them have living representatives. The 
chief families are as follows: 

1. Osmundaceae. This is the most primitive family. It comprises 
3 genera and about 20 species and occurs in both temperate and tropical 
regions. Osmunda is the principal genus, including over one-half the 
species. It is widely distributed, three species being found in temperate 
parts of North America. 

2. Schizaeaceae. This primitive family, including 4 genera and about 
160 species, is chiefly tropical. Schizaea and Lygodium are widely dis- 
tributed. Anemia, the largest genus, is found in tropical America, while 
Mohria is confined to eastern Africa. The family is represented in the 
Eastern and Southeastern United States by two species of Schizaea, one of 
Lygodium, and two of Anemia, all of which are rare. 



PTERIDOPHYTA 



277 



3. Gleicheniaceae. This small family of about 130 species includes 3 
genera, of which Gleichenia, with all the species but two, is of greatest 
importance (Fig. 232). It is confined to the tropics and subtropics of 
both the Eastern and Western Hemispheres. 




Fig. 233. Dicksonia antarctica, a tree fern cultivated in the Huntington Botanical Gardens 
at San Marino, California. 

4. Hymenophyllaceae. These are the "filmy ferns," small delicate 
forms chiefly tropical in distribution. There are about 400 species, nearly 
equally divided between Hymenophyllum and Trichomanes. Two species 
of Trichomanes are found in the Eastern United States. 

5. Dicksoniaceae. To this group belong 9 genera and about 125 species 
mainly tropical in distribution. The chief genera are Dicksonia, Cibo- 
tium, and Dennstaedtia.^ The first two are arborescent (Fig. 233). One 
species of Dennstaedtia is found in the Eastern United States. 

6. Cyatheaceae. This is a family of tree ferns, including 3 genera and 
about 700 species. Here belong Cyathea, Alsophila, and Hemitelia, all 
large genera widely distributed throughout the tropics. 

^ The position of this genus is uncertain. It is often placed in the Polypodiaceae. 



278 PLANT MORPHOLOGY 

7. Polypodiaceae. The Polypodiaceae constitute the highest and 
hirgest family of true ferns. Although chiefly tropical, it includes nearly 
all the ferns of temperate regions. About 150 genera and 6,000 species 
are known. The following list includes most of the large genera: Pteris, 
Adiantum, Athyrium, Cheilanthes, Dryopteris, Polystichum, Asplenium, 
Blechnum, Elaphoglossum, and Polypodium. 

Sporophyte. The sporophyte of the Filicales displays great variation 
in size, ranging from small delicate herbs to trees 18 m. or more in height. 
Most members of the group are terrestrial, but some are climbing, some 
epiphytic, and a few^ aquatic. The stem may be subterranean or aerial, 
erect or horizontal, and branched or unbranched. In most true ferns, and 
in all the common species of temperate regions, the stem is a creeping rhi- 
zome without aerial branches. The leaves of tropical species are ever- 
green. Those of temperate species, with few exceptions, die at the end of 
the growing season, new ones appearing each spring. The tree ferns of 
tropical regions have an erect, woody, unbranched stem bearing a ter- 
minal cluster of large leaves (Fig. 233). 

Some true ferns have simple leaves but most of them have large, char- 
acteristic, pinnately divided leaves often called fronds. Their leaflets, 
termed pinnae, are usually again divided, the smaller segments being 
known as pinnules. The leaves are generally firm and leathery but are 
often thin and membranaceous, being very delicate in the "filmy ferns" 
(Hymenophyllaceae). Stipular wings are present at the base of the pet- 
iole in the Osmundaceae but not in the other families. In unfolding from 
the bud, the leaves uncoil from the base toward the apex and continue to 
grow at the tip until they have reached their full size. This familiar 
behavior, known as circinate vernation, is very characteristic. The leaves 
have an elaborate system of branching veins, the branching being nearly 
always dichotomous and open, but sometimes reticulate (Fig. 240). 
Branching of the rhizome is usually monopodial but occasionally dichoto- 
mous; that of the roots is always monopodial. 

In nearly all the Filicales the root tip displays a large tetrahedral apical 
cell that undergoes very regular segmentation, cutting off cells from the 
three sides and also from the forward face to form the root cap (Fig. 234). 
The stem tip likewise grows by means of a large apical cell that nearly 
always is tetrahedral, cutting off segments in regular succession, but only 
from the three lateral faces. In the bracken (Pteridium aquilinum) a 
modified form of dolabrate apical cell is present, forming segments right 
and left. 

Vascular Anatomy. The anatomy of the roots and leaves is essentially 
similar to that of the spermatophytes. The leaves have an upper and a 
lower epidermis with stomata usually confined to the lower surface, meso- 



PTERIDOPHYTA 



279 



phyll, and vascular bundles. As a rule, the mesophyll is uniform. In 
nearly all the Filicales the root has a stele with two protoxylem groups. 
The stems of the Filicales display four different stelar types, being 
either a protostele, an amphiphloic siphonostele, an ectophloic siphono- 
stele, or a dictyostele. The dictyostele is the most common as well as the 




Fig. 234. Median longitudinal section through the root tip of Pteris gigantea, showing 
tetrahedral apical cell from which all the other cells have been derived; e, epidermis; c, 
cortex; s, stele. (After Hof.) 

most advanced type, but the amphiphloic siphonostele is also of rather 
frequent occurrence. The other two types are uncommon. 

The protoxylem consists of spiral tracheids — elongated cells with spiral 
thickenings on their walls. The metaxylem is made up almost entirely of 
scalariform tracheids. These are elongated cells, pointed at each end, 
with transverse bands of thickening resembling the rungs of a ladder. In 
nearly all the Filicales the development of the wood is mesarch, the 
protoxylem being surrounded on all sides by the metaxylem (Fig. 239). 
Generally the phloem surrounds the xylem, the vascular tissues thus 



280 



PLANT MORPHOLOGY 



showing an amphicribral arrangement. Sclerenchyma is usually prom- 
inently developed. There is no secondary thickening, even in the tree 

ferns. 

Except in protostelic stems, prominent leaf gaps are formed in connec- 
tion with the departure of loaf traces, as in all the orders of Filicinae. In 
addition to loaf saps, perforations not related to the departure of leaf 




Fig. 235. Cross section of the central portion of the rhizome of Gleichenia costaricensis, a 
protostele, X50. 

traces are sometimes developed in the vascular cylinder, especially in 
ferns with elongated rhizomes. In many forms accessory vascular 
strands are present, usually inside the vascular cylinder. In the common 
bracken (Pteridium aquilinum) we have a well-known example of the 
occurrence of medullary strands combined with a considerable amount of 
perforation of the vascular cylinder (Fig. 238). Accessory vascular 
strands are present also in many tree ferns. 

Protostele. The protostele, representing the most primitive vascular 
type, is found only in a few genera, such as Lygodium, Gleichenia, Hymeno- 
phyllum, and Trichomanes; but it occurs as the earliest developmental 
stage in most other true ferns. 

Gleichenia displays a typical protostele (Fig. 235). No pith is present, 



PTERIDOPHYTA 



281 



the xylem occupying the center of the stem. A cross section of the rhi- 
zome shows a more or less sclerenchj'matous cortex, a continuous and dis- 
tinct endodermis, a several-layered pericycle, and a narrow l)ut contin- 
uous band of phloem. The xylem forms a solid central mass consisting of 
groups of large scalariform tracheids intermixed with parenchyma. 
Numerous mesarch protoxylem groups are scattered throughout the meta- 













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E*^^y 


^^%M^r^m^cm^£^ 



Fig. 23G. Cross section of the central portion of tlie rliizome of Dennstacdtia pundilobula, 
an amphiphloic siphonostele, X80. In the center of the pith is a group of thick-walled 
sclerenchyma fibers. 

xylem. Leaf traces are connected directly with the stele, forming no 
gaps. 

Amphiphloic Siphonostele. This stelar type may be seen in such well- 
known ferns as Adiantum and Dennstaedtia, as well as in a number of 
others. Here the vascular tissues form a cylinder enclosing a pith, the 
xylem being surrounded both externally and internally by a complete 
zone of phloem (Fig. 236). 

A transverse section of the rhizome of Dennstaedtia punctilohula, the 
hay-scented fern of the Eastern United States, shows a thick outer cor- 
tical region composed of dark-colored sclerenchyma and a thin inner 
parenchymatous region. An outer endodermis delimits the cortex from 
the outer pericycle, which consists of two or three layers. Next come 
the outer phloem, the xylem, the inner phloem, the inner pericycle, the 
inner endodermis, and the pith. Both the outer and inner phloem are 



282 PLANT MORPHOLOGY 

made up almost entirely of sieve tubes but are separated from the xylem 
by a small amount of parenchyma. The xylem forms a narrow cyhnder 
composed of scalarif orm tracheids. Apparently no protoxylem is present. 
The inner pericycle usually comprises only one or two layers. The outer 
portion of the pith consists of parenchyma, the central part of scleren- 
chyma. The continuity of the vascular cylinder is interrupted by the 




Fig. 237. Cross section of the central portion of the rhizome of Osmunda cinnamomea, an 
ectophloic siphonostele, X 19. 

departure of leaf traces and the gap formed by each is closed above before 
the next trace is given off. The endodermis is continuous around the 
margins of the leaf gaps. Because the internodes are shorter in the rhi- 
zome of Adiantum, the leaf gaps are more numerous. Furthermore, the 
woody cylinder is wider and consists of both tracheids and parenchyma. 
Ectophloic Siphonostele. The ectophloic siphonostele differs from the 
amphiphloic in lacking internal phloem. Although found in Schizaea, it 
can be seen to better advantage in Osmunda. Here the rhizome is 
covered with persistent overlapping leaf bases. The outer cortex 
is extensive and consists mainly of dark-colored sclerenchyma, while the 
inner cortex is narrow and parenchymatous. The endodermis is distinct 
and continuous, even where the stele is interrupted by the outward pas- 



PTERIDOPHYTA 



283 



sage of leaf traces. The pericycle consists of one to four layers of paren- 
chyma forming a complete sheath. As seen in cross section, the vascular 
cylinder of Osmunda consists of a ring of mesarch xylem strands sep- 
arated by parenchymatous "rays" that pass outward from the large pith 
(Fig. 237). In some species the pith may contain sclerenchyma. Sur- 
rounding the xylem is a continuous layer of phloem made up chiefly of 
sieve tubes, while between the xylem and phloem are several layers of 
elongated parenchymatous cells continuous with the " rays." The xylem 




Fig. 238. Cross section of the rhizome of Ftcridium aquilinum, a dictyostele in which the 
vascular cylinder encloses two or more medullary bundles and two bands of dark, heavy- 
walled sclerenchyma fibers, X8. 

really consists of a cylindrical network forming a hollow cylinder, the 
meshes or "rays" being leaf gaps. 

Dictyostele. Among the lower families of Filicales, a dictyostele is pres- 
ent in Mohria and most species of Anemia, both members of the Schizaea- 
ceae. Dictyostelic stems are also found in some of the Dicksoniaceae, 
but the greatest number occur among the Cyatheaceae and Polypodia- 
ceae. The dictyostelic condition has been derived from the siphonostelic 
by the overlapping of leaf gaps, so that several or many separate vascular 
strands are seen in a cross section of the stem. In complex dictyosteles 
the vascular cylinder consists of a tubular network. 

A transverse section of the rhizome of Polypodium shows a number 
of small, widely separated vascular strands arranged in a circle. The 
rhizome of Pteridium, which is dorsiventral, consists of two series of 
strands — a circle of small peripheral ones enclosing two large central 
strands, or sometimes more than two as a result of branching (Fig. 238). 
The dorsal bundle is band-like and larger than the other peripheral ones. 



284 



PLANT MORPHOLOGY 



The bundles are amphicribral with mesarch xylem (Fig. 239). Each is 
surrounded by an endodermis that encloses a continuous pericycle usu- 
ally comprising only a single layer of cells. The ground tissue of the rhi- 
zome consists of parenchyma surrounded by an outer zone of thick-walled 
cells. In the central region are two transverse bands of sclerenchyma, 
one occurring above and one below the medullary strands. The lower 
band is larger than the upper one and slightly curved. 

Sorus. In most of the Filicales the sporangia are borne on the abaxial 
surface of ordinary foliage leaves. In the Schizaeaceae the sporangia are 




Fig. 239. Cross section of a vascular bundle from the rhizome of Pteridium aqiiilinum, 
X200; end, endodermis; per, pericycle; ph, phloem; px, protoxylein; mx, metaxylem. 

solitary, but in the other families they occur in groups called sori. Gen- 
erally the sori are arranged on either side of the midrib, but in many 
genera they are marginal or nearly so (Fig. 240). Ordinarily the leaf seg- 
ments that bear the sporangia are unmodified. Often, however, a 
marked differentiation exists between sterile and fertile leaflets, the latter 
being conspicuously contracted. This condition prevails in Osmunda, the 
Schizaeaceae, Onoclea, Blechnwn, etc. The same leaf may produce both 
sterile and fertile leaflets, or the entire leaf may be made up of either one 
kind or the other. 

Some ferns have naked sori, but usually each sorus is covered by a flap- 
like membrane called the indusium (Fig. 240). Although absent in the 
Osmundaceae and Gleicheniaceae, an indusium is present in the Schizaea- 
ceae, Hymenophyllaceae, Dicksoniaceae, Cyatheaceae (except Alsophila), 
and in most of the Polypodiaceae {Polypodium being a notable exception). 
Generally the indusium represents a special outgrowth of the leaf, but it 



PTERIDOPHYTA 



285 



may be formed by the inroUed leaf margin. Such a "false indusium" is 
present in the Schizaeaceae, Hymenophyllaceae, Dicksoniaceae, and in 
such well-known genera of Polypodiaceae as Pteris, Pteridium, Adiantum, 
Pellaea, Cheilanthes, and Notholaena (Fig. 240B, D). This condition 



K^\\^^ 



N 



S \ S \ \ \ \ \ \ \ "^v \ \ \ \ 






C E 

Fig. 240. Portion of the leaflets of five common ferns, illustrating differences in sori and 
indusia, X4. A, Woodwardia; B, Adiantum; C, Dryopteris; D, Pteridium; E, Poly podium. 
In B and D a false indusium is seen, while in E there is no indusium. 

may be regarded as primitive. In a number of true ferns the sori lose 
their individuality by a spreading of the sporangia over the leaf surface or 
along the leaf margin. Such "confluent sori" are seen in Pityrogramma, 
Elaphoglossum, Pteris, and Pteridium. 

With respect to the order of appearance of the sporangia within the 
sorus, three conditions are recognized, as follows: (1) The simple sorus, 



286 



PLANT MORPHOLOGY 




where all the sporangia arise at the same time and therefore are all of the 
same age, is found among the most primitive families, viz., Osmundaceae, 
Schizaeaceae, and Gleicheniaceae. (2) The gradate sorus, in which the 
sporangia arise in basipetal succession on an elongating receptacle, occurs 
in the Hymenophyllaeeae, Dicksoniaceae, and Cyatheaceae. (3) The 
mixed sorus, where sporangia of different ages are intermingled in the 
same sorus and show no developmental sequence, is the most advanced 
type. It is characteristic of all the Polypodiaceae except Woodsia and 

Onoclea, which are gradate. 

Sporangium. The sporangium in the Polypodiaceae 
is slightly flattened and has a long, slender stalk (Fig. 
241). It also has a rather long stalk in the Dicksoni- 
aceae and Cyatheaceae, but in the lower families it has 
a short, stout stalk or none. In all the Filicales the 
sporangium wall consists of a single layer of cells. A 
special feature is the presence of an annulus, a group 
or, more commonly, a ring of specialized cells that 
brings about the dehiscence of the sporangium. Its 
cells have all but their outer walls thickened. Drying 
causes a contraction of the thickened band, resulting in 
a state of tension that finally ruptures the sporangium 
wall. As the annulus bends backward, the spore mass 
is exposed. Then, suddenly, the annulus springs to its 
original position, hurling the spores into the air. 

The Osmundaceae have a rudimentary annulus (Fig. 
242). In both this family and the Schizaeaceae the 
annulus is apical, while in the Gleicheniaceae it is equatorial. Dehis- 
cence in all three families is longitudinal. In the Hymenophyllaeeae, 
Dicksoniaceae, and Cyatheaceae the annulus is oblique and the dehiscence 
is obliquely lateral. In the Polypodiaceae the annulus is vertical, extend- 
ing only about two-thirds of the way around the sporangium (Fig. 241). 
Dehiscence is transverse. 

The Filicales resemble the Ophioglossales and Marattiales in being 
homosporous, but differ in that they are leptosporangiate. This means 
that the sporogenous tissue is developed from the outer segment arising 
from the first periclinal division of the initial, rather than from the inner 
segment, as among eusporangiate pteridophytes. The sporangium initial 
consists of a single superficial or marginal cell that becomes papillate. A 
periclinal division separates an inner cell from an outer cell, and then three 
oblique walls appear in the latter in such a way that a tetrahedral apical 
cell is formed. This cuts off a variable number of segments that form a 
short stalk (Fig. 243A). The sporangium now enlarges above and, by 
means of a periclinal division, an outer cap cell is separated from an inner 



Fig. 241. Mature 
sporangium of one of 
the Polypodiaceae, 
showing the incom- 
plete vertical annu- 
lus, X150. 



PTERIDOPHYTA 



287 



primary sporogenoiis cell (Fig. 243B). The sporangium wall is developed 
by subsequent anticlinal divisions from the cap cell and the three upper- 
most stalk cells (Fig. 243C). In all the Filicales the sporangium wall 
remains one layer of cells thick. 

A unique feature is introduced by the formation of the tapetum from 
the primary sporogenous cell rather than from the wall tissue. The tape- 
tum arises from four cells, one of which is cut off each of the four faces of 
the primary sporogenous cell (Fig. 243Z), E). The tapetum may remain 




Fig. 242. Sporangia of some leptosporangiate ferns. A, B, C, Osmunda regalis, with 
rudimentary annulus; D and E, Anemia phyllitidis, with apical annulus; F and G, Glcichenia 
circinata, with equatorial annulus; H and /, Hymenophyllum dilatum, with oblique annulus. 
(After Wettstein.) 



single-layered, but in nearly all the Filicales its cells divide periclinally 
to form two layers (Fig. 243F, G). The innermost cell of the sporangium 
meanwhile undergoes division to form the spore mother cells (Fig. 
2A3F-H). When these round off, the tapetum disorganizes and forms a 
Plasmodium that later surrounds them. The number of spores formed in 
each sporangium exhibits considerable variation, but is relatively high 
(up to 512) in the lower families and relatively low (commonly 64 or less) 
in the advanced families. This tendency to reduce the spore output is a 
significant feature of Filicinean evolution. 

Gametophyte. The typical gametophyte of the Filicales is entirely 
aerial and consists of a flat, green, heart-shaped thallus usually about 6 
mm. in diameter (Fig. 244). Numerous unicellular rhizoids grow from 
its ventral surface into the soil. There is no endophytic fungus, except in 
the Gleicheniaceae and sometimes in the Schizaeaceae and Hymenophyl- 
laceae. In Hymenophyllum the gametophyte is an irregularly branched 



288 



PLANT MORPHOLOGY 



ribbon, while in Trichomanes and Schizaea it is filamentous and branched, 
resembling a moss protonema. 

The germinating spore gives rise to a short green filament and a rhizoid. 
Soon an apical cell with two cutting faces arises and a flat thallus develops. 
Then an apical cell with three cutting faces is formed and the median por- 
tion of the prothallium becomes slightly thickened, the wings remaining 





B 





Fig. 243. Development of the sporangium of Cyrtomium fakatum (Polypodiaceae), X400. 
A, young stage, showing tetrahedral apical cell cutting off segments to form a stalk; B, 
formation of cap cell and primary sporogenous cell; C, anticlinal division of cap cell; D, 
formation of first tapetal cell from primary sporogenous cell ; E, completion of tapetum ; 
F and G, later stages, the tapetum becoming two-layered; H, breaking down of tapetum 
and rounding off of spore mother cells. 

one-layered. Eventually a group of initials is formed in the apical notch, 
replacing the apical cell. 

In most of the Filicales the prothallium is monoecious, the sex organs 
arising from the ventral surface. The antheridia appear at a very early 
stage, often w^hen the prothallium is still filamentous, and continue to be 
produced for a long time. The first antheridia are often marginal as well 
as ventral in position. Later they become irregularly scattered over the 
entire ventral surface. The antheridia are not embedded in the prothal- 
lium but project beyond its surface. 



PTERIDOPHYTA 



289 



The antheridia of the Filicales differ greatly in development from those 
of the lower pteridophytes (Fig. 245). The initial is superficial and papil- 
late. The first wall is transverse, the antheridium developing from the 
outer cell. This then undergoes a second transverse division, cutting off a 
basal ring cell. Due to increasing turgidity of the upper cell, the wall 
formed, called the funnel wall, becomes concave and approaches the first 




Fig. 244. Ventral view of a fern prothallium (Polypodiaeeae), showing rhizoids and nu- 
merous antheridia in the older portion and three archegonia near the growing notch, X 35. 



wall, finally touching it (Fig. 245^). Next a do7ne wall appears, delimit- 
ing an outer cell from a central cell (Fig. 2455). The dome wall is hemi- 
spherical and nearly concentric with the outer surface of the antheridium. 
The appearance of a second funnel wall in the outer cell results in the for- 
mation of another ring cell and a cap cell, thus completing the sterile 
jacket (Fig. 245C-£'). The output of sperms in the higher families is 
commonly 32. As in the other ferns, the sperms are large, coiled, and 
multiciliate (Fig. 245F). 

The archegonia appear late in the development of the gametophyte, 
thus occurring in the median portion near the growing notch (Fig. 244). 



290 



PLANT MORPHOLOGY 



The initial is superficial but remains embedded. It divides transversely, 
the outer segment being the primary neck cell and the inner one dividing 
again to form the central cell and basal cell (Fig. 246^, B). The central 
cell gives rise to the axial row, which consists of a single binucleate neck 
canal cell, a small ventral canal cell, and the egg (Fig. 2A(dC-E). In the 
Osmundaceae and Gleicheniaceae the neck canal nuclei may be separated 
by a wall. The venter of the mature archegonium is embedded in the 






C E F 

Fig. 245. Stages in development of the antheridium of Nephrolepis (A to E), X700, and a 
sperm of Dryopteris (F), more highly magnified. A, cutting off a basal ring cell by funnel 
wall; B, appearance of dome wall, delimiting of outer cell from central cell; C, formation of 
second funnel wall; D, first division of central cell; E, mature antheridium with sperms form- 
ing in spermatogenous cells. (A to E, after Gilbert M. Smith; F, after Yamanotichi.) 

prothallium, but the neck, which usually curves slightly backward, pro- 
jects beyond the surface (Fig. 246F). The neck consists of four vertical 
rows of cells. 

Embryo. The Filicales are characterized by a striking regularity in the 
early divisions of the embryo. The first division of the fertilized egg is 
not transverse but vertical, i.e., parallel with the long axis of the archego- 
nium (Fig. 247A). By a division of each daughter cell at right angles 
to the plane of the first division, quadrants are formed (Fig. 247B). Of 
the two inner cells, the anterior one gives rise to the stem and the posterior 
one to the foot. Of the two outer cells (those nearer the neck of the arche- 
gonium), the anterior one forms the first leaf and the posterior one the 
primary root. Thus the arrangement of the quadrants differs conspicu- 
ously from that found among the eusporangiate ferns. The subsequent 
divisions are at first regular, resulting in a globular embryo in which the 
four primary organs may be easily distinguished (Fig. 247C). Later 
growth, however, is irregular, the leaf and root developing more rapidly 
and soon breaking through the calyptra (Fig. 247D). 



PTERIDOPHYTA 



291 





B 






Fig. 246. Development of the archegonium of Dryopteris, X350. A, division of arche- 
gonium initial into an inner and outer cell, the latter having again divided anticlinally to 
form two neck cells; B, formation of central cell and basal cell; C, division of central cell 
into ventral cell and neck canal cell; D, division of neck canal cell nucleus; E, nearly mature 
archegonium with egg, ventral canal cell, and binucleate neck canal cell; F, archegonium 
with egg ready for fertilization. 



Summary. The Filicales are homosporous and leptosporangiate. 
The sporangia are borne on the abaxial side of the leaves and are not in 
synangia. The sporangium wall, one layer of cells thick, has an annulus. 
The vernation is circinate. The gametophyte is flat, green, and thin 
(sometimes filamentous); with few exceptions it is without an endophytic 



292 



PLANT MORPHOLOGY 



fungus. The development of the antheridium is characteristic. The 
inner portion of the embryo forms the stem and foot, the outer portion the 
leaf and root. The Filicales are a highly organized group. Their prin- 




FiG 247. Stages in development of the embryo of a polypodiaceous fern, X225. A, 
two-celled stage; B, four-celled stage; C, slightly later stage; D, embryo showing differentia- 
tion of organs. 

cipal advance over the eusporangiate ferns lies in the development of their 
spore-producing structures. 

5. Hydropteridales 

The Hydropteridales include 2 families, 5 genera, and nearly 90 species. 
The two families are probably of separate origin and their inclusion in the 
same order is largely a matter of convenience. Both are more advanced 
than any of the families of Filicales, however, and in the same ways. All 



PTERIDOPHYTA 



293 



these plants live in water or in wet places and are appropriately called 
"water ferns." They constitute the most highly developed group of 
modern pteridophytes. 

1. Marsileaceae 

To the Marsileaceae belong 3 genera. MarsUea, with 65 species, and 
Pilidaria, with 6 species, are widely distriljuted, while Regnellidinm , with 




Fig. 248. Marsilea vestita. View of plants growing in swanipy ground. 



a single species, is confined to southern Brazil. They live on muddy flats 
or submerged in water, rooting in the mud. 

Sporophyte. In all three genera the stem is a slender, creeping, 
branched rhizome that produces erect leaves but no upright shoots. The 
leaves are arranged alternately in two rows along the upper side of the 
rhizome, while along the lower side roots are borne at the nodes. Each 
leaf of Marsilea has a long petiole and four terminal leaflets (Fig. 248). 
Regnellidium has two leaflets, while in Pilidaria leaflets are wanting, the 
whole leaf consisting merely of a petiole. As in the Filicales, the leaves 
exhibit circinate vernation and dichotomous venation. The stem devel- 



294 



PLANT MORPHOLOGY 



ops from a tetrahedral apical cell that cuts off three longitudinal rows of 
segments. The leaves arise from the two dorsal rows, the roots from 
the ventral row. The rhizome of Marsilea is an amphiphloic siphonostele 
without secondary thickening (Fig. 249). The rhizome of Pilularia is 
similar except that the vascular tissues are reduced. 




Fig. 249. Cross section of the rhizome of Marsilea quadrifolia, an amphiphloic sipho- 
nostele, X50. 

Sporocarp. The sporangia of the Marsileaceae are borne in special 
structures, called sporocarps, which occur on long or short stalks arising 
adaxially from the petiole of the leaf. They are usually borne singly, but 
in some species of Marsilea several or even many may be borne together. 
The sporocarp is a specialized leaf segment enclosing a group of sori. In 
Marsilea it is an ovoid or bean-shaped structure with a hard outer cover- 
ing (Fig. 251). It contains 14 to 20 sori. In Pilularia the sporocarp is 
spherical and contains 2 to 4 sori. In both genera the sori are arranged 
in two rows. Each is surrounded by an indusium and contains both 
microsporangia and megasporangia. The sporangium wall is only one 
layer of cells in thickness, as in the Filicales, but is without an annulus. 

The sporangia are leptosporangiate in development. Four cavities 



PTERIDOPHYTA 



295 



appear in the young sporocarp of Marsilea and from the layer of cells 
lining them the sporangia arise, each sorus coming from a single marginal 
cell (Fig. 250A). The sporangia appear in hasipetal succession on an 
elongating receptacle, the sorus thus being gradate, as in certain families 




Fig. 250. Sporangia of Marsilea quadrifolia. A, horizontal section through a young 
sporocarp, showing early development of two rows of sori, X25; B, longitudinal section 
through a young sorus and investing indusium, showing a developing megasporangium with 
sporogenous tissue and tapetum, and two younger microsporangia below, each with a pri- 
mary sporogenous cell, X400; C, megasporangium with young megaspores and tapetal 
Plasmodium, X300; D, megasporangium with enlarging functional megaspore, abortive 
megaspores, and tapetal plasmodium, X300; r, receptacle; h, vascular bundles; i, indusium; 
s, stalk of sporocarp. 

of the Filicales (see page 286). The tapetum, cut off from the sporog- 
enous tissue, becomes two-layered (Fig. 2505). In both kinds of sporan- 
gia 32 or 64 young spores are formed. In the microsporangia all of these 
mature, but in the megasporangium only one spore matures, the others 
degenerating (Fig. 250C, D). The functional megaspore greatly enlarges 



296 



PLANT MORPHOLOGY 



and develops a very thick cell wall, as in the other heterosporous pterido- 
phytes. In both kinds of sporangia the tapetum breaks down to form a 
multinucleate plasmodium that surrounds and nourishes the young 

spores. 

The sporocarp of Marsilea is remarkable on account of its longevity, 
some specimens having been known to have retained their viability for 
50 years. If placed in water after the hard outer covering has been 




Fig. 251. Germination of the sporocarp of Marsilea vestita, twice natural size. 
of mucilaginous ring to which the sori are attached. 



Extrusion 



cracked, germination is unusually prompt. Within an hour a mucilag- 
inous ring appears to which the sori are attached, each sorus being 
enclosed by its own indusium (Fig. 251). The spores germinate at once 
and the gametophytes develop with startling rapidity. 

Gametophytes. The male gametophyte of Marsilea reaches maturity 
within 10 to 20 hours after the microspore germinates. It does not 
emerge from the spore, but develops inside, as in SelagineUa and Isoetes. 
After a prothallial cell is cut off, the rest of the spore divides in half, each 
half becoming an antheridium (Fig. 252.4-C). Additional divisions 
result in the formation of two primary spermatogenous cells surrounded 
by a sterile jacket (Fig. 252D-F). Each of the primary spermatogenous 
cells gives rise to a group of 16 sperms (Fig. 252G-I). The sperms of 
Marsilea are corkscrew-like and multiciliate (Fig. 252 J). 

The female gametophyte of Marsilea is peculiar in that no internal 



PTERIDOPHYTA 



297 



tissue is developed and only one archegonium is formed. At the apex 
of the megaspore, where the wall is relatively thin, there is a papilla 
filled with dense cytoplasm in which the nucleus lies (Fig. 253^1). When 
the spore germinates, the nucleus divides and a small cell is cut off 
by a transverse wall (Fig. 253/^). The rest of the garnet ophyte acts as a 




G H I J 

Fig. 252. Male gametophyte of Marsilea quadrifolia. A, microspore with starch grains 
in cytoplasm; B, prothallial cell cut off; C, microspore divided into two antheridium initials, 
in each of which a jacket cell is being formed; D and E, additional jacket cells being cut off; 
F, two primary spermatogenous cells enlarging; G, H, I, stages showing increase in sperma- 
togenous cells to sixteen in each of the two antheridia; /, mature sperm, X 1,200; other 
stages, X350. {After Sharp.) 



food reservoir. The small cell is the archegonium initial, the larger one 
the nutritive cell. As development proceeds, the archegonium breaks 
through the megaspore wall. The mature archegonium consists of a 
large egg, a small ventral canal cell, a small neck canal cell, and a sterile 
jacket (Fig. 253/)). The neck is short, consisting of only two tiers of four 
cells each. Mucilage above the archegonium forms a deep funnel into 
which the sperms collect. 

Embryo. In the development of the embryo, the first wall is vertical, 
as in the Filicales, and then a quadrant stage appears (Fig. 2oZE). The 



298 



PLANT MORPHOLOGY 




Fig. 253. Female gametophyte of Marsilea quadrifolia. A, longitudinal section of 
megaspore; B, archegonium initial cut off at apex of spore; C, young archegonium with 
central cell surrounded by neck cells and sterile jacket; D, mature archegonium, showing 
large egg, small ventral canal cell, and neck canal cell; E, two-celled embryo; F, embryo, 
enclosed by cal.\ptra, showing differentiation into foot (/), root (r), leaf (I), and stem (s) ; 
A, XlOO; B to F, X250. 

arrangement of the segments is such that the two outer segments (those 
nearer the neck of the archegonium) give rise to the leaf and root, the two 
inner ones to the stem and foot, the leaf and stem developing from seg- 
ments on the same side (Fig. 253F) . With respect to the arrangement of 
the primary organs, the embryo of the Marsileaceae is similar to that of 
the Filicales. 



PTERIDOPHYTA 



299 



2. Salviniaceae 

To the Salviniaceae Ijelong 2 genera of widespread occurrence, Salvinia, 
with 11 species, and Azolla, with 4 species. Both are small plants that 
float on the surface of quiet water. Their fossil history does not extend 
beyond the Tertiary. 




Fig. 254. Floating plants of Salvinia rotundifolia (A), natural size, and of Azolla filicti- 
loides {B), twice natural size. 

Sporophyte. Salvinia has a slender, slightly branched stem bearing 
two kinds of leaves. The dorsal leaves are in four rows (Fig. 254A). 
They are 12 to 18 mm. long, entire, oval or oblong, flat, and overlapping. 
The ventral leaves, which occur in two rows, hang down into the water 



300 PLANT MORPHOLOGY 

and look like roots, being much dissected into filiform divisions. The 
ventral leaves are probably absorptive in function, true roots being want- 
ing. Azolla has pinnately branched stems covered with minute, crowded, 
overlapping leaves alternately arranged in two dorsal rows (Fig. 2545). 
Each leaf has two lobes, the upper lobe floating and the lower one sub- 
merged. The upper lobe contains cavities in which colonies of Anabaena 
live. Long, slender rootlets arise from the lower side of the stem. 

In both genera the leaves are folded in the bud, not circinate as in the 
Marsileaceae and Filicales. Each leaf has a single vascular bundle. The 
stem develops by means of an apical cell with two cutting faces. The vas- 
cular tissues are greatly reduced. The stem of Salvinia appears to be an 
ectophloic siphonostele, that of Azolla an amphicribral protostele. 

Sporocarps. The sporocarps of the Salviniaceae are globular and thin- 
walled, two or three occurring on a common stalk. In Salvinia they are 
borne in groups at the base of the ventral leaves, arising as outgrowths 
from them. In Azolla they are borne on lateral branches, chiefly in pairs, 
on the lower lobes of the first leaves to appear. In both genera the sporo- 
carps consist of an indusium enclosing a single sorus. The indusium 
becomes hard at maturity, forming a nut-like structure. 

The sporocarps are of two kinds, both occurring on the same plant. 
One contains only microsporangia and the other only megasporangia. In 
Salvinia the two kinds of sporocarps are of the same size and contain many 
sporangia, but in Azolla the megasporocarps are much smaller than the 
microsporocarps and contain only one megasporangium. At first each 
sporocarp of Azolla contains a young megasporangium with several 
younger microsporangia at its base. Only one kind of sporangium con- 
tinues its development, however, the other kind aborting (Fig. 255). In 
both genera the microsporangia are borne on long, slender stalks arising 
from a basal receptacle, while the megasporangia are short-stalked or 
nearly sessile (Figs. 255 and 256A). As in the Marsileaceae, the sorus is 
gradate. The sporangium wall is only one layer of cells thick and no 
annulus is formed. 

As in the Marsileaceae and Filicales, the development of the sporangia 
is leptosporangiate (Figs. 255 A and 2565). In the microsporangium of 
both Salvinia and Azolla 16 spore mother cells are formed and all of these 
give rise to tetrads, resulting in the formation of 64 microspores. In the 
megasporangium of both genera, however, only 8 spore mother cells are 
formed. These give rise to 32 megaspores, but only one of these matures, 
the rest degenerating (Fig. 256Z)). The functional megaspore enlarges 
until it finally completely fills the sporangium. It becomes very thick- 
walled. In both genera the tapetum, which is cut off from the sporog- 
enous tissue and consists of a single layer of cells, breaks down before the 



PTERIDOPHYTA 



301 



spores are ripe, forming a plasmodial matrix around them (Figs. 2555 and 
256C, D). Eventually this hardens. 

In the microsporangium of Azolla four to eight masses, called massulae, 
are organized from the tapetal plasmodium and within these the micro- 
spores are embedded. The massulae of some species of Azolla produce 
hair-like appendages (glochidia) with sagittate tips (Fig. 257A). They 




B C 

Fig. 255. Sporangia of Azolla caroliniana. A, young sporocarp, showing a young mega- 
sporangium and the developing indusium; B, megasporocarp, showing the terminal mega- 
sporangium with one functional and three abortive megaspores, and undeveloped micro- 
sporangia below; C, microsporocarp with developing microsporangia and an abortive 
megasporangium. {After Pfeiffer.) 



escape from the microsporangia and are carried to the megaspores, to 
which they become fastened by means of the glochidia. The microspores 
germinate within the massulae. In Salvinia they germinate while still 
within the microsporangia. The megaspores remain inside the megaspo- 
rangia, which break away and, in Salvinia, float on the surface of the 
water. 

Gametophytes. The male gametophyte of the Salviniaceae is peculiar 
in that the microspore produces a papillate outgrowth that forms one or 
two external antheridia, the internal portion functioning as a large nutri- 
tive cell (Fig. 2575). In both genera a small prothallial cell is cut off 
from the nutritive cell. In Salvinia, where there are two antheridia, each 
is enclosed by a sterile jacket and each produces four sperms. In Azolla 



302 



PLANT MORPHOLOGY 



there is only one antherirliiim and it produces eight sperms (Fig. 257C). 
The sperms are coiled and multiciliate. 

In the formation of the female gametophyte, the nucleus of the mega- 
spore divides near the apical end and a small lenticular cell is cut off. 
The larger cell later undergoes free-nuclear division, but no walls are 
formed and the cell becomes a food reservoir. The smaller cell gives rise 
to a tissue that breaks through the heavy megaspore wall and produces 




Fig. 256. Sporangia of Salvinia rotundifolia. A, longitudinal section through a young 
microsporocarp, XlOO; B, young microsporangium, showing primary sporogenous cell 
surrounded by tapetum, X600; C, older microsporangium, showing spores embedded in 
hardened tapetal plasmodium, X160; D, developing megasporangium, showing young 
functional megaspore surrounded by tapetal nuclei, the nonfunctional megaspores near the 
wall. X280. 

several archegonia (Fig. 257D). This tissue turns green and becomes 
rather extensive in Salvinia, but in Azolla is smaller and has little or no 
chlorophyll. The archegonia resemble those of Marsilea, except that the 
single neck canal cell is usually binucleate. 

Embryo. In both genera the fertilized egg, by means of two divisions 
at right angles to each other, gives rise to quadrants, but the first wall is 
longitudinal in Salvinia and transverse in Azolla. The relation of the 
four primary organs to one another is the same as in the other leptospo- 
rangiate ferns. 

Summary. The Hydropteridales are heterosporous and leptosporan- 
giate. The sporangia are borne in sporocarps representing either a mod- 
ified leaf segment (Marsileaceae) or a modified indusium (Salviniaceae). 
Both microsporangia and megasporangia occur in the same sporocarp 
(Marsileaceae) or in separate sporocarps (Salviniaceae). The sporan- 



PTERIDOPHYTA 



303 



gium wall is only one layer of cells thick and is without an anniilus. The 
vernation is circinate (Marsileaceae) or folded (Salviniaceae). The 
gametophytes are greatly reduced, developing largely within the spore 
wall. The development of the embryo is essentially the same as in the 
Filicales. The Hydropteridales are a specialized aquatic group, the two 
families apparently having been derived independently from the Filicales. 




B ^^^^^^^ C 

Fig. 257. Azolla fiUculoides. A, massula with glochidia and enclosed microspores, X250; 
B, germinating microspore, the antheridium initial dividing, X560; C, male gametophyte 
with small prothallial cell, large nutritive cell, and external antheridium, X560; D, female 
gametophjte, showing the large nutritive cell and the extruded tissue bearing archegonia, 
X65. (After Campbell.) 



COMPARISON OF THE CLASSES OF PTERIDOPHYTES 

The most important distinguishing characters of the four classes of 
pteridophytes are as follows: 

Psilophytinae. Leaves small, simple, spiral, generally without veins, 
often wanting. Roots absent. Stem mostly an exarch protostele. Leaf 
gaps absent. Definite strobili not organized. Sporangia solitary, ter- 
minal; on elongated branches and unilocular or (in Psilotales) on greatly 
reduced branches and bilocular or trilocular; tapetum wanting. Homos- 
porous. Prothallia (in Psilotales) tuberous, subterranean, not green. 
Sperms multiciliate. Embryo without a suspensor. 

Lycopodiinae. Leaves simple, usually small and numerous, generally 
spiral, with a single vein. Stem mostly an exarch protostele. Leaf gaps 



^ 304 PLANT MORPHOLOGY 

absent. Definite strobili usually present. Sporangia borne on sporo- 
phylls, solitary, adaxial, unilocular; trabeculae sometimes present. 
Ilomo'sporous (in Lycopodiales) or heterosporous. Prothallia in homos- 
porous forms tuberous, wholly or in part subterranean, and with chloro- 
phyll only in the aerial portion. Sperms biciliate or (in Isoetales) multi- 
eiliate. Embryo with or (in Isoetales) without a suspensor. 

Equisetinae. Leaves mostly small and simple, numerous, cyclic, with 
a single vein. Stems conspicuously jointed, longitudinally grooved. 
Stem an endarch siphonostele or (in Sphenophyllales) an exarch proto- 
stele. Leaf gaps absent. Strobili present. Sporangia generally borne 
in groups on sporangiophores, usually adaxial with reference to the sporo- 
phylls, which are freciuently absent. Homosporous or heterosporous. 
Prothallia (in Equisetales) flat, aerial, green. Sperms multiciliate. 
Embryo without a suspensor. 

Filicinae. Leaves mostly few, large, and divided, spiral, with numer- 
ous veins. Stem a protostele, siphonostele, or dictyostele; typically mes- 
arch. Leaf gaps present. Strobili absent. Sporangia numerous and 
generally abaxial on modified or unmodified leaflets, commonly in sori. 
Homosporous or (in Hydropteridales) heterosporous. Prothallia in 
homosporous forms flat, green, and aerial or (in Ophioglossales) tuberous, 
subterranean, and not green. Sperms multiciliate. Embryo nearly 
always without a suspensor. 

GENERAL CONCLUSIONS 

All pteridophytes have archegonia and multicellular antheridia, 
although these organs are somewhat reduced as compared with those of 
bryophytes. Fertilization is still conditioned by the presence of water. 
All pteridophytes display a distinct alternation of generations, but 
advance far beyond the bryophytes in the possession of an independent 
sporophyte with a leafy stem, true roots, and a well-developed vascular 
system. The sporophyte is nourished by the gametophyte only during 
the early stages of its development. 

Independent Sporophyte. In the evolution of the plant kingdom the 
first land plants to have established an independent sporophyte must 
necessarily have developed one with (1) a means of anchorage and of 
absorbing water directly from the soil and (2) a means of displaying green 
tissue to the light and air. In practically all existing pteridophytes the 
sporophyte carries on water absorption by roots and photosynthesis by 
leaves borne on a stem. Food manufacture has become primarily a func- 
tion of the sporophyte, leaving fertilization as the main function of the 
gametophyte. This arrangement permits the sporophyte to grow upward 
into the air. A large plant displaying leaves to the light and air requires 
a constant supply of water as well as a means of mechanical support. 



PTERIDOPHYTA 305 

These demands are met, in the pteridophytes, mainly by the vascular 
system. Thus the sporophyte, in achieving independence, has developed 
roots, stems, and leaves, with a system of conducting and supporting 
tissues extending throughout the plant body. 

Among existing pteridophytes the Psilotales are unique in having a 
sporophyte that shows little organization into vegetative organs. Their 
poorly developed leaves (without veins in Psilotum) and lack of roots, 
although probably related to a partially saprophytic existence, are to be 
regarded as primitive features, since they are shared with the extinct 
Psilophytales, a group which the Psilotales resemble in other respects as 
well. 

The size of the leaves must be considered in relation to the size of the 
stem. From this standpoint, small simple leaves, without a petiole, are 
found in nearly all members of the three lower classes, while large leaves, 
with a petiole and with a blade that is almost always divided into leaflets, 
are characteristic of the ferns. Moreover, ferns are the only pterido- 
phytes having leaf gaps, a feature setting them off in marked contrast to 
the lower classes and indicating a relationship to the seed plants, where 
leaf gaps are universally present. Except in the Equisetinae, where 
they are cyclic, the leaves of pteridophytes are fundamentally spiral in 
arrangement. 

The leaves of the Psilophytinae, except those of T7nesiptens, are vein- 
less, while those of the Lycopodiinae have a single unbranched vein. 
These groups are said to be microphyllous (small-leaved), for even the 
leaves of the Lepidodendrales and Isoetales, though larger than those of 
other lycopods, are narrow and have only one vein. The leaves of 
microphyllous pteridophytes probably represent emergences, or simple 
outgrowths from the stem. The Filicinae are megaphylloiis (large-leaved) 
and their leaves have many branching veins. Such leaves apparently 
have evolved from a lateral branch system that has become flattened and 
limited in growth. Thus the leaves of lycopods and those of ferns have 
probably had a different origin and so are not homologous. Most of the 
Equisetinae have small simple leaves with a single vein, but some of the 
fossil members have larger leaves with leaflets and branching veins. 
This indicates that the group was originally megaphyllous, the small 
leaves having been derived from larger ones by reduction. 

The Strobilus. A second contribution of the pteridophytes to the evolu- 
tion of the plant kingdom has been the organization of a strobilus. Orig- 
inally no distinction may have existed between sporophylls and foliage 
leaves, a condition found in the simpler species of Lycopodium. But 
gradually, as a result of "division of labor," sporophylls became less leaf- 
like and were organized to form a compact strobilus. Although not pres- 
ent in modern ferns, a strobilus is characteristic of nearly all the other 



306 PLANT MORPHOLOGY 

groups of living pteridophytes, as well as of a number of extinct forms. 
Its appearance is important because it is a feature carried on into the 

spermatophytes. 

In the Lycopodiinae and Filicinae the sporangia are borne in connection 
with some or all of the leaves. In the Lycopodiinae they are solitary and 
adaxial, in the Filicinae numerous and mainly abaxial. In the Psilo- 
phytinae the sporangia are terminal, either on the main stem (Psilophy- 
tales) or on a very short lateral branch (Psilotales). In the Equisetinae 
the sporangia are borne on sporangiophores. The sporangium of the 
pteridophytes is always epidermal in origin and may arise from a single 
cell or a small group of cells. In its development, all pteridophytes are 
eusporangiate except two orders of ferns, the Filicales and Hydropter- 
idales, which are leptosporangiate. With the exception of the Psilotales, 
a tapetum is present in all living pteridophytes. In the Lycopodiinae the 
tapetum is persistent, but in the Equisetinae and Filicinae it soon breaks 

down. 

Heterospory. The appearance of heterospory represents a third great 
forward step in evolution introduced by the pteridophytes. Among 
modern representatives it occurs only in Selaginella, Isoetes, and the five 
genera of the Marsileaceae and Salviniaceae. Heterospory was devel- 
oped, however, in many extinct forms. It introduces into the life history 
two kinds of spores, two kinds of gametophytes, and a great reduction in 
the gametophyte generation. Heterospory, introduced by the pterido- 
phytes, is an established feature of the spermatophytes. In fact, it 
makes seed formation possible. 

The gametophytes of homosporous pteridophytes may be tuberous and 
subterranean, as in the Psilotales, Lycopodiales, and Ophioglossales, 
where the absence of chlorophyll is associated with a saprophytic mode of 
nutrition; or they may be flat, green, and aerial, as in the Equisetales, 
Marat tiales, and Filicales. In practically all homosporous pteridophytes 
both kinds of sex organs are borne in comparatively large numbers on the 
same gametophyte. In the heterosporous forms, however, the gameto- 
phytes are always dioecious and reduced, both kinds developing largely or 
entirely within the spore wall. The male gametophyte produces only one 
or two prothallial cells and one or two antheridia. The female gameto- 
phyte usually has more vegetative tissue, but generally only one to several 
archegonia. 

The gametophyte of the homosporous pteridophytes, with much veg- 
etative tissue, must not only make its own food, but also enough for the 
embryo sporophyte that is dependent upon it. The development of such 
a gametophyte requires a considerable period of favorable external condi- 
tions. This handicap is largely avoided by the heterosporous pterido- 
phytes. Their gametophytes are formed inside the spores that produce 



PTRRIDOPHYTA 307 

them and do not emerge except, in some cases, to a very slight extent. 
Each lives on food stored within the spore. The advantage of heteros- 
pory lies in the fact that, since the gametophytes derive their nourish- 
ment from food made by the sporophyte, they are independent of such 
external conditions as might interfere with the growth of a free-living 
gametophyte. 

Except in the Anthocerotales, the sex organs of bryophytes are super- 
ficial structures, but in the pteridophytes they are embedded, either 
wholly or in part. Moreover, as compared with the sex organs of bryo- 
phytes, both the antheridia and archegonia of pteridophytes are reduced. 
The greatest reduction of spermatogenous tissue occurs in the heterospo- 
rous forms, reaching an extreme in Isoetes, where each antheridium pro- 
duces only four sperms. The most primitive archegonia, those with the 
greatest number of neck canal cells, are found in Lycopodium. In nearly 
all the other homosporous pteridophytes there are either two neck canal 
cells or, more commonly, only one, this being usually binucleate. In the 
heterosporous forms there is a single neck canal cell that may be either 
binucleate or uninucleate, according to the genus. Nearly all pterido- 
phytes have multiciliate sperms. Biciliate sperms, resembling those of 
bryophytes, are confined to Lycopodium, Phylloglossum, and Selaginella. 

Interrelationships. The Psilophy tales are the oldest known and most 
primitive group of vascular plants. Whether they were derived from 
ancestors resembling bryophytes or directly from alga-like forms is a mat- 
ter of difference of opinion. It is rather generally agreed, however, that 
the Psilophytales gave rise to the other pteridophytes, since transitional 
forms have been found. The Lycopodiinae, Equisetinae, and Filicinae 
separated early from the Psilophytales and each has subsequently pursued 
an independent course of evolution. The Psilophytales also gave rise to 
the Psilotales, a group that has made relatively little progress and one 
that stands apart from the other existing groups. 

The Lycopodiinae are a relatively primitive group in spite of the fact 
that some members have advanced to the condition of heterospory. 
They reached their climax in the Paleozoic and are now relatively unim- 
portant members of the flora. There is no evidence that they have given 
rise to any of the higher groups. The Equisetinae, more advanced than 
the Lycopodiinae, also made their greatest display during the Paleozoic. 
They are a peculiar group with many features not seen in any other vascu- 
lar plants. They also represent a line of evolution that ends blindly. 

The Filicinae are the most highly developed of all pteridophytes and 
show much progress among themselves. It is generally believed, on the 
basis of much morphological and paleobotanical evidence, that the ferns 
have given rise to the seed plants. The leptosporangiate ferns are essen- 
tially modern, while the eusporangiate ferns are more ancient and more 



308 PLANT MORPHOLOGY 

primitive The Paleozoic ancestors of the spermatophytes must have 
developed heterospory, ]>iit the only known heterosporous ferns are the 
two families of Hydropteridales, both of which are modern and highly 
speciali/ed. Yet, aside from heterospory, there are many resemblances 
between the eusporangiate ferns and the Cycadofilicales of the Paleozoic, 
which are the most primitive group of gymnosperms. 



CHAPTER VIII 
SPERMATOPHYTA 

The spermatophytes constitute the highest and largest division of the 
plant kingdom, numVjering approximately 196,000 species. They com- 
prise the two classes Gymnospermae and Angiospermae, the former being 
not only the older and more primitive group, but by far the smaller one 
today. Spermatophytes are found in all parts of the world and in the 
most diverse habitats. Although the angiosperms dominate the land 
vegetation, they include members that have become aquatic, epiphytic, 
and, through partial or complete loss of chlorophyll, saprophytic or 
parasitic. 

All spermatophytes are characterized by the production of seeds, a 
feature that at once distinguishes them from the lower groups. Like the 
pteridophytes, they are vascular plants with an independent sporophyte; 
but in spermatophytes the sporophyte attains its greatest complexity, 
while the gametophyte is obscure and so reduced that it is entirely depend- 
ent upon the sporophyte for its nutrition. 

Spermatophytes range in size from the minute floating duckweeds, 
some no larger than the head of a pin, to the giant redwoods of California 
and certain eucalypts of Australia, both of which may reach a height of 
100 m. All modern gymnosperms are woody plants, while the angio- 
sperms include both woody and herbaceous types. The stem undergoes 
lateral branching, the branches nearly always arising in the leaf axils. 
Most commonly the branching is monopodial. Elongation of the root 
and stem is accomplished by a terminal meristem, never by an apical cell. 
All seed plants are heterosporous. The microsporangia and megaspo- 
rangia are borne by members that are essentially foliar in nature but, 
although homologous with the sporophylls of pteridophytes, are nearly 
always more highly modified. Among spermatophytes it is customary to 
designate the microsporophyll as a stamen, the megasporophyll as a carpel, 
and the megasporangium as an ovule. ^ The megasporangium produces a 
single functional megaspore. Because the megaspore is not shed, the 
female gametophyte develops inside the megasporangium. This feature 
makes seed formation possible. 

In practically all gymnosperms the female gametophyte produces 
archegonia, but in angiosperms archegonia are eliminated. The male 

'■ Really the megasporangium is only part of the ovule, e.g., the nucellus. 

309 



310 



PLANT MORPHOLOGY 



garnet ophyte does not form antheridia, but gives rise directly to two 

sperms or their eciuivalent. Swimming sperms, universal throughout the 

bryophytes and pteridophytes, occur only in two orders of living gymno- 

sperms, being replaced in all other spermatophytes by nonmotile male 

cells. In all seed plants the sperms or male cells, as the case may be, reach 

the egg through the agency of pollen tubes, within which they develop. 

Following fertilization, the embryo develops inside the ovule, which 

becomes a seed. 

1. GYMNOSPERMAE 

The gymnosperms, numbering only about 700 living species, are an 
ancient group with a long geologic history. All the evidence points to 




Fig. 258. Diagram showing the geologic distribution and the relationships of the major 
groups of vascular plants. 

their origin from the ferns. They were abundant during the Paleozoic, 
while during the greater part of the Mesozoic they became the dominant 
group of land plants (Fig. 258). All existing gymnosperms are woody 
plants and most of them are trees. Gymnosperm means "naked seed," 
the seeds being borne not in a closed vessel, as they are in angiosperms, 



SPERM ATOPHYTA 311 

but freely exposed. The class includes seven orders: the Cycadofilicales, 
Bennettitales, Cycadales, Cordaitales, Ginkgoales, Coniferales, and 
Gnetales. Of these, the first, second, and fourth are entirely extinct. 

1. Cycadofilicales 

The Cycadofilicales' are the oldest and most primitive group of seed 
plants. Although some fossil remains have come from late Devonian 
deposits, the group did not become abundant and widespread until the 
Upper Carboniferous (Fig. 258). It declined greatly during the Permian, 
but persisted into the Triassic, when it soon became extinct. The 
Cycadofilicales are of great interest because, as their name implies, they 
are transitional between the ferns and the cycads. 

Sporophyte. The general aspect of the Cycadofilicales was distinctly 
fern-like (Fig. 259). Some forms resembled modern tree ferns, but most 
of them were smaller. Some appear to have been climbers. The leaves, 
when found as impressions in the rocks, are so fern-like that they cannot 
be distinguished from the leaves of true ferns except when found in associ- 
ation with stems, sporangia, or seeds. The stem anatomy is fern-like 
also, but the development of secondary wood is characteristic. This 
wood consisted of pitted rather than scalariform tracheids. Three stelar 
types were represented among the Cycadofilicales, each constituting a 
"stem genus." Heterangium was a protostele, Medullosa a polystele 
(with three separate steles), and Lyginopteris an ectophloic siphonostele 
(Fig. 260). In each case the primary xylem was mesarch, a fern char- 
acter. The leaf traces were double and direct; they were mesarch 
throughout. 

Microsporangium. The microsporangia of the Cycadofilicales were at 
one time regarded as the sporangia of ferns. The microsporophyll 
(stamen) resembled an ordinary fern frond having fertile and sterile 
pinnae. In the ''stamen genus" Crossotheca each fertile pinnule was 
more or less peltate and bore six to eight bilocular microsporangia on its 
lower side. This is designated as the "epaulet" type of stamen (Fig. 
261^4, B). Another, characteristic of the stamen genus Cahjmmatotheca, 
is known as the "cupule" type because the microsporangia were borne 
within a cup-like structure formed at the end of a naked branch (Fig. 
261C). In some of the Cycadofilicales the microsporangia occurred as 
synangia on the abaxial surface of fern-like leaves. 

Megasporangium. In none of the Cycadofilicales was a strobilus 
organized. Commonly the fern-like leaves were dimorphic, some being 
fertile and others sterile. The fertile leaves bore terminal ovules on their 
ultimate divisions (Figs. 196 and 259). The seeds were usually enclosed 
in a cupule (Fig. 262). As in other seed plants, the ovule consisted of a 

' Often called Pteridospermae. 



312 



PLANT MORPHOLOGY 




Fig. 259. Portion of restoration of Carboniferous swamp forest in the Chicago Natural 
History Museum. Plants belonging to the Cycadofilicales include Lyginopteris oldhamia, 
a climber leaning against the large tree near the center, Neuropteris decipiens, in left center, 
and Neuropteris heterophylla, in lower left center. 



central portion, the nucellus, surrounded by an integument except at its 
apex, where a narrow passageway, the micropyle, was formed. In the 
CycadofiUcales the integument commonly was free from the nucellus only 
in the upper portion of the ovule, but in some cases was wholly free. The 
nucellus was prominently beaked and contained a deep pollen chamber in 



SPERMATOPHYTA 



313 




Fig. 260. Cross section of small stem of Lyuiiiopteris oldhamia, X 5; a, sclerenchyma bands 
in outer cortex; h, inner cortex and phloem; c, double leaf trace; d, secondary xylem; 
e, primary xylem;/, pith. (From Arnold.) 







B C 

Fig. 261. Microsporangia of Cycadofilicales. A, diagrammatic longitudinal section of a 
fertile pinnule of Crossotheca, showing epaulet type with peltate limb and pendent sporangia, 
X3; B, cross section of same, showing bilocular sporangia; C, cupule type, Codonotheca, 
with sporangia on the inner surface of the valves, natural size. (A and B, after Kidston; 
C, after Sellards.) 



which the microspores accumulated. The Cycadofilicales must have had 
swimming sperms. 

Gametophytes. Pollen grains, found in pollen chambers, contain a 
tissue of numerous cells, all of which wei-e probably spermatogenous. 
The sperms seem to agree in form with those of existing cycads. No 
evidence of pollen tubes has ever been found and it is probable that they 



314 



PLANT MORPHOLOGY 



were not produced. Remains of the female gametophyte are fragmen- 
tary, but it is certain that archegonia were developed in the micropylar 
region of the nucellus, the microspores apparently coming directly in con- 
tact with them. No embryos have ever been found in the seeds, perhaps 
because they did not develop until after the seeds had been shed. 




Fig. 262. Diagrammatic longitudinal section of the ovule of Lyginopteris oldhamia 
{Lagenostoma lomaxii) with its investing cupule; n, central portion of nucellus; n', outer 
hardened portion with the pollen chamber between; i, inner fleshy layer of integument; 
h, vascular bundle; s, outer stony layer of integument; c, cupule. {After Oliver.) 



Summary. The Cycadofilicales are a group that was dominant in the 
Paleozoic. It is closely related to the Filicinae, the general habit, leaves, 
and microsporangiate structures being distinctly fern-like. The vascular 
anatomy is also fern-like, but with the addition of secondary wood. 
Three stelar types are represented. A primitive feature is the occurrence 
of mesarch xylem throughout the plant, an advanced feature, the presence 
of pitted tracheids. There is no strobilus and the sporophylls are leaf-like 
and not highly differentiated from foliage leaves. The microsporangia 
resemble fern sporangia in being numerous on the sporophylls, but the 
megasporangia, in forming seeds, show a great advance. The Cycadofili- 
cales are transitional between the ferns and cycads. They were probably 
ancestral to both the Bennettitales and Cycadales. 



SPERMATOPHYTA 



315 



2. Bennettitales 

The Bennettitales were a Mesozoic order, world-wide in distribution. 
They ranged from the Triassic to the Upper Cretaceous but reached their 
greatest display during the Jurassic (Fig. 258). The four principal genera 
are Cycadeoidea, Williamsonia, WilliamsonieUa, and Wielandiella. In 







Jg^ggH^iM ''■ -''^//^'.irfJ^^^^^^^^^^B^^^^B£||^B^^Rg'"sgl|y h 






""^•^H^H 


k, -'^''^'■■- ■■■-" - 





Fig. 263. Upper part of a large stem of Cycadeoidea. Some of the strobili are projecting 
and some have fallen out, leaving cavities. The specimen is about 60 cm. high. 

spite of their many distinctive features, the Bennettitales were probably 
direct descendants of the Paleozoic Cycadofilicales. 

Sporophyte. The Bennettitales were more diversified in habit than 
modern cycads. Few forms exceeded 2 m. in height and most of them 
were under 1 m. The stems of Cycadeoidea were mostly short, stout, and 
unbranched (Fig. 263), while those of Williamsonia were tall and colum- 
nar, often with a few lateral branches. They bore a crown of large fern- 



316 PLANT MORPHOLOGY 

like leaves at the summit. The stems were covered with an armor of 
persistent leaf bases and a mass of woolly scales, forming a ramentum, as 
in many tree ferns. The stems of WiUiamsoniella and Wielandiella were 
slender, dichotomously branched, and smooth, with a cluster of leaves at 
the points of forking. Except in WiUiamsoniella, the leaves of the 
Bennettitales were pinnately divided into many leaflets. 




Fig. 264. Cycadeoidea ingens. Photograph of a model of the strobilus in the Chicago 
Natural History Museum. 

The stem was an ectophloic siphonostele, a cross section showing a 
large pith, a thin vascular cylinder, and a thick cortex. The vascular 
bundles were collateral and endarch. Secondary wood, although scanty 
in amount, was always present. Most of the tracheids were scalariform, 
but in some cases were pitted. The leaf traces were single and direct, 
becoming mesarch after entering the leaves. 

Strobilus. In Cycadeoidea numerous strobili were borne on short stalks 
occurring among the leaf bases, each strobilus being axillary (Fig. 263). 
In Williamsonia the cones were long-stalked and borne in the apical crown 
of leaves. In the two other genera the strobili were borne singly in an 
upright position where the stem underwent forking. The order is charac- 
terized by bisporangiate strobili (mostly monosporangiate in William- 



SPERMATOPHYTA 



317 



sonia), the two kinds of sporophylls having the same relation to each other 
as have the stamens and carpels in a flower of the magnolia. Each cone 
consisted of four sets of members : an outer sheath of sterile bracts, a whorl 
of microsporophylls, stalked ovules, and interseminal scales (Fig. 26-i). 

In Cijcadeoidea, the microsporangiate structures had advanced but 
little beyond the fern condition. The microsporophylls (stamens), 10 to 




Fig. 265. Diagram of a longitudinal section of the strobilus of Cycadeoidea, showing hairy 
bracts below, two pinnate microsporophylls, and the central ovule-bearing axis. {After 
Wieland.) 

20 in number, were large, leaf-like, and pinnately divided, each division 
bearing two lateral rows of abaxial sporangia borne in synangia like those 
of the Marattiales (Fig. 265). The megasporophylls and interseminal 
scales were closely crowded together at the summit of the strobilus axis, 
forming a compact ovoid body. Each ovule was borne at the end of a 
stalk that probably represents a reduced sporophyll (Fig. 266x4). The 
stalks were more or less vertical, the middle one being the longest. The 
interseminal scales probably represent sterile sporophylls. The ovule had 
a basal cupule and a three-layered integument consisting of an outer 
fleshy, a middle stony, and an inner fleshy layer. The micropyle was 



318 



PLANT MORPHOLOGY 



long, and a prominent nucellar beak and pollen chamber were developed 
(Fig. 2665). Thus it seems certain that the Bennettitales had swimming 
sperms. Nothing is known of the gametophytes. The embryo was 
dicotyledonous and completely filled the seed, no endosperm having been 
present at maturity. 





A B 

Fig. 266. Strobilus and seed of Bennettites (Cycadeoidea). A, diagram of seed-bearing 
strobilus, showing sheathing bracts, long-stalked seeds, and interseminal scales; B, longi- 
tudinal section of seed; m, micropylar tube; «, nucellar beak; p, pollen chamber; s, inter- 
seminal scale; e, embryo space. (A, after Scott and others; B, after Wieland andLignier.) 

Summary. The Bennettitales are a group dominant in the Mesozoic 
and intermediate in some respects between the Cycadofilicales and the 
Cycadales. Characters common to the Cycadofilicales include branching 
of the stem, a ramentum, direct leaf traces, leaf-like microsporophylls with 
synangia, and the ovule structure. An advance is seen in the organiza- 
tion of a strobilus. Characters common to the Cycadales are the general 
habit of some of the genera, the leaves, and the vascular anatomy, the 
stem being an endarch siphonostele with relatively little secondary wood. 
Distinctive features of the group are the bisporangiate strobili and the 
occurrence of both fertile and sterile megasporophylls, the latter bearing 
solitary terminal ovules. The Bennettitales are considered too special- 
ized in their spore-bearing structures to have given rise to the Cycadales. 
Both groups seem to have had an independent origin from the Cycado- 



SPEHMATOPHYTA 



319 



filicales. The resemblance between the bisporangiate cj^cadeoid cone 
and the flower of the magnolia and its allies has suggested that the angio- 
sperms may have been derived from the Bennettitales, but there is little 
evidence to support this view (see page 411). 

3. Cycadales 

This order includes 9 living genera and about 100 species, all of which 
are tropical or subtropical in distribution. Four of the genera belong to 
the Western Hemisphere and five to the Eastern Hemisphere. Zamia, the 




Fig. 267. Dioon edule with a large female cone 28 cm. in diameter. 
1.5 m. long. 



The leaves are up to 



largest genus, has about 30 species; it ranges from Florida to Chili. 
Microcycas is confined to w^estern Cuba, Dioon and Ceratozamia to southern 
Mexico. Cycas, with 20 species, ranges from Japan to Australia and 
Madagascar. Bowenia and Macrozamia are found only in Australia, 
Sfangeria and Encephalartos only in Africa. The Cycadales are closely 
related to the Bennettitales and like them were probably derived directly 
from the Cycadofilicales. They flourished throughout the Mesozoic and 
reached their greatest display in the Lower Cretaceous, when they were 
much more widespread than they are today (Fig. 258). 

Sporophyte. The stems of cycads are either subterranean and tuberous 
or aerial and columnar. They bear a crowai of large fern-like leaves (Fig. 
267). The stems of the columnar forms are covered with an armor of 
persistent leaf bases. The stem is rarely branched and is commonly less 
than 3 m. high. The tallest species {Macrozamia hopei) sometimes 
reaches a height of 18 m. The leaves are pinnate (bipinnate in Bowenia) 



320 PLANT MORPHOLOGY 

and are borne in close spiral arrangement at the apex of the stem. They 
are rather tough and leathery and vary in length from 5 cm. to 3 m., 
depending on the species. The venation, often described as parallel, is 
really dichotomous, as in most ferns (Fig. 2G8A). In Cycas, however, the 
leaflets have no veins except a prominent midrib. The vernation is 




Fig. 268. Leaves of cycads. A, two leaflets of Zamia skinneri, showing dichotomous 
venation, one-half natural size; B, young leaves of Cycas circinalis, showing circinate 
vernation, one-fifth natural size. 

circinate in Cycas and either erect or somewhat circinate in the other 
genera (Fig. 2685). 

The stem of the Cycadales is like that of the Bennettitales in being an 
ectophloic siphonostele with a large pith, a thin vascular cylinder, and a 
thick cortex (Fig. 269). The vascular bundles of the stem are collateral 
and endarch, but the leaf traces, leaf veins, and bundles of the strobilus 
axis are mesarch and frequently amphicribral as well. Secondary wood 
is developed but is commonly small in amount. It consists of tracheids 
with bordered pits, except in Zamia and Stangeria, where the tracheids 
are scalariform, like those of ferns. The leaf traces of cycads are peculiar 
in being double and indirect. This means that, in passing from the stele 



SPERMATOPHYTA 321 

to the leaf, two leaf traces girdle the cortex in opposite directions, each 
passing about halfway around the stem in going from their point of origin 

to the leaf. 

The cones of the Bennettitales, each occurring in the axil of a leaf, are 
borne laterally along the stem. In Macrozamia and Encephalartos the 
cones are also lateral and axillary, although arising close to the stem tip. 




Fig. 269. Cross section of the stem of Zamia fioridatia, showing large pith, thin vascular 
cylinder, and thick cortex with portions of the girdling leaf traces, X4. 

In the other genera the original apical meristem is used up in the forma- 
tion of a cone, and a new meristem appears at its base. This produces a 
branch that soon becomes erect and gives rise to a new crown of leaves. 
Thus the first cone produced by a plant is terminal, but all the rest are 
morphologically lateral, although borne at the summit of the stem. All 
cycads are monosporangiate and dioecious. 

Staminate Strobilus. The staminate strobili are usually borne singly, 
but in Zamia, Macrozamia, and Encephalartos several or many may occur 
together (Fig. 2705). They are composed of an axis bearing many spi- 
rally aa-ranged microsporophylls (stamens) that are always compactly 
organized. The microsporophylls are not at all leaf-like, but are narrow 
below and broadened above into a sterile tip (Fig. 271). The microspo- 
rangia are abaxial and borne in sori of two to five, but not in synangia. 
They range in number from over a thousand on each sporophyll in Cycas 
to a comparatively few in Zamia. Their development is eusporangiate, 
the initials being hypodermal in origin rather than epidermal as in the 



322 



PLANT MORPHOLOGY 




Fig. 270. Cones of cycads. A, female cone of Cycas rcooluta, consisting of a loose rosette 
of megasporophylls; B, male cones of Zamia skinneri; C, female cone of Zamia skinneri 
with nearly ripe seeds. 



pteridophytes. There may be only one initial or a row or plate of several 
initials. The initials divide transversely, the outer segments being the 
primary wall cells and the inner ones the primary sporogenoiis cells. A 
wall several layers in thickness is developed (Fig. 272). The tapetum is 
cut off rather late, and so it is uncertain whether it is derived from the 



SPERMATOPHYTA 



323 



wall or from the sporogenous tissue. The number of spores produced by- 
each sporangium is high, ranging from 500 in Zamia to 30,000 in Dioon. 
Dehiscence occurs by means of a longitudinal slit. In general, the micro- 
sporangium of the cycads shows a striking resemblance to the sporangium 
of the Marattiales, particularly to that of Angiopteris (Fig. 229). 

Ovulate Strobilus. In most genera the ovulate cones are borne singly, 
but in Macrozamia and Encephalartos they may occur in groups of two, 
three, or more. They are com- 
posed of many spirally arranged, 
fleshy megasporophylls (carpels). 
In Cycas the megasporophylls are 
very loosely arranged to form a 
rosette that surrounds the stem tip, 
which later continues its growth 
upw^ard through the rosette (Fig. 
270A). In Dioon the megasporo- 
phylls form a loose cone, but in all 
the other genera they are compactly 
organized (Figs. 267 and 270C). 
The megasporophylls of cycads 
exhibit a striking reduction series, 
ranging from pinnate types with 
six or eight ovules, in most species 
of Cycas, to peltate types wdth 
only two ovules, in the other genera 
(Fig. 273). Throughout this series 
the megasporophylls become less 
and less leaf-like and the strobilus increasingly more compact. 

The main body of the ovule is the nucellus or megasporangium proper. 
It is surrounded by a single massive integument free from the nucellus only 
at its upper end and forming a narrow passageway, the micropyle (Fig. 
274). A prominent nucellar beak is developed, in the center of which a 
pollen chamber later arises. The integument consists of an outer fleshy, 
a middle stony, and an inner fleshy layer. Vascular strands are found in 
both fleshy layers. They are composed of mesarch xylem. Deep within 
the nucellar tissue a megaspore mother cell becomes differentiated. It 
gives rise to a linear tetrad of megaspores. Of these, only the innermost 
megaspore functions, the other three degenerating. 

Female Gametophyte. As in all seed plants, the megaspore germi- 
nates in situ, producing the female gametophyte. Its formation involves 
several stages. First, the megaspore enlarges and free-nuclear division 
occurs. Then, by further enlargement, a central vacuole is formed, 
resulting in a parietal placing of the nuclei. As nuclear division proceeds, 




Fig. 271. Microsporophylls of Cycas cir- 
cinalis, showing back (abaxialj and side 
views, one and one-half times natural size. 



324 PLANT MORPHOLOGY 

walls come in, forming a tissue. This tissue develops centripetally until 
it reaches the center of the gametophyte. Two regions are now differen- 
tiated — a region of smaller cells that develops archegonia, situated near 
the micropylar end, and a deeper region of larger, nutritive cells. 

As a rule, 3 to 5 archegonia are formed, but there may be as many as 10. 
In Microcycas, which is unique in this respect, as many as 200 archegonia 




Fig. 272. Longitudinal section of microsporangium of Zamia floridana, showing sporog- 
enous tissue surrounded by the tapetum (both shaded) and a wall five or six layers thick, 
X150. 

may appear. The archegonium initial is superficial and, by a periclinal 
division, an outer primary neck cell is differentiated from a central cell 
(Fig. 275 A, B). The former undergoes a vertical division, thus forming 
two neck cells, a constant feature throughout the Cycadales (Fig. 275C, 
D) . There are no neck canal cells. The central cell undergoes a marked 
enlargement. Its nucleus divides to form a ventral canal nucleus and an 
egg nucleus, but no wall is laid down between them. The ventral canal 
nucleus soon disorganizes and the egg is now ready for fertilization. An 
archegonial chamber is not present when the archegonia are young, but 
arises later. It is a depression formed by upgrowth of the adjacent tissue 
of the female gametophyte. 



UPERMATOPHYTA 



325 



Male Gametophyte. The first division of the microspore nucleus 
results in the formation of a small persistent prothallial cell that is cut off 
close to the microspore wall. The larger cell divides again to form the 
generative and tnbe cells, and in this condition the pollen grain is shed 
(Fig. 276.4, B). The pollen is transported by wind to the ovulate cone, 
whose sporophylls separate slightly at the time of pollination. A group 





/ 




A B 

Fig. 273. Megasporophylls of cycads. A, Cycas revoluta, showing pinnate blade with 
conspicuous leaflets; B, Cycas circinalis, the leaflets reduced to teeth; C, Macrozamia 
denisonii, side view of sporophyll, the blade reduced to a spine; D, Zamia floridana, with 
peltate sporophyll; A, B, C, two-fifths natural size; D, four-fifths natural size. 



of cells at the apex of the nucellus break down and form a droplet of liquid 
that exudes through the micropyle and to which some of the pollen grains 
adhere. As the droplet dries, the grains are drawn down into the pollen 
chamber formed by the disintegration of the cells that produced the drop- 
let. Then a pollen tube develops from each pollen grain, growing lat- 
erally into the nucellus. Its basal end advances downward, carrying the 
prothallial and generative cells with it. In the cycads the pollen tube is 
an absorbing organ, obtaining nourishment from the nucellar tissue, which 
is thereby destroyed. Soon after the pollen tube has begun to develop, 
the generative cell forms the stalk and body cells, the latter finally giving 



326 



PLANT MORPHOLOGY 



rise to two sperm mother cells (Figs. 27QC-E). From each of these a 
motile sperm is organized. The sperms of cycads are large and multi- 
ciliate, the cilia arising from a ])lepharoplast (Figs. 277 and 278). In 
Microcijcas 16 to 22 sperms are formed in each pollen tube. 

After the pollen tube has penetrated the nucellar tissue and entered the 
archegonial chamber, it ruptures and frees the sperms, which then make 
their way toward the archegonia (Fig. 278). An entire sperm enters an 




Fig. 274. Zamia floridana. Longitudinal section of the ovule shortly after pollination, 
X4; m, micropyle; n, nucellus; a, archegonium; g, female gametophyte; o, outer fleshy 
layer of integument; s, middle stony layer; i, inner fleshy layer. 



egg, but its nucleus soon separates from the cytoplasm and band of cilia, 
moving toward the egg nucleus and fusing with it. 

Embryo. The nucleus of the fertilized egg undergoes 8 to 10 simultane- 
ous divisions (only 6 in Bowenia) without the formation of cell walls (Fig. 
279 A). As a result, as many as over a thousand free nuclei may be pro- 
duced. This free-nuclear stage is common to all cycads, but differences 
now appear. In the Cycas type of embryogeny persistent cell walls are 
formed throughout the fertilized egg. In the Dioon type cell walls 
appear throughout the egg but soon disappear except at its base. In 
Zamia wall formation is confined to the basal portion, not even evanes- 
cent walls appearing in the main body of the egg (Fig. 279B). 

The cells at the base of the egg constitute the proemhryo. Even where 
persistent cell walls appear above, the upper portion functions as a large 
food reservoir, contributing no cells to the formation of the new plant. 



SPERMATOPHYTA 



327 



The proembryo soon becomes differentiated into three regions: an upper 
haiistorial portion in contact with the nutritive material above, a middle 
zone of elongating cells forming the suspensor, and a terminal group of 
cells constituting the embryo itself (Fig. 279C). The suspensor becomes 
enormously elongated and highly coiled, pushing the embryo deep within 





B 





Fig. 275. Development of the archegonium of Dioon edule, X85. A, archegonium 
initial; B, formation of primary neck cell and central cell; C, later stage with two neck 
cells; D, upper portion of archegonium with two neck cells and central cell nucleus. {After 
Chamberlain.) 



the tissue of the female gametophyte. When mature, the embryo has a 
short axis, the hypocotyl, terminating at the end next to the suspensor in a 
root tip, or radicle. This is enclosed in a hard covering, the coleorhiza. 
At the opposite end of the hypocotyl is a minute stem tip, the plumule, 
lying between a pair of seed leaves, or cotyledons. The presence of two 
cotyledons is a constant feature of the Cycadales. As the embryo devel- 
ops, food is stored in the vegetative tissue of the female gametophyte, 
forming ''endosperm." This is a feature of all gymnosperms. The 
stored food is later absorbed by the embryo when the seed germinates. 
The ripe seed is usually white, cream-colored, orange, or red. 



328 



PLANT MORPHOLOGY 




Fig. 276. Development of the male gametophyte of Dioon edule. A, microspore with 
nucleus in early prophase of first mitosis; B, shedding condition of pollen grain; C, beginning 
of pollen tube formation, showing prothallial cell {p), generative cell {g), and tube nucleus 
it); D, formation of stalk cell (s) and body cell (6) from generative cell; E, later stage, the 
body cell much elongated and with two blepharoplasts showing conspicuous radiations; 
A, B, C, X 1,235; D, X980; E, X618. (After Chamberlain.) 

The cycads are unique in that the seed germinates promptly, without 
going into a resting stage. The stony coat is broken by the coleorhiza 

enclosing the elongating root tip. The 
coleorhiza is soon destroyed by the root 
tip, which then rapidly grows down- 
ward into the soil. The stem remains 
inconspicuous, but a leaf soon develops. 
The cotyledons remain inside the seed 
coat, absorbing food from the "endo- 
sperm" and transferring it to the devel- 
oping seedling. 

Summary. The cycads resemble the 
ferns in their general habit, vascular 
anatomy, form and venation of the leaves, occurrence on the microspo- 
rophylls of abaxial sporangia in sori, structure of the microsporangia, and 
multiciliate sperms. All these characters, as well as the ovule structure, 





B 

Fig. 277. Side (.4) and top (B) 
views of a sperm of Zamia floridana, 
showing numerous cilia on a spiral 
band, X 100. (After Webber.) 



SPERM ATOPHYTA 



329 




Fig. 278. Reconstruction of the ovule of Dioon edule at the time of fertilization. Pollen 
tube on the left shows undivided body cell; the one in the middle shows two sperms and 
remains of prothallial and stalk cells; the one on the right shows two sperm mother cells. 
Two pollen tubes have discharged their sperms. A sperm has entered the egg on the left; 
the one on the right still shows tlie ventral canal nucleus. Two sperms, in the thick liquid 
discharged from the pollen tube just above them, are ready to enter the egg. The dark 
line below the nucellus is the megaspore membrane. {From Chamberlain.) 



are common to the Cycadofilicales, Bennettitales, and Cycadales. These 
three orders, considered together, are called cycadophytes. They re]!)re- 
sent a distinct line of evolution reaching far back into the Paleozoic. The 
stem of the Cycadales, like that of the Bennettitales, is an endarchsiphono- 
stele with a narrow zone of wood, the xylem in other aerial parts of the 
plant being mesarch. The dicotyledonous embryo is also suggestive of 
that of the Bennettitales. As compared with the other cycadophytes, the 



330 



PLANT MORPHOLOGY 



Cycadales display the following distinctive features: infrecjuent branching 
of the stem, indirect and double leaf traces, monosporangiate and appar- 
ently terminal strobili, reduced microsporophylls, and, except in Cycas, 
reduced peltate megasporophylls with only two ovules. 




Fig. 279. Early stages in embryogeny of Zamia floridana, X25. A, proerabryo with free 
nuclei; B, wall formation at base of proembryo; C, differentiation into suspensor and 
embryo. 



4. Cordaitales 

The Cordaitales comprise an extinct group of Paleozoic gymnosperms, 
contemporaneous with the Cycadofilicales, but plants of a very different 
aspect. They appeared late in the Devonian, made their greatest display 
during the Upper Carboniferous, and almost disappeared before the end 
of the Permian (Fig. 258). Although the Cordaitales differ in many ways 
from the Cycadofilicales, their resemblances are such as to indicate that 
both groups may have had a common origin. 

Sporophyte. The Cordaitales were tall trees with slender, branched 
stems often reaching a height of 30 m. (Fig. 280). The branches were 
covered with simple leaves that were generally long and narrow and borne 
in spiral arrangement. A cross section of the stem, which was an ecto- 



SPERMATOFHYTA 



331 



phloic siphonostele, shows a large pith, a thick or thin vascular cylinder, 
and a small cortex. In contrast to the mesarch collateral bundles of the 
Cycadofilicales, those of the Cordaitales were endarch and collateral. 
Secondary wood was well developed, its tracheids having bordered pits in 




Fig. 280. Restoration of Dorycordaites, showing the roots, branching stem, simple leaves, 
and clusters of strobili borne on lateral branches. The stem was actually much longer 
than shown here. (After Grand' Eury.) 

several rows, as in the modern coniferous genus, Araucaria. The leaf 
traces were double, collateral, and endarch, becoming mesarch in the 
leaves. In most cases the venation of the leaves was dichotomous. 

Strobili. A feature of the Cordaitales was the presence of small strobili 
of two kinds, both occurring on the same plant and borne in clusters on 
lateral branches (Fig. 281). The strobili, about a centimeter in length, 
were completely ensheathed by sterile bracts. The staminate strobili 
were composed of spirally arranged microsporophylls and bracts, the lat- 



332 PLANT MORPHOLOGY 

ter representing sterile microsporophylls (Fig. 282 A). The microsporo- 
phylls (stamens) were long-stalked and bore two to five erect microspo- 
rangia at the tip. 

The ovulate strobili were composed of bracts and ovules, the latter 
borne on secondary axes in the axils of the bracts (Y'lg. 282/i). Thus the 
strobilus was compound. The integument of the ovule had an outer 




Fig. 281. Cordaites laevis. Restoration of foliage-bearing branch with numerous strobili; 
a large bud is shown at the right. {From Arnold, after Grand'Eury.) 

fleshy and a middle stony layer, but no inner fleshy layer. The nucellus 
was entirely free from the integument, its peripheral region being trav- 
ersed by one set of vascular strands, another set occurring in the outer 
fleshy layer of the integument. A prominent nucellar beak and pollen 
chamber were developed, indicating that the sperms were swimming. 

Gametophytes. As in the Cycadofilicales, a group of cells has been 
found within the pollen grain, but it is uncertain whether they represent 
vegetative or spermatogenous tissue. The female gametophyte was simi- 
lar to that of modern gymnosperms. There were two archegonia, sepa- 
rated by a beak-like upgrowth of gametophyte tissue. This is also a 
feature of the modern genus Ginkgo. The seeds were very similar to those 
of the Cycadofilicales. In neither group have seeds wuth embryos been 
found. 



SPERMATOPHYTA 



333 



Summary. The Cordaitales, a dominant Paleozoic group, seem not 
to have been derived from the CycadofiHcales, although both groups may 
have had a common origin. The stem is an endarch siphonostele. The 
primitive features of the group, which they share with the Cycadofilicales 
and the other cycadophytes, include a large pith, mesarch leaf bundles, 





V ii 



A B ' 

Fig. 282. Strobili of Cordaianthus, X8. A, longitudinal section of staminate strobilus, 
showing sterile bracts and microsporophylls bearing terminal sporangia; B, longitudinal 
section of ovulate strobilus, showing sterile bracts and two stalked axillary ovules. (After 
Renault.) 



the ovule structure, and swimming sperms. The advanced features are 
the branching habit, thick vascular cylinder, form of the leaves, and com- 
pound ovulate strobili. The Cordaitales seem to have given rise to both 
the Ginkgoales and Coniferales. 

5. Ginkgoales 

The Ginkgoales are represented by only one living species. Ginkgo 
biloba, a native of western China. It is widely cultivated but virtually 
unknown in the wild state. The order, probably derived from the Cordai- 
tales during the Carboniferous, has been recognized as far back as the 
Permian. Its members were most abundant during the Mesozoic, par- 
ticularly in the Jurassic, when the order had almost a world-wide dis- 



334 



PLANT MORPHOLOGY 




Fig. 283. 
D.C. 



A large tree of Ginkgo biloba on the grounds of the White House, Washington, 



tribution (Fig. 258). The most important Mesozoic genera were Baiera 
and Ginkgo itself, both represented by many species. 

Sporophyte. Ginkgo is a tree with the general habit of a conifer (Fig. 
283). It is excurrent when young, becoming round-topped in old age. 
Under favorable conditions, it may reach a height of 30 m. There are 
two kinds of branches: long branches of unlimited growth bearing scat- 



SPERMATOPHYTA 



335 



tered leaves, and short branches of limited growth bearing a few leaves in 
a cluster. The leaves are deciduous. They have a long petiole and a 
broadly wedge-shaped lilade (Fig. 284). The blade is typically bilobed 
but may be entire or each lobe may be partially divided into several nar- 




FiG. 284. Leaf blade of Ginkgo biloba, showing dichotomous venation, natural size. 




Fig. 285. Cross section of long stem ot Glnkyo biloba, showing small pith, thick vascular 
cylinder, and thin cortex, X 15. 

row segments. The leaves are highly variable, even on the same tree. 
They exhibit dichotomous venation. 

The stem of Ginkgo is an ectophloic siphonostele. The long stems have 
a small pith, a thick vascular cylinder, and a thin cortex, as in the Conit- 



336 



PLANT MORPHOLOGY 



erales (Fig. 285). The dwarf ])ranches, on the other hand, have a large 
pith, a thin vascular cylinder, and a thick cortex, as in the Cycadales. 
The leaf traces are double and pass directly into the petiole. Mesarch 
bundles occur only in the cotyledons, those in all other parts of the plant 
being collateral and endarch. The tracheids of the secondary xylem have 
bordered pits in one or two rows. 




Fig. 286. Staminate strobili of Ginkgo biloha. A, clusters of strobili borne on dwarf 
shoots, two-thirds natural size; B, enlarged view of a dwarf shoot with young leaves and 
four strobili. 



The strobili of Ginkgo are monosporangiate and dioecious, as in the 
Cycadales. They are borne at the end of the dwarf shoots, each in the 
axil of a leaf. 

Staminate Strobilus. The staminate strobili are composed of a central 
axis bearing many spirally arranged microsporophylls (stamens) forming 
a loose, catkin-like cluster (Fig. 286). There are no sterile bracts among 
the sporophylls. The microsporophyll consists of a long stalk ending in a 
knob that bears two, or occasionally three or four pendent microsporan- 
gia. The knob is a reduced blade. Ginkgo has continued the "epaulet" 
type of stamen found among the Cycadofilicales. The microsporangium 
is eusporangiate in development, the initial being single and hypodermal. 
The wall consists of four to seven layers of cells, the tapetum being derived 
from the outermost layer of sporogenous tissue. Dehiscence occurs by 
means of a longitudinal slit. 



SPERMATOPHYTA 



337 




Fig. 287. Ovulate strobili of Ginkgo biloba. A, dwarf shoot bearing cluster of young 
leaves and ovulate strobili, two-fifths natural size; B, enlarged view of three strobili, each 
bearing two ovules. 




Fig. 288. Ginkgo biloha. A, longitudinal section of young ovule, showing "collar," 
integument, nucellus, and spore tetrad, X15; B, enlarged view of tetrad of niegaspores 
surrounded by nutritive tissue, X400. 

Ovulate Strobilus. The ovulate strobili are greatly reduced. Each 
consists of a long stalk that bears mostly two terminal ovules, only one of 
which ordinarily matures as a seed (Fig. 287). At the base of each ovule 
is a peculiar "collar." This probably represents a vestigial megasporo- 
phyll, since in rare instances it may become leaf-like. Sometimes three 
or four ovules may be boVne on the same stalk. The ovule is character- 
ized by a single massive integument. As in the Cycadales, this is three- 



I 



338 



PLANT MORPHOLOGY 



layered, consisting of an outer fleshy, a middle stony, and an inner fleshy 
layer. A prominent nucellar beak and pollen chamber are present (Fig. 
288-4). Vascular strands are present in the outer fleshy layer, the inner 
set being suppressed. The megaspore mother cell is deep-seated but may 
be easily recognized, as it becomes invested with a glandular digestive 




Fig. 289. Archegonium of Ginkgo biloba. A, micropjlar end of female gametophyte, 
showing two archegonia, X40; fi, median longitudinal section of archegonium surrounded 
by tissue of the female gametophyte, showing central cell and two neck cells, X 100. 

tissue. A linear tetrad is formed and only the innermost megaspore is 
functional (Fig. 2885). 

Female Gametophyte. As in the Cycadales, the development of the 
female gametophyte is initiated by free-nuclear division, but the nuclei 
are parietally placed from the beginning. Wall formation is centripetal. 
A remarkable condition is seen in that the vegetative tissue of the female 
gametophyte develops chlorophyll and becomes bright green. As it 
develops, it encroaches upon the nucellar tissue and destroys nearly all 
of it. As a rule, only two archegonia appear, developing as in the cycads. 



SPERMATOPHYTA 



339 



except that a small ephemeral ventral canal cell is formed. A prominent 
beak arises between the archegonia, a feature also of the Cordai tales 
(Fig. 289). 

Male Gametophyte. The development of the male gametophyte 
begins with the cutting off of two prothallial cells, of which the first is 
ephemeral, the second persistent. The remaining large cell divides 
unequally to form the generative and tube cells. This is the shedding 




Fig. 290. Early embryogeny of Ginkgo biloba, X75. A, free-nuclear stage; B, cellular 
stage; C, later stage with meristematic tissue at lower end. 



condition of the pollen. The further development of the male gameto- 
phyte takes place as in the Cycadales. The pollen tubes are extensively 
branched and function as haustoria. The generative nucleus gives rise to 
the stalk and body nuclei, but these are not separated by a cell wall. A 
body cell is organized and divides to form two sperm mother cells, within 
each of which a large swimming sperm is formed. The sperm has a band 
of cilia like that of the cycads. 

Embryo. In the development of the proembryo of Ginkgo, the first 
stage is one of free-nuclear division, 256 nuclei being formed (Fig. 290 A). 
This is followed by a stage of wall formation, the entire egg becoming 
filled with tissue (Fig. 290B). The lower third of the proembryo now 
becomes meristematic, while the upper two-thirds remains dormant (Fig. 
290C). The growing region gives rise to a short massive suspensor and a 
terminal embryo, the latter developing two cotyledons. On the whole, 
the embryo of Ginkgo is primitive. The ripe seed of Ginkgo is brownish 
yellow and about 2.5 cm. in diameter. 



34.0 PLANT MORPHOLOGY 

Summary. Ginkgo hiJoha is the sole survivor of an order that was 
widespread and abundant during the Mesozoic. It has retained the 
primitive reproductive features of its ancestors, its advance being wholly 
in vegetative characters. The stem is an endarch siphonostele, almost 
all traces of mesarch structure having disappeared. The pith is not large. 
Characters that the Ginkgoales have in common with the Cordaitales 
include the branching habit, thick vascular cylinder, venation of the 
leaves, structure of the stamens, ovule structure, and swimming sperms. 
The distinctive features of the Ginkgoales include the form of the leaves 
and the structure of the strobili. The ovulate strobili and megasporo- 
phylls are greatly reduced, the microsporophylls less so. The Ginkgoales 
may have been derived from the Cordaitales, but are themselves a blindly 
ending line. 

6. Coniferales 

The Coniferales constitute the largest order of living gymnosperms, 
including 6 families, 40 genera, and over 500 species. They are dis- 
tributed throughout the North and South Temperate Zones, with only a 
few representatives in the tropics, where they occur at high altitudes. 
Conifers have been found as fossils as far back as the Permian, having 
probably been derived from the Cordaitales (Fig. 258). During the 
Mesozoic they became numerous and diversified into as many families 
as are represented today. As a group, the Coniferales reached their 
climax during the Lower Cretaceous. 

Families. In the following outline, all the genera occurring in North 
America are named, together with several others of particular interest. 

1. Abietaceae. This is a family almost entirely confined to the North- 
ern Hemisphere. It comprises 9 genera and about 230 species. The 
largest genus is Pinus, with 90 species. Other important genera are 
Cedrus, Larix, Picea, Tsuga, Pseudotsuga, and Abies. Of the foregoing, 
all but Cedrus are represented in North America. 

2. Taxodiaceae. Here belong 9 genera and 16 species, nearly all grow- 
ing in the Northern Hemisphere. Only Sequoia and Taxodium are found 
in North America. Aletasequoia, formerly known only in the fossil state, 
has recently been discovered growing in western China. 

3. Cupressaceae. This family includes 10 genera and approximately 
120 species. It is represented in both hemispheres. The following 
genera are found in North America: Libocedrus, Thuja, Cupressus, Cham- 
aecyparis, and Juniperus. The largest genus, Juniperus, has 60 species. 

4. Araucariaceae. This family has only 2 genera. Both Agathis, with 
20 species, and Araucaria, with 12 species, are of wide distribution in the 
Southern Hemisphere. 

5. Podocarpaceae. This family belongs almost exclusively to the 



SPERMATOPHYTA 



341 




Fig. 291. Ovulate strobili of Pinus contorta. A, portion of leafy shoot with young cone 
and two one-jear-old cones, three-fourths natural size; B, young cone at time of polli- 
nation, twice natural size. 

Southern Hemisphere. It includes 6 genera and approximately 100 spe- 
cies. The largest genera are Podocarpus, with 70 species, and Dacrydium, 
with 20. 

8. Taxaceae. Here belong 4 genera and 14 species, mainly occurring 
in the Northern Hemisphere. There are only two genera in North Amer- 
ica, Taxus and Torreya. 



342 PLANT MORPHOLOGY 

Sporophyte. Almost all conifers are trees, but a few are shrubs. Typ- 
ically they display the excurrent habit, with a tall straight trunk giving 
rise to numerous wide-spreading branches. Nearly all conifers are ever- 
green, retaining their leaves for from 3 to 10 years. The only deciduous 
forms in North America are Larix and Taxodium. The largest conifers 
are the two species of Sequoia, both native to California. Sequoia 
sempervirens, the redwood of the coastal region, sometimes reaches a 
height slightly in excess of 100 m., a diameter of 6 m.,^ and an age of 
1,300 years. Sequoia gigantea, the big tree of the Sierra Nevada, attains a 
maximum height of somewhat less than 100 m., a diameter of 8 m.,^ and 
an age of about 3,500 years. This species is regarded by some botanists 
as distinct enough to constitute a separate genus, Sequoiadendron. 

In almost all genera only stems of unlimited growth and with scattered 
leaves are present. In Pinus, Cedrus, and Larix, however, both long and 
dwarf (spur) shoots occur. In Pinus the long shoots bear scale-like 
leaves and in the axil of each arises a spur shoot bearing needle-like foliage 
leaves (Fig. 291). Only Pinus monophylla produces one leaf on a spur; 
most species have either two or three leaves ; Pinus quadrifolia has four ; 
and some species have five. In Cedrus and Larix foliage leaves are borne 
both on the long and dwarf shoots and the number of leaves on the latter 
is much larger than in Pinus, being usually 30 to 50. In Pinus the entire 
spur falls away with the leaves, but in Cedrus and Larix only the leaves 
drop off, new ones appearing on the old spur, as in Ginkgo. 

The leaves of conifers are small and always simple. Their arrangement 
is spiral, except in the Cupressaceae, Avhere it is cyclic. The needle-like 
type of leaf, as seen in Pinus and the other Abietaceae, is the dominant 
one throughout the order (Fig. 291), but other types are also found. 
Broad, flat leaves occur in Agathis and in many species of Podocarpus and 
Araucaria. Scale-like leaves are characteristic of nearly all the Cupres- 
saceae (Fig. 292). Where the adult foliage is scale-like, often the juvenile 
leaves, appearing on seedlings, are needle-like. The flat leaves of conifers 
have several or many parallel veins, while the needle-like and scale-like 
leaves have but a single vein. 

A pine leaf is adapted to endure severe environmental conditions (Fig. 
293). On the outside is a single-layered epidermis with heavily cutinized 
cell walls and deeply sunken stomata. Beneath the epidermis are one or 
more hypodermal layers also with thick walls. The mesophyll is compact 
and peculiar in that the cells have infolded walls. It generally contains a 
number of resin ducts. The central tissue of the leaf, enclosed by an 
endodermis, contains one or two vascular bundles, the number depending 
on the species. The xylem and phloem, nearly equal in amount and 

' These diameters are measured at a height of about 3 m. above the greatly swollen 
base. 



SPERMATOPHYTA 



343 




Fig. 292. Leafy branch of Cupressus macrocarpa with ripe cones, about one-half 
natural size. 




Fig. 293. Cross section of a leaf of Piniis niyra, X75. The thick-walled epidermal and 
hypodernial layers surround the mesophyll, with infolded cell walls, and containing several 
resin ducts. The endoderniis encloses the transfusion tissue and two vascular bundles, 
in which the jiyleni lies below the phloem. 



344 



PLANT MORPHOLOGY 



largely iset'oudary in origin, show a collateral arrangement. They are 
enclosed by a zone of "transfusion tissue." 

The stem of the Coniferales is an ectophloic siphonostele and exhibits 
an advanced type of vascular anatomy (Fig. 294). It has a small pith, a 
thick vascular cylinder, and a thin cortex. The organization of the con- 
ducting tissues is collateral and endarch. . Only traces of mesarch struc- 
ture remain, as in the cotyledons of certain genera. There is a great 




Fig. 294. Cross section of a six-year-old stem of Piniis monophyUa. showing small pith, 
thick vascular cylinder, and thin cortex, XH. The phloem occupies a narrow zone out- 
side the xylem, the latter showing radiating vascular rays and numerous resin ducts. 

development of secondary wood, which consists almost entirely of tra- 
cheids. The tracheids have bordered pits, those of the Araucariaceae 
being mostly in two or three rows, instead of in a single row as in the other 
conifers. In Taxus the tracheids bear spiral thickenings in addition to 
the bordered pits. The phloem of conifers consists chiefly of sieve tubes. 
There are no companion cells. The leaf traces may be double, as in the 
Abietaceae, or single, as in the Cupressaceae. The conifers are char- 
acterized by the presence of resin canals, which are long intercellular cav- 
ities lined with resin-secreting cells. They commonly occur in all parts 
of the plant, being especially abundant in the leaves and in the cortex of 
the stem. In Pinus and many other genera they may also be present in 
the wood. In Taxus resin canals are lacking. 

The strobili are normally monosporangiate. The majority of conifers 
are monoecious, but some are dioecious, e.g., Juniperus, Araucana, Podo- 
carpus, the Taxaceae, and a few others. The arrangement of the sporo- 



SPERMATOPHYTA 



345 



phylls follows that of the leaves, lieing cyclic in the Cupressaceae and 
spiral in the other families. 

Staminate Strobilus. In all the Coniferales the staminate strobilus is 
simple, with few to many microsporophylls arising directly from the cone 
axis (Fig. 295A). Bracts are not present. The stamen is a reduced 
structure, generally consisting of a slender stalk and an expanded sterile 
tip. The microsporangia are abaxial and most commonly borne in pairs 
(Fig. 295^, C). Each stamen bears 
2 microsporangia in the Abietaceae 
and Podocarpaceae, 2 to 5 in the 
Taxodiaceae, 2 to 6 in the Cupressa- 
ceae, 6 to 15 in the Araucariaceae, 
and 4 to 8 in the Taxaceae. In the 
Araucariaceae and Taxaceae the mi- 
crosporangia are pendent on a peltate 
stamen, as in Ginkgo. The micro- 
sporangia of conifers are eusporan- 
giate, developing either from a single 
hypodermal initial or from a layer of 
initials. The wall is composed of 
several layers of cells, the innermost 
layer forming the tapetum. 

Ovulate Strobilus. A definite ovu- 
late cone is present in all the families 
of Coniferales except the Podocarpa- 
ceae and Taxaceae, where it is greatly 
reduced, in some of the Podocarpa- 
ceae to one or two ovules and in most 
of the Taxaceae to one. The ovulate strobilus differs from the staminate 
in being compound, that is, the ovules are borne on secondary axes, as in 
the Cordaitales. The main axis of the cone bears a number of bracts and 
in the axil of each is an "ovuliferous scale" (Figs. 295i) and 296^). The 
bract is homologous with the microsporophyll of the staminate cone, but 
the nature of the scale is puzzling. The view most generally held is that it 
represents a greatly reduced axillary shoot bearing a pair of leaves that are 
fused along their margins. In fact, in abnormal cones the ovuliferous 
scale is sometimes replaced by a spur shoot bearing two leaves. In the 
Abietaceae the bract and scale are free, being united only at the base. 
Although in the mature cone the bract is usually smaller than the scale 
and inconspicuous, in some cases, as in Pseudotsuga and species of Larix 
and Abies, it is large and very prominent. In the Taxodiaceae, Cupres- 
saceae, and Araucariaceae the bract and scale are coalescent. 

In families having a well-developed ovulate strobilus, this nearly 




Fig. 295. Pinus nigra. A, longitudinal 
section of staminate strobilus, each 
microsporophyll bearing a pair of abaxial 
microsporangia, X4; B, side view of a 
microsporophyll, X 10; C, abaxial view of 
same; D, ovuliferous scale, showing two 
adaxial ovules, natural size. 



346 



PLANT MORPHOLOGY 



always becomes dry and woody at maturity but, in some genera, as in 
Jumpcrus, the ripe cone is berry-like. In the Abietaceae each cone scale 
bears two basal ovules on the adaxial side (Fig. 295D), but in the other 
families the number is variable, being two to seven in the Taxodiaceae, 
one to many in the Cupressaceae, and only one in the Araucariaceae. 

The ovules are inverted in the Ab- 
ietaceae, Araucariaceae, and almost 
all the Podocarpaceae; they are 
erect in the Cupressaceae, Tax- 
aceae, and almost all the Tax- 
odiaceae. 

The ovule has a single integu- 
ment (Fig. 297A). In the Abieta- 
ceae the integument is fused to the 
nucellus below but is free at the 
apex, while in the other families, 
with few exceptions, the integu- 
ment and nucellus are either en- 
tirely free or slightly united below. 
The ovule of the Coniferales does 
not develop a nucellar beak and 
pollen chamber, their absence being 
related to the fact that swimming 
sperms are not produced. The in- 
tegument consists of an outer fleshy 
layer, a middle stony layer, and an 
inner fleshy layer. The outer layer 
is thin and usually disappears as 
the seed ripens. Both inner and 
outer sets of vascular strands have 
been eliminated. The megaspore 
mother cell is solitary and deep- 
seated (Fig. 296B). It forms a 
linear tetrad. As in other gym- 




B 

Fig. 296. Pinus nigra. A, longitudinal 
section of young ovule, showing integument 
(i), ovuliferous scale (s), bract (6), and 
megaspore mother cell (m), deeply embedded 
in the nucellus, X25; B, megaspore mother 
cell surrounded by nucellar tissue, its 
nucleus in prophase of the first reduction 
division, X400. 



nosperms, the innermost megaspore alone is functional. 

In the Podocarpaceae and Taxaceae an outer fleshy covering grows up 
around the ovule, uniting with it in Podocarpus and Torreya, but remain- 
ing separate in Taxus. This fleshy structure has been interpreted by 
some botanists as a second integument, by others as the ovuliferous scale. 

Female Gametophyte. As in the Cycadales and Ginkgoales, the devel- 
opment of the female gametophyte involves several stages, as follows: 
(1) free-nuclear division accompanied by the formation of a large central 
vacuole that results in parietal placing of the nuclei; (2) wall formation; 



SPERMATOPHYTA 



347 



(3) centripetal growth until the gametophyte is cellular throughout. A 
deeper region of nutritive tissue is usually differentiated fromamicropylar 
region of smaller cells in which the archegonia develop. The number of 
archegonia is highly variable. In the Abietaceae it is usually 2 to 5, but 
in the other families the number may be much higher, reaching 200 in 
extreme cases. 




A B 

Fig. 297. Ovule and archegonium of Pinus lambertiana. A, longitudinal section of 
ovule, showing female gametophyte with two archegonia, XIO; B, mature archegonium 
with two neck cells, a small ventral canal cell, and a large egg with a conspicuous nucleus 
and many small food bodies, X85; m, micropyle; i, integument; n, nucellus; p, pollen tube; 
a, archegonium; g, female gametophyte. 

Archegonial development follows the same pattern as in the Cycadales. 
Sometimes only two neck cells are formed but generally there are many 
more, the number varying from 4 cells in one tier to about 12 or more in 
several tiers. In the Abietaceae a definite ventral canal cell is formed 
that soon disorganizes (Fig. 2975). In the other families there is only a 
ventral canal nucleus. In Torreya it is doubtful whether even this 
is present, the nucleus of the central cell apparently functioning directly 
as the egg nucleus. A special feature of the Taxodiaceae and Cupres- 
saceae is the formation of an ''archegonium complex," an organization 
of several archegonia in contact with one another and enclosed within 
a common jacket. 



348 



PLANT MORPHOLOGY 



Male Gametophyte. The amount of vegetative tissue arising in the 
male gametophyte varies according to the family. The most primitive 
condition is seen in the Araucariaceae and Podocarpaceae, where two 
prothallial cells are cut off, these soon dividing to form a tissue of many 




Fig. 298. Male gametophyte of Pinus nigra, X600. A, microspore; B, C, D, successive 
stages in development of the pollen tube; E, pollen tube; p, prothallial cells; t, tube nucleus; 
g, generative cell; s, stalk cell; b, body cell. (After Coulter and Chamberlain.) 



cells. In the Abietaceae two prothallial cells are formed but both of 
them are ephemeral (Fig. 298A-C). Finally, in the Taxodiaceae, Cupres- 
saceae, and Taxaceae, no prothallial cells are formed. The generative 
and tube cells are nearly always differentiated before the pollen is shed 
(Fig. 298Z)). All the conifers are wind-pollinated. In the Abietaceae, 
with the exception of a few genera, e.g., Larix, Tsuga, and Pseudotsuga, 
the pollen grains develop a pair of wings that grow out from the wall (Fig. 
298). The Podocarpaceae (except Dacrydium) also have winged pollen 
grains, but those of the other families are wingless. 



SPERMATOPHYTA 349 

The young ovulate cone is ready for pollination soon after emerging 
from the l^ud (Fig. 291). Its scales separate slightly and a pollination 
droplet exudes through the micropyle of each ovule. This droplet 
catches some of the pollen grains and, upon evaporating, draws them 
down into contact with the nucellus. Following pollination, the scales 
close and the cone begins a long period of growth. The pollen grains soon 
germinate. The apical end of each pollen tube grows downward into the 
nucellus, not laterally as in the cycads. 

The tube nucleus moves into the pollen tube, while the generative cell, 
remaining at its basal end, soon gives rise to the stalk cell and body cell 
(Fig. 298£'). These pass into the tube and considerably later the body 
cell divides to form two male cells that are usually equal in size and are 
always nonciliated (Fig. 299A). In the Abietaceae the two male nuclei 
are surrounded by cytoplasm derived from the body cell but are without 
a cell wall. They remain inside the body cell until just before the time 
of fertilization. 

The pollen tube comes in close contact with the archegonium and the 
tip ruptures, discharging its contents into the egg. One of the male nuclei 
approaches the egg nucleus and the tw^o come together (Fig. 2995). If 
the second male nucleus also enters the egg, it soon disintegrates. The 
cytoplasm surrounding the male nucleus mingles with the egg cyto- 
plasm. In conifers with highly organized male cells, after entering the 
egg, the protoplast escapes from the cell wall and the cytoplasm remains 
in contact with the male nucleus, finally forming a conspicuous sheath 
around the fusing nuclei. The male and female nuclei do not fuse in the 
resting condition, but each forms a group of chromosomes that become 
arranged on a common spindle (Fig. 299C, D). Completion of the mitosis 
gives rise to the first two nuclei of the proembryo. 

In Pinus and Juniperus the interval between pollination and fertiliza- 
tion is slightly more than a year, but in most other conifers it is less, some- 
times only a month or two. During this time the development of the 
female gametophyte and the growth of the pollen tube take place. 

Embryo. In Pinus four free nuclei are formed within the fertilized 
egg as a result of two successive mitotic divisions (Fig. 300A, B). These 
nuclei move to the base of the egg, where they become arranged in a hori- 
zontal plane. Each nucleus divides and walls come in, forming two tiers 
of four cells each (Fig. 300C', D). The cells of the upper tier, which 
remain open above, divide again and then the cells of the lower tier divide. 
The proembryo now consists of four tiers of cells with four cells in each 
tier (Fig. 300£', F). The lowest tier gives rise to four embryos; the next 
tier forms the primary suspensor cells; the next one constitutes the 
"rosette tier," which may later give rise to four embryos also; while the 
four upper cells are part of the general cytoplasm of the egg, which serves 
as a large food reservoir. The nuclei of the upper cells soon disintegrate. 



350 



PLANT MORPHOLOGY 










Fig. 299. Fertilization in Pinus. A, lower end of pollen tube, showing small tube nucleus, 
stalk cell, and two large male nuclei, X250; B, two male nuclei within the egg, the larger 
one in contact with the egg nucleus, X95; C, male and female chromatin groups inside egg 
nucleus; X360; D, chromosomes derived from male and female nuclei on a common 
spindle, X 360. (From Haupt.) 



SPERMATOPHYTA 



351 




Fig. 300. Early embryogeny of Pinus monophylla. A, 2-nucleate proembryo; B, 4- 
nucleate stage; C, two of the four nuclei at base of egg; D, 8-celled proembryo; E, three 
tiers of four cells each; F, four tiers of four cells each; G, cells of suspensor tier elongating; 
H, later stage of Pinus banksiana, the four proembryos separating; r, rosette cells; s, 
primary suspensor cells; ei, 62, es, secondary suspensor cells; A and B, X75; C to G, X 125; 
H, XQO. {H, after Buchholz.) 



352 PLANT MORPHOLOGY 

Following the establishment of the embryonal, suspensor, and rosette 
tiers, the four cells of the lowest tier divide and interpose another tier 
between them and the primary suspensor cells, thus forming secondary 
suspensor cells (embryonal tubes) . This behavior may be repeated once 
or twice again. The primary suspensor cells elongate and thrust the 
lower cells into the female gametophyte tissue. The four rows of lower 
cells now separate and the secondary suspensor cells elongate. Elonga- 
tion of the suspensors, both primary and secondary, continues to such an 
extent that they become coiled and twisted (Fig. 300//). 

Each of the four terminal cells gives rise to a separate embryo. Mean- 
while one or more cells of the rosette tier may divide to form a rosette 
embryo. These ordinarily do not develop very far, however, before they 
disintegrate. As a result of receiving unequal amounts of food, the four 
primary embryos grow at different rates. The largest finally survives and 
the others become aborted. Thus the mature seed, with rare exceptions, 
has only one embryo. 

In the Abietaceae, Taxodiaceae, and Cupressaceae four free nuclei are 
formed in the egg before walls appear. In the other families a larger num- 
ber of nuclei are produced — as many as 32 or 64 in the Araucariaceae. 
After the appearance of walls, the proembryo, as a rule, consists of four 
tiers of cells in the Abietaceae, but of only three tiers in the Taxodiaceae, 
Cupressaceae, and Araucariaceae. In many conifers the cells of the 
lowest tier of the proembryo do not separate to form four embryos, as 
they do in Pinus, but remain together to form a single embryo, while the 
cells of the rosette tier collapse instead of giving rise to rosette embryos. 
In these conifers several embryos usually begin to develop in the same 
ovule, but each comes from a different fertilized egg. 

Thus polyembryony in the Coniferales is of two types: (1) cleavage 
polyembryony, where multiple embryos arise from the splitting of a single 
embryo; and (2) simple polyembryony, where more than one fertilized egg 
in the same ovule gives rise to an embryo. Each type is characteristic of 
particular genera. In the Abietaceae, for example, cleavage polyembry- 
ony is a feature of Pinus, Cedrus, and Tsuga, while simple polyembryony 
is characteristic of Larix, Picea, Pseudotsuga, and Abies. In both types of 
polyembryony only one embryo in each seed reaches maturity, the others 
degenerating. 

When mature, the conifer embryo consists of a hypocotyl terminating 
in a radicle at the suspensor end and a minute plumule at the opposite end, 
the plumule being surrounded by two or more cotyledons (Fig. 301) . The 
number of cotyledons ranges from 2 to 18, but more conifers are dicotyle- 
donous than polycotyledonous. The Abietaceae show the largest number 
and the greatest variability. The ripe seed of conifers is generally brown. 
The outer and inner fleshy layers of the integument become very thin, the 



SPERMATOPHYTA 



353 




c J^ II 

h i — 

e — «t — 



outer one often disappearing entirely. Thus the seed coat consists essen- 
tiall}^ of the middle stony layer. The embryo goes into a condition of 
dormancy in which it usually remains for many months, although the 
seeds of many conifers will germinate without undergoing a resting period. 
As in the cycads and Ginkgo, food is stored in the vegetative tissue of the 
female gametophyte, generally designated as endosperm. In germination 
the entire embryo emerges from the 
seed coat, which is carried out of the 
ground on the tips of the cotyledons. 

Summary. The Coniferales were 
probably derived from the Cordai- 
tales during the Paleozoic and repre- 
sent a parallel line of evolution to the 
Ginkgoales, which seem to have had 
a similar origin. These three orders, 
comprising the coniferophj^tes, con- 
stitute a line of descent as old as the 
cycadophytes. The Coniferales have 
retained fewer primitive reproductive 
features than the Ginkgoales, their 
chief advance being in ovule struc- 
ture and the loss of swimming sperms. 
In both orders the stem, freely 
branching, is an endarch siphonostele 
with almost no mesarch structure 

left. Moreover, the pith is small and the vascular cylinder thick. The 
leaves of conifers are characteristically small and without dichotomous 
venation. Although the microsporophylls are grouped to form a simple 
strobilus, the ovulate strobilus, except where greatly reduced, is com- 
pound, a feature also of the Cordaitales. 

The six families of conifers represent various degrees of progress from 
a common ancestry. Their advance has been in different directions, as 
follows:^ (1) The arrangement of leaves and sporophylls is spiral in all the 
families except the Cupressaceae, where it is cyclic. (2) All families have 
distinct ovulate cones except the Podocarpaceae and Taxaceae. (3) The 
bract and scale in the ovulate cone are separate in the Abietaceae but 
united in the other families. (4) Winged pollen grains are present only in 
the Abietaceae and Podocarpaceae. (5) A considerable amount of 
vegetative tissue is present in the male gametophyte of the Araucariaceae 
and Podocarpaceae. Two ephemeral prothallial cells are formed in the 
Abietaceae, none in the three other families. (6) A ventral canal cell is 
formed in the Abietaceae, only a ventral canal nucleus in the other fami- 

1 In each case the condition to be regarded as primitive is stated first. 



A B 

Fig. 301. Embryo (A) and longitudinal 
section of the seed (B) of Pinus edulis, 
X4; 0, outer seed coat; i, inner seed 
coat; c, cotyledons; s, stem tip; h, 
hypocotyl; r, root tip; e, endosperm. 



354 PLANT MORPHOLOGY 

lies. (7) An arehegoniiim complex is present only in the Taxodiaceae and 
Ciipressaceae. This is an advanced character. 

The oldest families of conifers are the Abietaceae and Araucariaceae. 
Although it is uncertain which is the more ancient, much evidence from 
the vascular anatomy of both living and extinct forms indicates that the 
Abietaceae are the ancestral stock of conifers. The Araucariaceae seem 
to have given rise to the Podocarpaceae and Taxaceae. The Taxodiaceae 
and Cupressaceae are younger than the other families and have probably 
sprung from the Abietaceae. 

7. Gnetales 

This is the highest order of gymnosperms. It includes 3 peculiar genera 
of diverse habit and distribution. Ephedra, with about 35 species, inhab- 
its arid parts of the Mediterranean region, tropical and temperate Asia, 
and western North and South America. Welwitschia, with a single spe- 
cies, is found only in arid parts of western South Africa. Gnetum, with 
30 species, occurs in the tropics of South America, Asia, and Africa. The 
fossil record of the Gnetales is very fragmentary and does not extend 
beyond the Tertiary. 

Sporophyte. The species of Ephedra are low, much-branched shrubs, 
seldom exceeding 2 m. in height, with long-jointed green stems bearing 
opposite or whorled scale-like leaves (Fig. 302.4). Some of the species are 
trailing. Welwitschia is a large turnip-shaped plant with a tuberous stem 
about 1 m. in diameter and about one-third as tall (Fig. 303). It bears a 
single pair of terminal, elongated, strap-shaped leaves with parallel vena- 
tion. They trail along the ground, reaching a length of 3 m. or more and 
becoming split into numerous segments. Except for the cotyledons, 
these are the only leaves the plant ever has. Most of the species of 
Gnetum are woody vines, but a few are shrubs or small trees. They have 
oval, leathery, opposite leaves that are net-veined and 5 to 8 cm. long 
(Fig. 304A). All three genera are cyclic in the arrangement of their 
leaves and sporophylls. 

The most distinctive feature of the vascular anatomy of all three genera 
is the occurrence of true vessels (tracheae) in the secondary wood. These 
are present in addition to tracheids. Resin canals are absent. The 
endarch condition prevails throughout the plant body, all traces of 
mesarch structure apparently having been eliminated. 

The strobili of Ephedra and Gnetum are usually monosporangiate and, 
as a rule, the two kinds occur on separate plants. The strobili of Wel- 
witschia are also functionally monosporangiate but, in the staminate 
strobilus, each set of stamens surrounds an abortive ovule, thus indicating 
an ancestral bisporangiate condition (Fig. 305C). In all three genera 



SPERMATOPHYTA 



355 



both kinds of strobili are compound, the sporophylls arising on secondary 
axes borne in the axils of bracts. 

Staminate Strobilus. The staminate strobilus consists of an axis bear- 
ing a series of bracts arranged in opposite pairs. These are connate in 




Fig. 302. A, Ephedra viridis, showing portion of staminate plant with strobili, two-fifths 
natural size; B and C, Ephedra antisiphilitica, showing a staminate {B) and an ovulate (C) 
strobilus, enlarged. {B and C, after Watson.) 



Gnetum and imbricate in the two other genera. In the axil of each bract, 
except the lower ones, is a "staminate flower," representing a simple 
strobilus. This consists, in Ephedra and Gnetum, of a stalk bearing two 
(or, in Ephedra, up to six or eight) terminal microsporangia and a pair of 
basal scales (Figs. 302B and 304C). In Welwitschia the staminate flower 
is composed of two opposite pairs of basal scales investing a whorl of six 



356 



PLANT MORPHOLOGY 



united stamens that surround a sterile ovule (Fig. 305 A, C). Each sta- 
men bears three terminal microsporangia forming a synangium. In all 
three genera the scales at the base of each staminate flower are sometimes 
designated as a "perianth." 

Ovulate Strobilus. The ovulate strobilus of Ephedra is simpler than 
that of either of the other genera, consisting of an axis bearing several 




Fig. 303. Welwitschia mirahilis. Female plant in foreground; male plant in background. 
{From a photograph furnished by the Chicago Natural History Museum.) 



opposite pairs of bracts and an erect terminal ovule or, in some species, of 
two or more ovules (Fig. 302C). In Welwitschia and Gnetum the strobilus 
has a long axis with many ovules borne in the axils of bracts (Figs. 304^ 
and 305i^). In each genus an "ovulate flower" includes a single ovule 
invested by a pair of scales that constitute a "perianth." The ovule has 
two integuments, the inner one forming a long tubular micropyle (Figs. 
305D and 306A). The outer integument has an outer fleshy layer and an 
inner stony layer. There is a set of vascular strands in the fleshy layer of 
the outer integument. In Ephedra a pollen chamber is formed by the 
breaking down of some of the nucellar tissue but there are no swimming 
sperms. In Gnetum, but not in Welwitschia, there is a slight tendency 
toward the formation of a pollen chamber. 

Female Gametophyte. The female gametophyte differs greatly among 
the three genera of Gnetales. In Ephedra it develops as in the Coniferales, 



SPERMATOPHYTA 



357 



with free-nuclear division followed by the formation of compact, small- 
celled, nutritive issue (Fig. 306). The archegonia, usually numbering 
two or three, appear in the micropylar region. They have many tiers of 
neck cells. A ventral canal nucleus is cut off but a wall does not separate 
it from the egg nucleus. 




Fig. 304. Strobili of Gnetum latifolium. A, leafy branch with staminate strobili, natural 
size; B, part of staminate strobilus, enlarged; C, an expanded staminate "flower," X5; D, 
branches with ovulate strobili, natural size; E, part of ovulate strobilus, enlarged. {After 
Bliime.) 



In WelwitscMa the development of the female gametophyte proceeds as 
far as the formation of walls, but this is incomplete and the cells are multi- 
nucleate. Nuclear fusions are said then to occur in most of the cells until 
they become uninucleate. No archegonia are formed. The cells in the 
micropylar region are multinucleate and become free eggs, sending out 
tubes into which their nuclei pass. These tubes penetrate the nucellus, 



358 



PLANT MORPHOLOGY 



where they meet the pollen tubes. Then an egg nucleus fuses with one of 
the male nuclei, after which it passes back into the female gametophyte. 
The cells in the lower region, after becoming uninucleate, continue to 
multiply, even after fertilization has taken place. They form a nutritive 
tissue. 




Fig. 305. Welwitschia mirabilis. A, young staminate strobili, natural size; B, ovulate 
strobilus, natural size; C, staminate "flower" with bracts removed, showing the six stamens 
united below and the sterile ovule with a long twisted micropylar tube, X8; D, longi- 
tudinal section of ovule, showing inner integument forming micropylar tube and outer 
integument forming a wing, X3. {A, B, C, after Hooker; D, after Church.) 



In Gnetum the female gametophyte begins its development with free- 
nuclear division. However, there is no wall formation except, in some 
species, in the basal region, where a small-celled nutritive tissue is formed. 
Each nucleus in the micropylar region is a potential egg nucleus, several 
usually becoming organized as eggs but only one being fertilized. After 
fertilization, the gametophyte becomes cellular throughout. At first the 
cells are multinucleate but later become uninucleate, as in Welwitschia. 

Male Gametophyte. In Ephedra the microspore cuts off two prothal- 
lial nuclei but only the first is organized as a cell (Fig. 307). These are 



SPERMATOPHYTA 



359 



persistent. The generative nucleus and tube nucleus are differentiated, 
the former giving rise to the stalk and body nuclei. These are not 
formed as cells. In this condition the pollen grain is shed. In the pollen 
tube two male nuclei are formed. The pollen grains of Gnetum and Wel- 




A B 

Fig. 306. Ephedra trifurca. A, longitudinal section of an ovule, showing outer integument 
(o), inner integument (i) forming a long micropylar tube, pollen chamber (p), and female 
gametophyte (g) with two archegonia, X42; B, two archegonia just before division of the 
central-cell nucleus, X112. (After Land.) 

witschia contain an ephemeral prothallial nucleus, a tube cell, and a gen- 
erative cell. A stalk cell and body cell are not formed, but later the 
nucleus of the generative cell gives rise directly to two male nuclei, as in 
angiosperms. 



360 



PLANT MORPHOLOGY 



Embryo. In all three genera of the Gnetales the embryo is dicotyle- 
donous but its development varies. In Ephedra the division of the fer- 
tilized egg nucleus results in the formation of eight free nuclei, around 
each of which there is organized a cell that becomes an independent pro- 
embryo (Fig. 308A). Only one or, rarely, two of these reach maturity. 
The proembryo sends out a suspensor tube, at the tip of which a cell is cut 




Fig. 307. Male gametophyte of Ephedra trifurca. A, pollen grain with two prothallial 
nuclei (p), generative nucleus (g), and tube nucleus (t); B, division of generative nucleus; 
C, shedding condition with two prothallial nuclei, stalk nucleus (s), body nucleus (b), and 
tube nucleus; D, pollen tube with tube nucleus (0, stalk nucleus (s), and two male nuclei 
(m); A, B, C, X 1,500; D, X500. {After Land.) 



off. This gives rise to the embryo (Fig. 3085). In Welwitschia and 
Gnetum the fertilized egg behaves as one of the proembryonal cells of 
Ephedra, the embryo arising from the fertilized egg without any free- 
nuclear division. This condition is characteristic of angiosperms. 

Summary. The occurrence of vessels in the secondary wood, of com- 
pound staminate strobili, and the prolongation of the inner integument of 
the ovule into a micropylar tube are unique features that distinguish the 
Gnetales from all other gymnosperms. The presence of true vessels is an 
angiosperm character. Others include the elimination of archegonia and 
formation of free eggs (except in Ephedra), the elimination of free-nuclear 
division in the embryogeny (except in Ephedra), the formation of two 



SPEKMATOPHYTA 



361 



male nuclei directly from the generative nucleus (except in Ephedra), and 
the presence of compound strobili with simple strobili ("flowers") bear- 
ing a "perianth." Such strobili suggest the inflorescences of certain 
angiosperms. The origin of the Gnetales is unknown. They are not 





Fig. 308. Proembryonal cells of Ephedra trifurca, X440. A, three of the eight free 
proembryonal cells; B, a proembryonal cell in which the embryo initial cell (e) is differ- 
entiated from the suspensor cell (s). {After Land.) 

closely related to any of the other gymnosperms, while any direct relation- 
ship to the angiosperms is very doubtful, although both groups may have 
had a common ancestry. The three genera do not seem to be closely 
related to one another and may have had a separate origin. Ephedra 
shows certain resemblances to the Cordaitales that suggest its derivation 
from that group. 



CHAPTER IX 
SPERMATOPHYTA (CONTINUED) 

2. ANGIOSPERMAE 

The angiosperms are the largest and most conspicuous group of modern 
plants, numbering about 195,000 species. They are also the youngest 
group, and are thought to have been derived from the gymnosperms, or 
possibly to have had an independent origin from the pteridophytes. 
They appeared in the Lower Cretaceous and from the Upper Cretaceous 
to the present time have been the dominant group of land plants (Fig. 
258). Angiosperms are found in practically all terrestrial habitats where 
plants may exist. Some occur in fresh water and even in the sea, the 
aquatic habit being secondarily acquired. A relatively few forms, having 
little or no chlorophyll, are saprophytic or parasitic. 

Although many angiosperms are woody, the majority are herbaceous. 
The woody condition is considered to be the more primitive, and the 
herbaceous one to have been derived from it. The seeds of angio- 
sperms are borne in a closed vessel, the ovary, and not, as in gymno- 
sperms, on the exposed face of an open carpel (or equivalent structure). 
The ovary represents the basal portion of a single closed carpel or of two 
or more united carpels. It ripens to form a fruit, which contains the 
seeds. Angiosperms are often called "flowering plants," as the presence 
of flowers is one of their most outstanding features. 

The two great groups (subclasses) of angiosperms are the Dicotyle- 
doneae and the Monocotyledoneae, distinguishable on the basis of the 
following combination of characters, but with individual exceptions to 
each: The dicotyledons have seeds with two cotyledons, stems with a 
hollow cylinder of vascular tissue and with a functioning cambium, leaves 
with netted veins forming an open system, and floral parts chiefly in fours 
or fives. The monocotyledons have seeds with one cotyledon, stems with 
scattered vascular bundles and without a functioning cambium, leaves 
with parallel veins forming a closed system, and floral parts typically in 
threes. 

The Dicotyledoneae include 240 families and approximately 155,000 
species, the Monocotyledoneae 45 families and about 40,000 species. 
The Dicotyledoneae comprise the Archichlamydeae, whose flowers are 
naked, apetalous, or choripetalous, and the Metachlamydeae, whose 
flowers are sympetalous. The Archichlamydeae include some members 

362 



SPERMATOPHYTA 



363 



having floral parts more or less spirally arranged, while the flowers of the 
Metachlamydeae are all definitely cyclic. The Archichlamydeae are 
generally regarded as the ancestral stock from which both the Metachla- 
mydeae and the Monocotyledoneae have been derived. 

Vegetative Organs 

The sporophyte of angiosperms presents an enormous diversity in size 
and habit, ranging from tiny herbs to tall trees 100 m. in height. Most 
angiosperms grow erect upon the ground, but some are trailing, climbing. 




Fig. 309. Cross section of a leaf of lilac {Syringa vulgaris), X250. Beginning at the top, 
the tissues are the upper epidermis, the palisade layer, the spongy tissue with numerous 
intercellular spaces, the lower epidermis in which three stomata are seen, and, in the 
center, a small vein. 

or epiphytic. Their stems are usually branched, but may be unbranched; 
they are generally aerial, but may be subterranean. As in gymnosperms, 
branching of the stem is lateral, never dichotomous. The leaves are 
typically broad and thin, but display extreme variation in size, shape, 
and other features. Their arrangement on the stem may be either spiral 
or cyclic. They may be simple or divided into leaflets (compound), pet- 
iolate or sessile, net-veined or parallel-veined, deciduous or evergreen. 
In net-veined leaves the veins form an obvious reticulum and their ulti- 
mate veinlets end freely to form an open system. In parallel-veined 
leaves the larger veins run parallel to one another and, if connected by 
cross veinlets, these form a closed system. 

Leaf Structure. In spite of their diversity in external features, the 
leaves of angiosperms are rather uniform in structure. Leaves arise as 
lateral outgrowths from the embryonic region of a stem tip and develop by 
intercalary growth. A cross section of a typical mature leaf reveals the 
following tissues (Fig. 309) : the epidermis, usually a single external layer 
of colorless cells with cutinized outer walls and containing numerous 



364 PLANT MORPHOLOGY 

stomata; the palisade tissue, generally comprising one or two layers of 
green cells, vertically elongated, and lying beneath the upper epidermis; 
the spongy tissue, a loose region of green cells and large intercellular 
spaces; the veins, vascular bundles that traverse the spongy tissue. The 
arrangement of the conducting tissues in the veins is collateral, the xylem 
lying above the phloem. Cambial activity, if present, is weak. The 
mesophyll, including the palisade and spongy tissues, is the photosyn- 
thetic tissue of the leaf. 

Root Tip. The root tip comprises the rootcap, embryonic region, 
region of elongation, and region of maturation. The rootcap is a protec- 
tive sheath. The embryonic region includes the apical meristem, of very 
limited extent, characterized by active cell division. In the region of 
elongation the newly formed cells increase in length, while in the region of 
maturation they become differentiated to form permanent tissues. In 
many roots three or four distinct growing regions, or histogens, can be 
recognized (Fig. 310). The outermost layer of cells, nearly continuous 
around the embryonic region, is the dermatogen, which gives rise to the 
epidermis. Inside the dermatogen is the periblem, consisting of several 
layers that form the cortex. In the center of the root tip is the plerome, 
which produces the stele. In the monocotyledons a calijptrogen forms the 
rootcap and lies just behind it. 

The plerome arises from a group of initials situated at its very tip. 
Just beyond lie another group of initials, often constituting a single layer. 
In the dicotyledons these form the periblem, while a third layer, beyond 
and in contact with it, gives rise to both the dermatogen and rootcap. 
In the monocotyledons the middle group of initials produce both the 
periblem and dermatogen, while the outermost layer of initials, the calyp- 
trogen, independently forms the rootcap (Fig. 310). 

Root hairs, arising in the region of maturation, are slender tubular 
extensions of the epidermal cells. They greatly increase the absorbing 
surface of the root. New ones are formed as the root increases in length. 
The older ones finally disappear. 

Mature Root. The structure of the mature root is rather uniform 
throughout the angiosperms (Fig. 311). As in other vascular plants, the 
root represents a primitive type of vascular organization, being typically 
an exarch radial protostele. The stele is surrounded by an extensive 
cortex whose innermost cells, the endodermis, have more or less thickened 
walls. Lying immediately inside the endodermis is a layer of pa- 
renchyma, or occasionally several layers, forming the pericycle. 

The process of lignification, progressing centripetally from the proto- 
xylem strands, often does not reach to the center of the root, whose cells 
then remain parenchymatous and form a pith. Such a condition is com- 
mon in monocotyledons, while in dicotyledons a pith is typically absent. 



d 



per 



SPERMATOPHYTA 



365 



per 




Fig. 310. Median longitudinal section of a root tip of onion {Allium cepa), showing 
histogens, X 170; d, dermatogen; per, periblem; pl, plerome; re, rootcap. At the tip of the 
plerome is a layer of cells that gives rise to the periblem and dermatogen. Below this 
layer is the calyptrogen, which produces the rootcap. 



366 



PLANT MORPHOLOGY 



The primary wood that forms after the protoxylem is differentiated is 
metaxylem. In dicotyledons the number of protoxylem strands is com- 
monly 4 or 5, while in monocotyledons it is generally more, often 15 or 20. 
Phloem occurs as separate strands lying between the groups of protoxylem 




Fig. 311. Cross section of the central portion of a root of baneberry (Actaea alba), 
showing primary tissues and beginning of formation of secondary tissues by the cambium, 
which lies between the secondary xylem and phloem, X200; end, endodermis; per, peri- 
cycle; pp, primary phloem; sp, secondary phloem, px, protoxylem; mx, metaxylem; sx, 
secondary xylem. 

elements. Branch roots arise in the pericycle directly opposite the proto- 
xylem strands. They then grow outward through the cortex (Fig. 312). 
Except in fibrous roots, a cambium arises between the primary xylem 
and phloem and cuts off secondary vascular tissues — secondary xylem on 
the inside and secondary phloem on the outside (Fig. 311). Soon a more 
or less continuous cylinder of secondary vascular tissues is formed. 



SPERM ATOPHYTA 



367 



Stem Tip. An embryonic region, a region of elongation, and a region 
of maturation are present in a stem tip, but are more extensive and much 
less clearly defined than in a root tip, overlapping to a consideral)le 
extent. Immediately behind the apical meristem the leaf primordia 
arise superficially as lateral outgrowths and develop in acropetal succes- 




FiG. 312. Cross section of a root of willow (Salix), showing a branch root pushing outward 
through the cortex, X55. 

sion (Fig. 313). A meristem may arise in the axil of each leaf primordium 
while it is still very small, giving rise to a lateral bud, or the meristem may 
not appear until later. 

Generally the dermatogen is clearly recognizable, but often the line of 
demarcation between the periblem and plerome is not. For this reason, 
and because it is usually difficult or impossible to relate the origin of the 
epidermis, cortex, and stele to distinct cell regions or "histogens," a 
newer and more satisfactory concept of the structure and growth of the 
stem apex is that it is made up of two distinct "growth zones." The 
outer zone, or tunica, consists of one or more (up to four or five) superficial 
layers of small uniform cells that divide anticlinally, so that each layer 
remains distinct. Periclinal divisions occur only in connection with leaf 
and bud formation. The inner zone, or corpus, comprises the central 
tissue, the cells of w^hich are larger and divide in all planes and so are 



368 



PLANT MORPHOLOGY 



irregular in size and arrangement. Thus the two zones differ both in 
position and mode of growth. Surface growth predominates in the 
tunica, while volume growth is characteristic of the corpus. Frequently 
these zones are not clearly marked off from each other and vary consider- 




1 



Fig. 313. Median longitudinal section of the stem tip of Coleus blumei, X200. In the 
center is the apical meristem (a) with a leaf primordium (b) on either side. In the axil 
of each of the older leaves (c) is a lateral meristem (d) that will produce an axillary bud. 

ably in form and relative extent. Moreover, the relative contributions 
of the tunica and corpus to the three regions of the mature stem are 
usually difficult to determine and differ according to the species. 

Mature Stem. As in the leaf, the epidermis of the young stem becomes 
cutinized and contains many stomata. The cortex consists mainly of 
green parenchyma, but sclerenchyma may be differentiated as develop- 
ment proceeds. In some stems a well-marked endodermis is present, but 
generally this layer is not clearly differentiated.. In stems that increase 



SPERMATOPHYTA 



369 



in diameter a cork cambium or phellogen arises beneath the epidermis, 
forming cork tissue. This finally replaces the epidermis as a protective 
covering. Communication between the atmosphere and the living tissues 
beneath the cork is maintained through lenticels. 

The stele is bounded externally by the pericycle, which usually consists 
of several layers of cells. Some of these may remain parenchymatous, 












Fig. 314. Conducting tissues, X200. Phloem elements: a sieve tube and a row of com- 
panion cells from a squash stem, as seen in longitudinal {A) and transverse (5) sections. 
Xylem elements: spiral (C), annular (D), and pitted vessels {E), and a wood fiber (F) from 
a stem of castor bean. 



while others become sclerenchymatous. The development of the primary 
xylem is endarch. Among gymnosperms, with the exception of the 
Gnetales, tracheids are the only conducting elements present in the xylem, 
but in angiosperms vessels are the chief elements. Tracheids are derived 
from single cells, vessels from a row of cells whose end walls break down. 
Both are lignified, the lignin being localized to form spirals, rings, an 
irregular network, or it may be so abundant that the walls are pitted 
(Fig. 314C-E). Spiral and annular elements are characteristic of proto- 
xylem, reticulate and pitted elements of metaxylem and secondary xylem. 
In addition to vessels, the secondary xylem of angiosperms may consist, 
largely or in part, of tracheids, wood fibers, and wood parenchyma (Fig. 
, 314F). The phloem is made up of sieve tubes, companion cells, and often 



370 PLANT MORPHOLOGY 

also of fibers and parenchmya (Fig. 314.4, B). Companion cells do not 
occur in pteridophytes and gymnosperms. As in the Filicinae and gym- 
nosperms, leaf gaps are formed in the vascular cylinder in connection with 
the departure of leaf traces. 

Stelar Types. The stem of most woody dicotyledons is like that of 
gymnosperms in being an ectophloic siphonostele, the vascular tissues 

Cortex Epidermis p^,^^^ 

Voiscular rav \ ^^-^^^^^^Z .Xylem 

\^(gP^^~^^iJM>»^LiZ^^^^fc_/ fibers 
P'+l^x J^lttz^m^YmM^l^\>. Cambium 



Fig. 315. Cross section of a >oung stem of magnolia {Magnolia grandiflora), showing 
vascular cylinder surrounded by the cortex and enclosing the pith, X 17. 

forming a nearly continuous cylinder enclosing the pith (Fig. 315). This 
cylinder, consisting of xylem and phloem in collateral arrangement, is 
traversed by numerous vascular rays. As in the root, the cambium, a 
meristematic layer of cells, arises between the primary xylem and phloem. 
Through cambial activity, the stems of woody dicotyledons undergo a 
great deal of secondary thickening, increasing in diameter from year to 
year. 

The stems of herbaceous dicotyledons are like those of woody dicotyle- 
dons except that the vascular tissues are greatly reduced in amount, 
either as a result of diminished cambial activity, resulting in a continuous 
but narrow vascular cylinder, or by the breaking up of the cylinder into 
separate bundles to form a dictyostele. In such stems the vascular bun- 
dles are at first separated by wide bands of parenchyma connecting the 
pith with the cortex. The cambium may extend across these ''pith rays " 



SPERMATOPHYTA 



371 



and later give rise to secondary xylem and phloem, thus forming a contin- 
uous vascular cylinder, or it may produce only parenchyma between the 
bundles, which then remain separate. In many herbs the interfascicular 
cambium fails to develop at all, the secondary vascular tissues then 
being produced within the bundles. 

Thus the stems of herbaceous dicotyledons illustrate various degrees 
of reduction from the more highly organized but more primitive condition 




Fig. 316. Cro.ss section of a young stem of Indian corn {Zea mays), showing scattered 
vascular bundles, X8. 

seen in the stems of typical woody dicotyledons. This strongly indicates 
that the woody stem is the more ancient type from which the herbaceous 
stem has been derived, probably in response to climatic changes. 

The stems of most monocotyledons display a characteristic type of 
dictyostele with scattered vascular bundles (Fig. 316). With rare excep- 
tions, a cambium is wanting, and so no secondary thickening ordinarily 
occurs. In a few monocotyledons, such as Dracaena, Aloe, and Yucca, a 
special kind of secondary thickening takes place. Here a cambium arises 
in the pericycle or inner cortex and forms a cylinder of new vascular 
tissues. 

In most monocotyledons the arrangement of the conducting tissues in 
each vascular bundle is collateral, as in dicotyledons, but frequently it 
is amphivasal, the xylem surrounding the phloem (Fig. 317). The young 
stem of a monocotyledon is usually a siphonostele with collateral bundles. 
The monocotyledons represent, in their stem structure, the final stages in 



372 



PLANT MORPHOLOGY 



a reduction series that begins with the gymnosperms and woody dicotyle- 
dons and passes through the herbaceous dicotyledons, where every inter- 
mediate condition is seen. This reduction series indicates the general 
trend of evolution as it seems to have taken place in the spermatophytes. 




Fig. 317. Cross section of an amphivasal bundle from the rhizome of sweet flag {Acorns 
calamus), a monocotyledon, showing the xylem completely surrounding the phloem, X 500. 



The Flower 

A strobilus is a group of sporophylls borne on a more or less elongated 
axis. A flower is essentially a strobilus in which the sporophylls (stamens 
and carpels) are usually borne on a shortened axis (receptacle) and are 
usually surrounded by a perianth. This distinction is untenable, how- 
ever, because some flowers have an elongated receptacle and some have 
no perianth. For convenience, any organization of sporophylls may be 
designated as a strobilus in pteridophytes and gymnosperms and as a 
flower in angiosperms. Such a distinction is arbitrary. A flower and a 
strobilus are morphologically equivalent structures. 

Practically all gymnosperms, except the Bennettitales, have monospo- 
rangiate strobili, while most angiosperms have bisporangiate ("perfect") 
flowers. In many cases the monosporangiate ("imperfect") condition 
has arisen by the suppression of stamens in the one kind of flower and of 
carpels in the other, the reduced organs often being represented by 



SPERMATOPHYTA 



373 



vestiges. The two kinds of flowers may occur on the same plant (monoe- 
cious condition) or on separate plants (dioecious condition). 

The Perianth. In a typical flower the perianth consists of two differen- 
tiated sets of parts, the outer set being the calyx and the inner set the 




stamen 



receptacle 



Fig. 318. Floral structure of the large-flowered trillium {Trillium grandiflorum) . A, a 
single flower, two-thirds natural size; B, four of the stamens and the pistil, twice natural 
size; C, cross section of the ovary, the dotted lines indicating the junction of the carpels, 
X 10; D, the floral diagram. 



corolla (Fig. 318). The calyx is made up of sepals, the corolla of petals. 
Ordinarily the sepals are scale-like and green, while the petals are larger 
and either white or of some other color than green. Both may be small 
and inconspicuous, however, as in the rushes {J uncus), or large and 
showy, as in the lilies {Lilium) . In some flowers the perianth consists of a 
single set of parts. These may be greenish and scale-like, as in the beet 
{Beta), or large and showy, as in the anemone. In either case the flower 
is said to be apetalous and the single whorl is arbitrarily designated as the 
calyx. This is based on the assumption that the corolla is the missing 



374 PLANT MORPHOLOGY 

set, which may or may not be true. In fact, the single whorl apparently 
often represents a perianth that has never become differentiated into a 
calyx and corolla. Naked flowers are those which are entirely without a 
perianth. It may have been lost through degeneration or may never have 
been developed. 

Sepals and petals are leaf-like in both form and structure. Phylogenet- 
ically they may either have been derived by sterilization from sporophylls. 




Fig. 319. Flower of Magnolia yrandijlora, a primitive type, with numerous stamens and 
carpels borne in spiral arrangement on an elongated receptacle, one-lialf natural size. 

may represent modified foliage leaves, or possibl}^ at least in some flowers, 
the sepals may have evolved from leaves and the petals from stamens. 
Often the foliage leaves and perianth parts intergrade, making it difficult 
to delimit the flower from the vegetative shoot that bears it. In some 
flowers, notably in the water lily (Nymphaea), the petals intergrade with 
the stamens. 

Most flowers have a regular (actinomorpMc) corolla, composed of petals 
alike in size and shape, the flower as a whole exhibiting radial symmetry 
(Fig. 318). This represents a relatively primitive condition. Many 
flowers have an irregular {zijgomorphic) corolla, with not all the petals 
alike, thus showing bilateral symmetry. This tendency reaches its high- 
est expression in flowers having spurs, sacs, or pouches, as in the Legu- 
minosae, Labiatae, and Orchidaceae. 

Establishment of Whorls and Definite Numbers. Primitive flowers, 
like those of the magnolia and buttercup (Ranunculus), have a convex, 
elongated receptacle bearing indefinitely numerous stamens and carpels 



SPERMATOPHYTA 375 

in spiral arrangement (Fig. 319). Such a condition is similar to that of a 
strobilus. In most flowers, however, the receptacle does not elongate but 
generally broadens at the apex, the floral parts arising from it in a series 
of whorls. The members of one whorl usually alternate with those of the 
next whorl (Fig. 318D). Commonly there are two whorls of perianth 
parts, two whorls of stamens, and one whorl of carpels. Such flowers are 
said to be pentaajdic. Where one whorl of stamens is wanting, this 
being nearly always the inner one, the flower is tetracyclic. With the 
establishment of a cyclic arrangement of floral parts, the members of each 
set are reduced to a definite number that is often the same in all whorls. 
In monocotyledons the number of parts in each whorl is generally three, 
while in dicotyledons it is usually five but often four. In many flowers 
the number of carpels is less than the number of parts in any of the other 
whorls. 

Zonal Development. A striking feature of floral evolution has been the 
tendency for the members of the same whorl to develop as a single organ. 
Thus, in some flowers, the carpels are separate, each forming a simple 
pistil, while in most flowers the carpels are organized to form a compound 
pistil (Figs. 3185, C, and 326). Similarly, in many flowers, the petals are 
wholly or partly united to form a corolla tube and the sepals are united to 
form a calyx tube. Obviously a sijncarpous flower (one with united car- 
pels) is more advanced than an apocarpous one (one with separate car- 
pels) , and a sympetalous flower (one with united petals) is more advanced 
than a choripetalous one (one with separate petals) . In some flowers the 
stamens are united to form a tube, but this condition is uncommon. 

It should be understood that, in all flowers where members of the same 
set are united, the parts do not arise separately and later fuse, but origi- 
nate together from a common meristem and develop as a single organ. 
There is a zonal development from the receptacle that involves all mem- 
bers of the same set, so that they are united from the beginning. Some- 
times the primordia of the individual members arise separately but are 
soon carried upward by zonal development from below. This results in a 
compound pistil with separate styles or in a corolla tube with free tips. 

Hypogyny, Perigyny, and Epigyny. In most sympetalous flowers the 
stamens are free above but are attached below to the corolla tube. Here 
zonation involves the members of two different sets. A still more 
advanced condition occurs where the receptacle enters into a zonal devel- 
opment with other floral sets. In hypogynous flowers all the sets arise 
independently from a more or less convex receptacle, the ovary being 
entirely free and situated above the place of attachment of the sepals, 
petals, and stamens (Fig. 320). In perigynous flowers the ovary is also 
free, but the receptacle is more or less concave, forming a disk-like or cup- 
like structure from the rim of which the sepals, petals, and stamens arise. 



376 



PLANT MORPHOLOGY 



Here zonation involves the three outer floral sets. In epigynous flowers 
the upward growth of the receptacle involves the ovary as well as the other 
floral parts, so that the ovary is embedded in the receptacle and the sepals, 
petals, and stamens seem to arise from its summit. In hypogynous and 
perigynous flowers the ovary is superior, while in epigynous flowers it is 
inferior. Hypogyny represents the most primitive and most common 
condition, epigyny the most advanced. Perigyny is intercnediate and 
least common. 

In perigynous and epigynous flowers the structure surrounding the 
ovary and bearing the sepals, petals, and stamens on its rim is generally 




ABC 

Fig. 320. Diagram.>s illustrating hypogyny (.4.), perigyny (B), and epigyny (C). In each 
flower the receptacle is stippled. 

regarded as a zonal upgrowth of the receptacle because, in the develop- 
ment of the flower, such an upgrowth actually occurs. Another view, 
supported by evidence from vascular anatomy, is that the structure 
referred to is made up of the fused basal portions of the sepals, petals, and 
stamens, to which, in epigynous flowers, the carpels are also united. 
Although such fusion cannot be seen in floral development (ontogeny), it 
is assumed to have occurred during the course of floral evolution. 

Floral Development. A longitudinal section through a very young 
flower bud reveals the fact that the floral parts arise at the tip of the 
receptacle as rounded protuberances of meristematic tissue. They arise 
in much the same way as foliage leaves from a vegetative stem tip. Ordi- 
narily their appearance is acropetal, the sepals coming first, next the 
petals, then the stamens, and finally the carpels. This sequence is shown 
in the buttercup (Ranunculus), for example, a primitive flower that is 
apocarpous and hypogynous and one in which the stamens and carpels 
arise in spiral succession on an elongated receptacle (Fig. 321). As in all 
flowers, the apical meristem does not continue to grow indefinitely, but 
sooner or later becomes transformed into carpels. 

The usual order of appearance of floral parts is modified in certain 
flowers, especially where one set is being suppressed. In the shepherd's- 
purse iCapsella), one of the Cruciferae, the petals appear after the other 



SPERMATOPHVTA 



377 




Fig. 321. Floral development in the buttercup (Ranunculus), X50. A to D, successively 
older stages; 6, bract; s, sepal; p, petal; st, stamen; c, carpel; o, ovule. 







Fig. 322. Floral development in shepherd's-purse (Capsella bursa-pastoris), X 100. 
A to D, successively older stages; s, sepal; st, stamen; c, carpel; p, petal. The order of 
appearance differs from that of a typical flower in that here the petals appear last. 



378 



PLANT MORPHOLOGY 



parts have arisen but, of course, in their proper place between the sepals 
and stamens (Fig. 322). This flower, when mature, has small petals. 
In the fleabane {Erigeron) and other members of the Compositae the 
sepals are the last members to appear. They remain vestigial. This 
flower shows the epigynous type of development (Fig. 323). 




Fig. 323. Floral development in fleabane (Erigeron). A, very young and older inflores- 
cence, the flowers arising on the convex receptacle, X75; B to E, successive stages in the 
development of a single flower, X200; b, bract; fl, flower; p, corolla; st, stamen; c, carpel; 
s, calyx. 

The Stamen. The stamen of angiosperms is the same structure as in 
gymnosperms, a microsporophyll. Generally it is differentiated into a 
terminal, club-like, spore-bearing portion, the anther, and a slender stalk, 
the filament (Fig. 318B). A cross section of a young anther usually shows 
four microsporangia, but the number may vary among different angio- 
sperms from one to many (Fig. 32-iA). As a rule, the microsporangia 
extend the entire length of the anther. Later, by the breaking down of 
the intervening tissue between each pair of microsporangia, two large 
cavities may be formed (Fig. 3245) ; or the four microsporangia may 



SFERMATOPHYTA 



379 



remain separate. When the stamen is mature, the microsporangia, 
regardless of their number, are called pollen sacs. 

As in gymnosperms, the development of the microsporangia is euspo- 
rangiate, A very young anther is made up of uniform meristematic tissue 




Fig. 324. Cross section of a young and of a mature anther of lily {Lilium). A, young 
anther, the four microsporangia with sporogenous tissue, X60; B, mature anther with two 
pollen sacs containing pollen grains, X30. The tapetum, surrounding the sporogenous 
tissue and conspicuous in A, has broken down in B, while the endothecium has developed 
bands of thickening. {B, after Chamberlain.) 



surrounded by an epidermis. As seen in cross section, four lobes soon 
appear and a conducting strand becomes differentiated in the center. 
The cells forming the hypodermal layer are probably all potentially spo- 
rogenous but, as a rule, are actively so only in four regions, viz., under the 
lobes. Only one longitudinal row of hypodermal cells may be differen- 
tiated under each lobe as sporangium initials, as in the Malvaceae and 



380 



PLANT MORPHOLOGY 




Fig. 325. Early development of the microsporangium of lochroma lanceolatum, X400. 
Each stage also shows outline of entire anther, X32. A, cross section of portion of young 
anther with hypodermal initial cells (shaded) ; B, division of initials to form primary 
parietal cells (outer shaded layer) and primary sporogenous cells; C, later stage, showing 
two layers of parietal and of sporogenous cells (latter heavily shaded) and differentiation 
of inner portion of tapetum (lightly shaded) ; D, later stage, showing anther wall composed 
of epidermis, endothecium, and middle layers; also sporogenous tissue (heavily shaded) 
surrounded by tapetum (lightly shaded). 

most members of the Compositae, but ordinarily a plate including several 
or many hypodermal cells appears (Fig. 325A). 

In the development of a microsporangium, the formation of a periclinal 
wall in each initial separates the outer primary parietal cells from the inner 
primary sporogenous cells (Fig. 325B). The former, lying immediately 



SPERM A TOPH YTA 38 1 

beneath the epidermis, then undergo further perichnal divisions, usually- 
forming about three to five layers of parietal tissue (Fig. 325C, D) . The 
outermost parietal layer, lying next to the epidermis, is the endothecium. 
As a rule, by the development of fibrous bands of thickening, the endo- 
thecium becomes hj^groscopic and assists in the dehiscence of the anther 
(Fig. 324B). The innermost layer of parietal tissue forms part of the 
tapetum, the rest of which is derived from the cells immediately in con- 
tact with the sporogenous cells on their inner side. Sometimes the 
tapetum becomes two- or three-layered. An interesting feature is the 
division of the tapetal nuclei to form two or more free nuclei in each cell 
(Fig. 32oZ)). The middle layers and tapetum generally disappear before 
the maturing of the spores, the ripe sporangium wall usually consisting 
only of the epidermis and endothecium. A tapetal Plasmodium, sur- 
rounding the microspores, is seen in several groups, such as the Compositae 
and Helobiales. 

The cells forming the primary sporogenous layer generally undergo two 
or three divisions to form the microspore mother cells, which then greatly 
enlarge and assume a spherical form. The next two divisions, during 
which the number of chromosomes is reduced one-half, result in the for- 
mation of tetrads. At this time the tapetum disorganizes. The tetrads 
are mostly tetrahedral in dicotyledons and isobilateral in monocotyledons. 
A linear arrangement of microspores is rare, but occurs in the milkweeds 
(Asclepias) and a few other forms. 

Upon separation from the tetrads, the microspores have developed a 
two-layered cell wall consisting of an outer exine and an inner inline (Fig. 
334). Although commonly the exine is thickened, the cell wall usually 
has one or more thin places where the exine is not formed and through 
which the pollen tube may later emerge. Its outer surface usually bears 
warts or spines, or is variously sculptured. Ordinarily the microspores 
become free from one another at maturity, but in some angiosperms {e.g., 
Typha and Rhododendron) the members of the tetrad do not separate, 
while in a few others, notably in the milkweeds (Asclepias) and certain 
orchids, all the spores in a sporangium cling together and escape as a mass, 
which is called a pollinium. 

As a rule, the anther dehisces by means of two longitudinal slits (Figs. 
318B and 324J5), but sometimes by terminal slits or pores, by hinged 
valves, or irregularly. 

The Carpel. The carpel of angiosperms is really a megasporophyll but, 
instead of bearing the ovules freely exposed, as in gymnosperms, it sur- 
rounds them. Where two or more carpels are wholly or partly united, 
forming a compound pistil, the flower is said to be syncarpous. Where 
the carpels are free, each constituting a simple pistil, the flower is apocar- 
pous. The enlarged, hollow, lower portion of the pistil, the ovary, encloses 



382 



PLANT MORPHOLOGY 



one or more ovules (Fig. 318^). Generally a slender stalk-like style arises 
from the ovary. In some compound pistils the styles as well as the 
ovaries are united, while in others the styles are wholly or partly free 
(Fig. 32G). The style may be hollow but usually is solid. The tip of the 
style, termed the stigma, is not a morphological unit, but merely an 
exposed and often expanded portion of the tissue that lines the ovarian 
cavity and extends upward through the style. In some flowers the stig- 
matic surface extends down the outside of the style. 




ft 




Fig. 326. Pistils showing various degrees of union between the carpels. A, five separate 
carpels in the flower of stonecrop (Sedum), X3; B, pistil of garden pink (Dianthiis) with 
two carpels having united ovaries and free styles, X3; C, pistil of geranium {Pelargonium) 
with five united carpels having free stylar tips, X6; D, pistil of nightshade (Solanum) with 
two completely united carpels, X6. 



An ovary may contain a single cavity (locule) or two or more cavities 
separated from one another by partitions. The ovules may be attached 
to the walls of the ovary or to the partitions between the locules, in either 
case being foliar in origin. In some cases the receptacle grows upward 
into the ovarian cavity and bears the ovules either terminally, laterally, 
or in both ways. Such ovules are cauline in origin. 

The carpel of angiosperms is generally regarded as the equivalent of an 
infolded leaf bearing ovules along its fused margins. In fact, in many 
apocarpous flowers the carpel arises as an open structure that encloses the 
ovules as development proceeds. Although a foliar organ, the carpel is 
not a transformed foliage leaf. It is a sporophyll — an organ with its own 
evolutionary history reaching far back into a pteridophyte ancestry. 
Sporophylls and foliage leaves have undergone a parallel evolutionary 
development. Another view regarding the nature of the carpel is that it 
is a greatly reduced branch system. This theory is based on its supposed 



SPERMATOPHVTA 



383 



evolution from the sporangium-bearing leaf of the ferns, which is often 
interpreted as a modified branch system (see page 305). 

The Ovule. In the development of an ovule, at first a small rounded 
protuberance appears (Fig. 327). This is the nucellus, or megasporan- 
gium proper. At its base an integument then arises as a ring of tissue, the 
nucellus meanwhile increasing in prominence. Later, if a second integu- 
ment is to be formed, it arises outside the first one. The integument or 
integuments grow out beyond the nucellus, leaving a narrow passageway, 






Fig. 327. Successive stages in the development of an anatropous ovule, the last repre- 
senting a section through a mature ovule. (After Gray.) 





A B c 

Fig. 328. Directions of ovules: A, orthotropous; B, campylotropous; C, anatropous. 
(After Coulter.) 

the micropyle. In the Archichlamydeae and Monocotyledoneae two 
integuments are generally present, but in nearly all the Metachlamydeae 
there is a single massive one. As a rule, the ovule is borne on a short 
stalk, the funiculus, the part of the ovary to which it is attached being the 
placenta. The basal portion of the ovule is called the chalaza. 

When mature, ovules may be erect {orthotropous), curved {campylotro- 
pous), or inverted {anatropous) (Fig. 328). There are also intermediate 
conditions. The first represents the most primitive condition and is 
characteristic of most cauline ovules. It is found among the Urticaceae, 
Polygonaceae, Xyridaceae, and a few other relatively primitive families 
of Archichlamydeae and Monocotyledoneae. The second type is also 
uncommon, being found among the Chenopodiaceae, Caryophyllaceae, 
Cruciferae, and Gramineae. The third condition is most advanced and 
most common. Anatropous ovules are found in many of the Archi- 
chlamydeae and Monocotyledoneae, and almost exclusively in the 



384 



PLANT MORPHOLOGY 




Metachlamydeae. In anatropous ovules having two integuments, the 
outer one is united on one side with the funiculus. 

The development of the ovule is eusporangiate. Generally a single 
hypodermal initial is differentiated at the apex of the nucellus, but occa- 
sionally there are two or more initials, especially among the lower families 
of Archichlamydeae. Ordinarily the initial, by a periclinal division, gives 
rise to an outer primary parietal cell and an inner primary sporogenous cell, 

as in the microsporangium (Fig. 329). The 
parietal cell may divide periclinally once or 
twice again, or it may remain undivided. 
In practically all the Metachlamydeae, and 
exceptionally in the two other groups of 
angiosperms, wall tissue is eliminated, the 
hypodermal initial functioning directly as 
the megaspore mother cell (Figs. 330A and 
333A). In all other angiosperms the pri- 
mary sporogenous cell is the megaspore 
mother cell. It gives rise, by two successive 
divisions, usually to a linear tetrad, the re- 
duction in the number of chromosomes tak- 
ing place at this time (Fig. 330B-D). A 
T-shaped arrangement of the megaspores is 
not infrequent. 

The Female Gametophyte. As in gym- 
nosperms, the female gametophyte (embryo 
sac) develops within the tissues of the ovule 
and similarly is nearly always formed by the 
innermost megaspore, the other three degen- 
erating. The functional megaspore greatly 
enlarges, encroaching vipon and absorbing the 
abortive megaspores as well as more or less of 
the surrounding nucellar tissue (Fig. 330E). Typically the megaspore 
nucleus gives rise to eight nuclei by three successive divisions (Figs. 
S30F-H and 331). Thus, as in gymnosperms, the development of the 
female gametophyte is initiated by free-nuclear division, but in angio- 
sperms the nuclei are almost always definitely eight in number. There is 
no wall formation at this stage, but the free nuclei exhibit a striking 
polarity, four being at one end of the embryo sac and four at the other 
end. This polarity is established after the first nuclear division, when a 
large vacuole appears between the daughter nuclei (Fig. 330G). 

One nucleus from each polar group now comes to the center of the 
embryo sac. These two nuclei, called polar nuclei, come in contact with 
each other and generally unite at once to form the fusion nucleus, or some- 




B 

Fig. 329. Early development 
of the megasporangium of the 
willow (Salix), showing single 
hypodermal initial (A) and the 
two cells derived from it (B): 
the primary parietal cell (outer 
shaded one) and the primary 
sporogenous cell. (After 
Chamberlain.) 



SPERM ATOPHVTA 



385 




G H 

Fig. 330. Megasporogenesis and early development of the embryo sac of Anemone patens, 
X200. A, young ovule with megaspore mother cell; B, first meiotic division; C, comple- 
tion of first division; D, linear tetrad of megaspores; E, functional megaspore enlarging; F, 
division of mega.spore nucleus; G, 2-nucleate embryo sac; H, 4-nucleate embryo sac. 
(From preparations supplied by Dr. George H. Conant.) 

times remain distinct (Fig. 331). The three nuclei left at the micropylar 
end of the embryo sac become organized as naked cells, forming the egg 
apparatus. Of these, one is the egg and the two others are synergids. 
Ordinarily the egg lies between the synergids and slightly exceeds them in 
size; its nucleus is farther from the micropyle than their nuclei are. 
Usually the three cells forming the egg apparatus are pyriform. The 



386 



PLANT MORPHOLOGY 



synergids are generally interpreted as potential eggs normally incapable 
of being fertilized. The three nuclei at the chalazal end of the embryo 
sac, which is the one opposite the micropylar end, are usually organized 
as small naked or walled cells called antipodals. The antipodals, some- 
times ephemeral, are usually somewhat persistent, rarely giving rise later 



ow 




Fig. 331. Ovule of Anemone patens with mature embryo sac, X200; ow, ovary wall; oc, 
ovary cavity; st, stalk of ovule; i, integument; m, micropyle; es, embryo sac; s, synergid; e, 
egg;/, fusion nucleus; a, antipodal cell. 

to an extensive tissue. The antipodals are said to be nutritive in function 
and probably represent vegetative cells of the female gametophyte in 
various stages of disappearance. 

Variations in Embryo -sac Development. The development of the ordi- 
nary type of embryo sac is characterized by two important features: 



SPERMATOPHYTA 387 

(1) Five successive nuclear divisions intervene between the megaspore 
mother cell and the formation of the egg. (2) The embryo sac is derived 
from a single megaspore, the innermost one. Numerous deviations from 
this type of development are seen throughout the angiosperms. The prin- 
cipal ones are as follows (Fig. 332) : 

Oenothera. In Oenothera and other members of the Onagraceae, a 
linear tetrad of megaspores is formed in the usual way but, with few 
exceptions, the outermost (micropylar) megaspore, rather than the inner- 
most (chalazal) one develops into the embryo sac. Only four successive 
nuclear divisions intervene between the megaspore mother cell and the 
egg. The nucleus of the functional megaspore gives rise to two nuclei, 
both of which remain at the micropylar end of the embryo sac, a vacuole 
appearing below them. Each again divides and, of the four nuclei thus 
formed, three are organized into the egg apparatus, while the fourth 
becomes a polar nucleus. Because of the absence of a fifth nuclear divi- 
sion, there is no second polar nucleus and there are no antipodals. 

Allium. This type of embryo sac occurs not only in certain other 
genera of Liliaceae, such as Scilla and Trillium, but also in numerous 
genera belonging to many other families. The megaspore mother cell 
divides into two cells of which the upper one soon degenerates, while the 
lower one undergoes three successive free-nuclear divisions to form the 
embryo sac. When mature, this displays the usual kind of eight-nucleate 
organization. It is apparent that only four nuclear divisions occur 
between the megaspore mother cell and the egg and that two megaspore 
nuclei participate in the formation of the embryo sac. 

Peperomia. In this genus, one of the Piperaceae, all four megaspore 
nuclei are involved in the formation of the embryo sac, no walls being 
formed between them. The four nuclei are arranged in a cross-like man- 
ner, with a large vacuole between them; two successive nuclear divisions 
follow, the resulting 16 free nuclei being arranged in various ways, 
depending on the species. The egg apparatus consists of the egg and a 
single synergid. In Peperomia pellucida eight nuclei form the fusion 
nucleus and six degenerate, while in Peperomia hispidula 14 nuclei form 
the fusion nucleus. 

A^arious modifications of the Peperomia type are seen in certain other 
families. Thus, in Gunnera, three nuclei form the egg apparatus, seven 
unite to form the fusion nucleus, and six degenerate. In the Penaeaceae 
and certain species of Euphorbia the 16 free nuclei are arranged in four 
groups of four each. One member of each group becomes a polar nucleus, 
the four polar nuclei fuse, while the three remaining nuclei in each group 
become organized as cells. The three cells at the upper end of the embryo 
sac constitute the egg apparatus, the other cells finally degenerating. 



388 



PLANT MORPHOLOGY 



Megaspore I I HI IS" Y Mature 

Mother Cell Division Division Division Division Division Embryo Sac 




o 

Ordinary Type 






o 

Oenothera 








Allium 









Peperomia 






Penaea 





(0 01 





Fritiilaria 










Plumbagelia 




o 

Plumbago 









Adoxa 






Fig. 332. Principal types of embryo-sac development in aiigiosperms. 



SPERMATOPHYTA 389 

Fritillaria. In Fritillana, Tulipa, Ltlium, and certain other Liliaceae, 
as well as members of other families, a characteristic development occurs 
(Fig. 333). The four megaspore nuclei, arranged in a linear row, are not 
separated by walls. As the embryo sac enlarges, the three lower mega- 
spore nuclei migrate to the chalazal end. All four nuclei now begin to 
divide, but before the division is complete, the three lower nuclei fuse. 
As a result, a second four-nucleate stage appears, the two micropylar 
nuclei being separated from the two chalazal ones by a large vacuole. 
The micropylar nuclei are haploid, the chalazal ones triploid. After 
another free-nuclear division occurs, the upper group of four nuclei give 
rise to the egg, two synergids, and a haploid polar nucleus, the lower group 
to three antipodals and a triploid polar nucleus. Sometimes only two 
antipodals are formed. 

Plumhagella. The embryo sac of Plumbagella, one of the Plumbagina- 
ceae, closely resembles that of Fritillaria. Four megaspore nuclei arise 
without any wall formation. The three lower nuclei pass to the chalazal 
end of the embryo sac, a large vacuole appearing between them and the 
micropylar nucleus. The three chalazal nuclei fuse. Both nuclei now 
divide and the embryo sac usually remains four-nucleate. The egg is 
organized from one of the two haploid nuclei, an antipodal cell from one 
of the two triploid nuclei. The fusion nucleus is formed by the union of 
the two remaining nuclei, one of which is haploid and the other triploid. 

Plumbago. In Plumbago and several other genera of the Plumbagina- 
ceae, a unique type of embryo sac is seen. The four megaspore nuclei, 
formed without the appearance of walls and arranged in a cross-like man- 
ner, undergo one more division. One of each pair of nuclei becomes a 
polar nucleus, the second member of the micropylar pair is organized into 
the egg, while the three other nuclei degenerate. The mature embryo 
sac has only two nuclei — that of the egg and a fusion nucleus formed by 
the union of the four polar nuclei. 

Adoxa. The type of development seen in this genus and in Sambucus, 
both members of the Caprifoliaceae, has been reported in members of 
many other families, but some of these {e.g., Lilium) have been shown to 
belong to other types, while many others are doubtful. In Adoxa no 
cell-wall formation accompanies the two divisions of the megaspore 
mother cell, the four nuclei dividing again to form an eight-nucleate game- 
tophyte. Thus the egg is separated from the megaspore mother cell by 
only three free-nuclear divisions and all four megaspore nuclei participate 
in the formation of the embryo sac. This has the ordinary type of 
mature organization. 

Male Gametophyte. In angiosperms the male gametophyte is reduced 
even more than in gymnosperms. No prothallial cells are produced. 
Before the anther dehisces, the microspore nucleus divides to form the 



390 



PLANT MORPHOLOGY 




D -- H ^^-^ 6 

Fig. 333. Development of the embryo sac of Fritillaria biflora X200. A, entire ovule 
with megaspore mother cell; B, 2-nucleate stage; C, first 4-nucleate stage; D, the four 
megaspore nuclei beginning to divide; E, completion of division, resulting in second 
4-nucleate stage; F, the two micropjlar nuclei being haploid and the two chalazal ones 
triploid; G, the two micropylar nuclei and one of the chalazal nuclei dividing again; H, 
mature embryo sac with egg, two synergids, and haploid polar nucleus at micropylar end 
and triploid polar nucleus and two antipodal cells at chalazal end. 



SPERMATOPHYTA 



391 



generative nucleus and tube nucleus, the former becoming organized as a 
small, naked generative cell lying within the larger tube cell (Fig. 334). 
The generative cell is usually elliptical, lenticular, or spindle-shaped. 
The tube nucleus is generally large, with a large nucleolus and little 
chromatin. The generative nucleus is usually smaller, with a small 
nucleolus or none, and with considerable chromatin. The generative 
cell gives rise directly to two male cells, dividing either within the pollen 
grain or, somew^hat more frequently, in the pollen tube. The male cells 
show considerable variation in form but are 
never ciliated. In most angiosperms the male 
cells remain intact, while in some the mem- 
brane around each seems to disappear, leav- 
ing their nuclei free. The male nuclei often 
become vermiform, especially after entering 
the embryo sac. 

Fertilization. As in gymnosperms, pollina- 
tion must precede fertilization but, because 
the ovules of angiosperms are enclosed in an 
ovary, the pollen grains cannot come in con- 
tact with them. Pollen is transferred by vari- 
ous agencies from the anther to the receptive 
surface of the style (the stigma), where it 
germinates, putting forth a long pollen tube 
that grows down the inside of the style and 
into the cavity of the ovary. Branching 
pollen tubes, characteristic of gymnosperms, 
are found in only a few angiosperms, notably 
among members of the amentiferous orders. 

Where there is a stylar canal, the pollen tube usually grows down 
through it, but where the style is solid, as is more commonly the case, the 
tube secretes enzymes that digest a passageway to the ovary. The 
nucleus and cytoplasm of the tube cell, as well as the generative cell, pass 
down the pollen tube as it develops. The tube nucleus usually lies at the 
tip of the advancing pollen tube and apparently is concerned with its 
development. While the tube is developing, or frequently before the 
pollen grain is shed, the generative cell gives rise to two male cells. 

Upon reaching the cavity of the ovary, the pollen tube grows along the 
ovary wall until it reaches one of the ovules, which it then enters, ordi- 
narily through the micropyle. After penetrating the intervening nucellar 
tissue, the tip of the pollen tube ruptures and its contents are discharged 
into the embryo sac. The tube nucleus soon disintegrates, but both male 
cells (or male nuclei, as the case may be) enter the embryo sac. One of 
the male nuclei penetrates the egg and unites with the female nucleus, thus 




Fig. 334. Section of a pollen 
grain of lily {Lilium auratum) 
in the shedding condition, 
X750. The smaller, naked 
generative cell lies within the 
larger tube cell, each having 
its own nucleus. 



302 



PLANT MORPHOLOGY 



effecting fertilization (Fig. 335). Although distinct male cells may be 
present in the pollen tube, or even in the embryo sac, there is evidence 
indicating that, in most angiosperms, only a male nucleus enters the cyto- 
plasm of the egg. Immediately after fertilization, the egg becomes sur- 
rounded by a cell wall. 

The second male nucleus entering the embryo sac now unites with the 
nucleus resulting from the fusion of the two polar nuclei, thus forming the 
primary endosperm nucleus. This unique behavior, which has been called 

"double fertilization," has been observed in 
so many angiosperms that it must be regarded 
as characteristic of the group as a whole. 
Usually one of the synergids is destroyed by 
the entrance of the pollen tube, while the other 
synergid, as well as the antipodal cells, gen- 
erally disappear soon after fertilization has 
taken place. 

Although ordinarily the pollen tube enters 
the ovule through the micropyle (porogamy) , 
in some of the more primitive Archichla- 
mydeae it may penetrate the lower end of the 
ovule. This behavior, known as chalazogamy , 
has been observed in the Casuarinaceae, 
Juglandaceae, Corylaceae, Urticaceae, and 
Euphorbiaceae. In certain other angiosperms 
the pollen tube may follow an intermediate 
route, entering the ovvile through the integu- 
ment (mesogamy). 
The behavior of the polar nuclei is variable, depending on the species. 
Generally they unite before the pollen tube enters the embryo sac, forming 
the fusion nucleus (Fig. 331). To this the male nucleus later is added. 
Sometimes, as in Fritillaria and Lilium, the fusion of the polar nuclei is 
delayed until the male nucleus has joined them, all three then fusing 
simultaneously (Fig. 335). Sometimes the polar nuclei remain at oppo- 
site ends of the embryo sac until the pollen tube has entered. Then the 
male nucleus fuses with the micropylar polar nucleus, the other one join- 
ing them later. 

Because typically a male nucleus unites with two haploid polar nuclei, 
the primary endosperm nucleus, of course, is triploid. In some forms, 
however, as in the Onagraceae, it is diploid, being formed by a union 
between the male nucleus and one polar nucleus; while in such genera as 
Peperomia, Fritillaria, Lilium, Plumbago, and Penaea various degrees of 
polyploidy are attained, the primary endosperm nucleus arising from the 




Fig. 335. Fertilization in 
Fritillaria bifiora, X 250. One 
naale nucleus is in contact 
with the egg nucleus, while 
the other has joined the two 
polar nuclei. 



SPERMATOPHYTA 



393 



fusion of four or more nuclei of the embryo sac, with the addition of the 
male nucleus. 

Following fertilization, the petals and stamens wither and drop off, and 
often the sepals do likewise. As the ovules are transformed into seeds, 
the ovary enlarges to form a fruit. Thus normally fertilization provides 
a stimulus that has far-reaching 
effects. 

Endosperm. Typically endo- 
sperm arises from a triple-fusion 
nucleus, division of which usually 
precedes that of the fertilized egg. 
Sometimes each of the nuclear di- 
visions is accompanied by the for- 
mation of a wall, so that a tissue is 
formed at once (Fig. 33(3A). More 
commonl}^, however, the formation 
of endosperm is initiated by free- 
nuclear division (Fig. 336fi) . These 
free nuclei are usually parietally 
placed but sometimes fill the em- 
bryo sac. Unless the endosperm 
is absorbed by the embryo while 
still in the free-nuclear stage, wall 
formation then takes place, often 
simultaneously throughout the en- 
dosperm, resulting in a compact 
tissue without intercellular spaces. 
Reserve food becomes stored in its 
cells, generally in large quantities. 
This may be deposited as hemicel- 
lulose on the cell walls, which often become very thick, as in the date 
and many other palms. In some angiosperms, as in many of the Podo- 
stemaceae and Orchidaceae, the endosperm is absent or greatly reduced, 
often being represented by only a few free nuclei. The endosperm may 
persist in the seed as a food-storage tissue or may be entirely absorbed by 
the developing embryo. 

The integument or integuments of the ovule are transformed into the 
testa of the seed. The nucellus is almost or entirely destroyed during the 
development of the seed, but in such forms as the Centrospermales it 
persists and gives rise to a tissue, called perisperm, which becomes the 
chief food-storage region. In the Nymphaeaceae the seed contains both 
endosperm and perisperm. 





A B 

Fig. 336. Two methods of endosperm 
formation. A, Silphium laciniatum, nu- 
clear division followed immediately by wall 
formation, X300; B, FritiUaria biflora, 
endosperm arising by free-nuclear division, 
X 150. In A, the fertilized egg (above) has 
not yet divided; in B, it has divided once. 



39-i PLANT MORPHOLOGY 

The nature of the endosperm in angiosperms is very confusing. In 
gymnosperms it is obviously vegetative tissue of the female gametophyte, 
and thus necessarily arises before fertilization. In angiosperms it arises 
after fertilization, ordinarily from a triple fusion of nuclei, one of which is 
male, another female (since it is sister to the egg nucleus), and a third 
vegetative. Some have regarded endosperm as gametophyte tissue stim- 
ulated to develop by nuclear fusions. Others have considered it to be 
sporophyte tissue, the twin of the embryo. Since it is ordinarily triploid, 
however, it cannot be either gametophyte or sporophyte in the strict 
sense of the terms. It might better be regarded as undifferentiated tissue 
continuing the growth of the female gametophyte and stimulated to 
develop by nuclear fusions. The union of the male nucleus with the polar 
nuclei cannot be regarded as an act of fertilization because (1) the effect 
of the fusion is merely to fvu-nish a growth stimulus; (2) more than a single 
male and female nucleus is involved; and (3) the product of the triple 
fusion is not a new individual. 

Embryo. The development of the embryo from the fertilized egg does 
not, as in nearly all gymnosperms, begin with free-nuclear division, but 
each division is accompanied by the formation of a cell wall. Since 
embryogeny differs in dicotyledons and monocotyledons, except in the 
earliest stages, a representative example of each will be described. 

Capsella. The sequence of embryonic stages can be followed easily in 
the common shepherd's-purse (Capsella), a dicotyledon belonging to the 
Cruciferae. Here the zygote, by a series of transverse divisions, gives rise 
to a proembryo of varying length (Fig. 337A, B). The terminal cell (the 
one farthest from the micropyle) forms practically all the embryo, while 
the other cells give rise to the suspensor. The basal cell of the suspensor 
is much larger than the others. The terminal cell undergoes three suc- 
cessive divisions, each at right angles to the preceding one, thus resulting 
in the formation of octants (Fig. 337C, D). The first division is always 
vertical but the second vertical and the horizontal divisions may occur in 
either order. Of the eight cells now constituting the embryo, the upper 
tier of four cells eventually gives rise to the cotyledons and stem tip, the 
basal tier to all the hypocotyl except its tip. 

The suspensor elongates, becoming 8 to 10 cells in length and pushing 
the embryo downward. A peripheral layer of primary epidermal cells, 
the dermatogen, is now cut oE by periclinal walls appearing in all 8 cells of 
the embryo (Fig. 337^). Additional longitudinal and transverse divi- 
sions occur in the inner cells and soon the periblem, comprising the cells 
eventually to produce the cortex, is differentiated from the plerome, which 
gives rise to the stele (Fig. 337F). The plerome is complete at the tip of 
the hypocotyl but the periblem and dermatogen are not. They are com- 
pleted at the expense of the adjacent cell of the suspensor. This divides 



SPERMATOPHYTA 



395 




Fig. 337. Successive stages in early development of the embryo of Capsella bursa-pastoris, 
a dicotyledon, X500. A, two-celled proembryo; B, three-celled proembryo; C, proembryo 
with longitudinally divided terminal cell and enlarged basal cell; D, terminal cell divided 
to produce octants, four cells lying beneath the four shown; E, cutting off of dermatogen 
by periclinal walls; F, differentiation of periblem and plerome, the former indicated by 
lighter shading; the cell at the upper end of the suspensor is the hypophysis; G, completion 
of periblem by cell cut off the hypophysis; H, later stage, showing cell divisions throughout 
the embryo; 7, completion of dermatogen from middle tier of cells derived from the 
hypophysis. 



transversely into two cells. The upper cell, called the hypophysis, con- 
tributes to the embryo, while the lower one is added to the suspensor. 

The hypophysis divides transversely, the cell next to the embryo com- 
pleting the periblem and the other cell undergoing tvv'o longitudinal divi- 
sions at right angles to each other to form a plate of four cells (Fig. 
337G, H). In a later stage, each of these four cells divides transversely, 



390 PLANT MORPHOLOGY 

the upper tier completing the dermatogen and the lower tier forming the 
first layer of the rootcap (Fig. 337/). This stage is further marked by the 
appearance of the two cotyledons, one on each side of the stem tip, which 
lies at the upper end of the hypocotyl. Thus the stem tip is terminal and 
the cotyledons are lateral. 

Many dicotyledons follow the general course of embryogeny as seen in 
Capsella, but there are a number of departures from it. In the Nymphae- 
aceae, for example, a globular proembryo is developed and generally no 
suspensor is formed. In the Myrtaceae a massive proembryo fills the 
micropylar end of the embryo sac and several embryos may be differen- 
tiated from it. In the Ilubiaceae and Leguminosae the suspensor is 
enormously elongated. 

Sagittaria. The development of the embryo of Sagittaria, one of the 
Alismaceae, is representative of the more primitive families of monocotyle- 
dons. Here the proembryo is a filament of three cells^ — a large basal cell, a 
middle cell, and a terminal cell (Fig. 338A, B). The basal cell enlarges 
considerably but does not divide, constituting the greater part of the sus- 
pensor. The middle cell, by a series of transverse and vertical divisions, 
forms the stem tip, hypocotyl, root tip, and the rest of the suspensor (Fig. 
338C-F). The terminal cell gives rise to the cotyledon. It divides first 
by a vertical wall and then by walls in the two other planes, thus forming 
octants (Fig. 338Z), E). The dermatogen arises in the cotyledon by the 
formation of periclinal walls and proceeds toward the root end of the 
embryo (Fig. 338(t). Later the periblem and plerome are differentiated. 
The stem tip arises as a depression in the side of the embryo, thus being 
lateral in position rather than terminal as in the dicotyledons (Fig. 
338i/, 7). 

A number of modifications of the Sagittaria type of embryogeny have 
been noted. For example, the Araceae have a massive proembryo and 
lack a suspensor. The Liliaceae have a filamentous proembryo that soon 
becomes massive. In the Orchidaceae the body regions are not differen- 
tiated and, in many forms, a large suspensor becomes a haustorial organ. 

In Agapanthus, a member of the Liliaceae, dicotyledonous embryos are 
occasionally produced. The proembryo is more or less massive. Its tip 
broadens and a peripheral cotyledonary zone gives rise to two growing 
points. The entire zone then grows upward, resulting in the formation of 
a cotyledonary ring surrounding a depression from which the stem tip 
develops. If both growing points continue to develop equally, a dicoty- 
ledonous embryo results, but if only one continues, a monocotyledonous 
embryo is formed. This indicates that dicotyledony is more primitive 
than monocotyledony. It also suggests that the stem tip is really ter- 
minal and the cotyledons lateral, even though only one cotyledon is 
produced. 



SPERMATOl'HYTA 



397 



Apomixis. Irregularities in the normal process of sexual reproduction 
occur occasionally in some angiosperms, constantly in others. Apomixis 
is a condition in which sexual reproduction in the flower is replaced by 
some form of asexual reproduction. It may take the form of partheno- 




FiG. 338. Successive stages in early development of the embryo of Sagittaria variabilis, a 
monocotyledon. A and B, three-celled proembryos, showing synergid {syn), basal cell 
(a), middle cell (b), and terminal cell (c) from which the cotyledon is derived; C, division 
of middle cell into two cells, one of which (s) gives rise to the stem tip; D, slightly older 
stage; E, formation of four cells from the terminal cell; F, further development of the 
middle region; G, differentiation of dermatogen in the terminal region; H, further develop- 
ment of dermatogen and differentiation of middle region into hypocotyl (h) and stem tip 
(s); 7, later stage. {After Schaffner.) 

genesis, which is the development of an embryo from an unfertilized egg. 
This has been observed in a number of angiosperms, such as Thalictrum, 
Antennaria, Alchemilla, Erigeron, and Taraxacum. In all these and sim- 
ilar cases the reduction of chromosomes fails to take place in connection 



398 PLANT MORPHOLOGY 

with megaspore formation. Consequently the egg is a diploid cell and 
fertilization is unnecessary. Haploid parthenogenesis is very rare, having 
been reported in only a few plants. 

The development of an embryo from other cells of the emh)ryo sac than 
the egg, called apogamy, has been observed in Antennaria, Alchemilla, 
Allium, Iris, and other forms, but is not known to occur constantly in 
nature. Here a synergid or an antipodal gives rise to an embryo, which 
may be diploid or haploid, depending on whether or not meiosis occurred 
in the division of the megaspore mother cell. Sporophytic budding occurs 
when cells of the nucellus or integument project into the embryo sac and 
give rise to embryos. It has been reported in Citrus, Coelebogyne, Funkia, 
and other angiosperms, where it frequently accompanies apogamy. 

The Fruit. The fruit develops as the seeds ripen and always encloses 
them. Like the flower, it has no morphological individuality. A true 
fruit consists merely of a ripened ovary, while an accessory fruit includes 
in addition one or more associated parts of the flower, such as the calyx or 
receptacle. The ripened ovary wall is the pericarp. When a fruit devel- 
ops from an inferior ovary, its wall consists of the pericarp united with the 
receptacle. At maturity, fruits may be dry or fleshy ; when dry, they may 
be dehiscent or indehiscent. Sometimes the pericarp becomes fleshy on 
the outside and stony within. Some fruits develop from simple pistils, 
others from compound pistils. An aggregate fruit arises from a group of 
separate ovaries, belonging to a single flower, that become more or less 
consolidated. A multiple fruit is similar, except that it is derived from the 
ovaries of a number of flowers. The development of a fruit without fer- 
tilization is called parthenocarpy . Parthenocarpic fruits are nearly 
always seedless. 

The Seedling. In practically all angiosperms the embryo goes into a 
state of dormancy as the seed matures, this being accomplished by the 
withdrawal of most of the water present and by important chemical 
changes. In the presence of favorable external conditions, germination 
occurs by the resumption of growth of the embryo and of other processes 
within the seed. 

In many seeds, particularly those of dicotyledons, the endosperm is 
completely absorbed by the embryo while the seed is ripening, the reserve 
food being thus transferred to the embryo itself, principally to the cotyle- 
dons. In seeds containing endosperm, this is absorbed by the embryo 
in germination. 

When germination begins, the root tip pushes through the testa and 
grows downward into the ground, giving rise to the primary root. The 
hypocotyl may remain short or may elongate considerably, depending on 
the kind of germination. Where it remains short, the cotyledon or cotyle- 
dons remain inside the testa, the plumule soon giving rise to a shoot that 



SPERMATOPHYTA 399 

pushes upward. Where the development of the primary root is accom- 
panied by elongation of the hypocotyl, the cotyledon or cotyledons are 
pulled out of the testa and are carried above the ground, where they often 
expand and function as foliage leaves. In such seedlings the develop- 
ment of the plumule into the shoot is usually considerably delayed. 

Chief Orders of Angiosperms 

It would be beyond the scope of the present work to include an exten- 
sive account of the classification of angiosperms, a subject of concern 
mainly to the taxonomist. A general survey of the chief orders, however, 
will demonstrate the complexity of the group and illustrate its principal 
evolutionary trends, many of which have already been mentioned. The 
orders in each of the three series do not represent a phylogenetic sequence 
but merely different levels of progress. The interrelationships of many 
groups is obscure, so that the tracing of lines of descent is difficult and will 
not be attempted here. 

1. Archichlamydeae 

The Dicotyledoneae include the Archichlamydeae and Metachlamyd- 
eae. The Archichlamydeae are the primitive stock of angiosperms from 
which both the Metachlamydeae and Monocotyledoneae have been 
derived. Their flowers are naked, apetalous, or choripetalous, the parts 
being usually cyclic but frec^uently more or less spiral. 

Piperales. The Piperales comprise 4 families and about 1 ,200 species of 
mostly tropical herbs and shrubs, nearly all belonging to the Piperaceae. 
The peppers {Piper} and peperomias are the best-known examples. The 
flowers, borne in spikes, are perfect or imperfect, mostly naked, typically 
trimerous but usually reduced, hypogynous, and mostly apocarpous. 

Salicales, Juglandales, and Fagales. These orders, with several others 
of minor importance, were once grouped together as the Amentiferae, 
because their flowers are borne in aments or catkins. The Salicales 
include a single family, the Salicaceae, to which belong the willows (Salix) 
and poplars (Populus). The Juglandales also comprise a single family, 
the Juglandaceae, including the walnuts (Juglans) and hickories {Carija). 
The Fagales contain two families, the Corylaceae and Fagaceae. To the 
Corylaceae belong the birches (Behda), alders (Alnus), etc., and to the 
Fagaceae the beeches {Fagus), chestnuts (Castanea), and oaks (Quercus). 
The Amentiferae are woody plants with imperfect flowers. In the Salica- 
ceae and Corylaceae both kinds of flowers are in aments, but in the Juglan- 
daceae and Fagaceae only the staminate flowers are. In all families 
except the Salicaceae, the flowers of some members have a simple bract- 
like perianth. The pistillate flowers are hypogynous in the Salicaceae 
and epigynous in the other families; they are syncarpous in all. 



400 PLANT MORPHOLOGY 

Urticales. This is an order of about 1,500 species distributed among 
4 families: the Ulmaceae and Moraceae, which are mostly woody, and the 
Cannabinaceae and Urticaceae, which are mostly herbaceous. Repre- 
sentative members of the Ulmaceae are the elms {Ulmus); of the Mora- 
ceae, the mulberries {Morns) and figs (Ficus); of the Cannabinaceae, 
hemp {Cannabis) and hop {Hutnulus); of the Urticaceae, the nettles 
{Urtica). The flowers are mostly imperfect, apetalous, hypogynous, and 
syncarpous. The stamens equal the perianth segments in number. The 
ovary is unilocular, usually having a single ovule. 

Santalales. These are parasitic herbs and woody plants, numbering 
about 1,200 species, mostly tropical. The Loranthaceae and Santalaceae 
are the largest of 8 families. The mistletoes belong to the Loranthaceae. 
The flowers are perfect or imperfect and epigynous, mostly with a petaloid 
perianth consisting of a single whorl but sometimes differentiated into a 
calyx and corolla. The stamens equal the sepals in number. There are 
generallj^ three united carpels, mostly forming a unilocular ovary. 

Aristolochiales. This is a small order of herbs and woody plants, the 
principal family, the Aristolochiaceae, numbering about 200 species. 
The chief genus is Aristolochia. The flowers are perfect and epigynous, 
with a highly developed petaloid perianth consisting of a single whorl of 
united parts. The ovary is multilocular and has an indefinite number of 
ovules. 

Polygonales. This is a small order of 800 species of herbs and woody 
plants, all belonging to the Polygonaceae. Representative genera are 
smartweed {Polygonum), dock {Rumex), rhubarb {Rheum), and buck- 
wheat {Fagopyrum). The small flowers, mostly borne in spikes, are per- 
fect or sometimes imperfect, regular, hypogynous, and syncarpous. The 
perianth, consisting of a single whorl, is bract-like. The unilocular ovary 
contains a single ovule. 

The preceding orders, with some others of less importance, constitute 
the apetalous series of the Archichlamydeae. They are characterized by 
flowers that, with few exceptions, have a simple perianth which is not 
differentiated into a distinct calyx and corolla but consists of a single 
whorl of parts. Some members have naked flowers. Most of these 
orders are of uncertain relationships. Some may be primitive, while 
others are doubtless reduced. 

The following orders, constituting the choripetalous series of the Archi- 
chlamydeae, typically have a perianth consisting of two distinct whorls — 
calyx and corolla — the members of which are separate and distinct. 

Centre spermales. This assemblage is often broken up into two orders, 
the Chenopodiales and Caryophyllales. It includes about 3,500 species 
of herbs grouped into 10 families, of which 4 are of greatest interest, viz., 



SPERMATOPHYTA 401 

the Chenopodiaceae, to which belong the goosefoots (Chenopodium), beet 
(Beta), spinach (Spinacia), etc.; the Amaranthaceae, including pigweed 
(Amaranthus) and coxcomb (Celosia); the Portulacaceae, represented by 
Portulaca; and the Caryophyllaceae, containing the carnation and pinks 
(Dianthus), catchfly (Silene), chickweed (Stellaria), etc. 

The flowers are mostly perfect, regular, mostly hypogynous, syncar- 
pous, and usually pentamerous. The perianth may consist of either one 
or two whorls. The ovary is mostly unilocular. The Centrospermales 
represent a transition between the apetalous and choripetalous dicotyle- 
dons, as the lower families have a bract-like undifferentiated perianth, 
while the higher families have a distinct calyx and corolla, the latter being 
very showy. A characteristic feature is the presence of abundant peri- 
sperm in the seed. This takes the place of endosperm as a food-storage 
region. The Centrospermales show some resemblances to the Poly- 
gonales, and the two orders may be related. 

Ranales. The Ranales are a great genetic order, comprising 16 families 
and about 5,000 species of herbs, shrubs, and trees. The largest family, 
the Ranunculaceae, has 1,200 species, and includes such common forms as 
buttercup {Ranunculus), Hepatica, Anemone, Clematis, columbine {Aqui- 
legia), larkspur {Delphinium), and peony {Paeonia). Other important 
familes are the Nymphaeaceae, Berberidaceae, Magnoliaceae, and 
Lauraceae. 

This order is ill defined. The flowers are perfect and mostly regular, 
but some are irregular. The perianth, usually consisting of a distinct 
calyx and corolla, is often undifferentiated and petaloid. Although the 
floral parts are often indefinitely numerous and wholly or partly spiral, 
there is a strong tendency toward the establishment of a cyclic condition 
with definite numbers, especially in the perianth. Hypogyny and apo- 
carpy are features of the order, perigyny and syncarpy being infrequent. 
In the Berberidaceae and Lauraceae the carpels are reduced to one. 
The Ranales are generally regarded as a primitive order that has given 
rise both to the more specialized orders of dicotyledons and to the 
monocotyledons. 

Papaverales. This order represents a specialized offshoot from the 
Ranales. There are 6 families and about 3,600 species, the principal fam- 
ilies being the Papaveraceae, Fumariaceae, and Cruciferae. The Crucif- 
erae, with 3,000 species, includes such well-known forms as the mustards 
{Brassica), radish {Raphanus), Alyssum, stocks {Matthiola), shepherd's- 
purse {Capsella), etc. The flowers of the Papaverales are mostly regular, 
hypogynous or sometimes perigynous, and syncarpous. The flowers are 
cyclic except that the stamens are spiral in some members. The pistil 
usually consists of two united carpels, syncarpy being the chief point of 



402 PLANT MORPHOLOGY 

difference between this order and the Ranales. The Cruciferae are a 
distinct family whose members can be recognized by the floral formula 
4-4-6-2. 

Sarraceniales. This is a small order of insectivorous plants, compris- 
ing 3 families, and represented by the pitcher plants {Sarracenia and 
others) and the sundews (Drosera). As in the Papaverales, the flowers 
are regular, hypogynous, and syncarpous. The two orders seem to have 
undergone a parallel development, the chief difference between them 
being the placentation. 

Resales. The Rosales constitute the great central order of Archi- 
chlamydeae. They include 16 families and 15,000 species of herbs, 
shrubs, and trees distributed throughout the world. The three chief 
families are the Saxifragaceae, Rosaceae, and Leguminosae. The Legu- 
minosae, with 12,000 species, is the second largest family of dicotyledons. 
The Saxifragaceae are represented by the saxifrages (Saxifraga), goose- 
berries and currants (Rihes), and Hydrangea. The Rosaceae include the 
roses (Rosa), strawberries {Fragaria), raspberries and blackberries 
(Ruhus), cherries and plums (Prunus), hawthorns (Crataegus), pear 
and apple (Pijrus), etc. The Leguminosae include the acacias, locust 
(Robinia), lupines (Lwpinus), clovers (Trijolium), beans (Phaseolus), peas 
(Pisum), etc. 

The Rosales overlap the Ranales, on the one hand, with regularity, 
hypogyny, apocarpy, and indefinite numbers of stamens and carpels, but 
advance far beyond them, on the other hand, with irregularity, epigyny, 
syncarpy, and definite numbers. The perianth is typically pentamerous. 
The Rosaceae have regular flowers that are perigynous or epigynous and 
usually have several carpels. The Leguminosae have mostly irregular 
flowers that are hypogynous or somewhat perigynous and have a single 
carpel. 

Geraniales. This large order of herbs and woody plants, containing 
about 9,000 species, is broken up into 20 families. Half of the species 
belong to the Euphorbiaceae, of which the largest genus is Euphorbia. 
Some familiar genera belonging to other families are Geranium, Pelargo- 
nium, Oxalis, Linum, and Citrus. The flowers are regular or irregular, 
hypogynous, syncarpous, and pentamerous throughout or often reduced. 
In the Euphorbiaceae the flowers are imperfect and apetalous, while in 
some members they are naked. The stamens of the Geraniales are rarely 
more than twice as many as the petals and usually equal to them in num- 
ber. This is the first definitely cyclic and isocarpic order in the Archi- 
chlamydeae, but it shows a tendency to reduce the number of carpels. 

Sapindales. The Sapindales, with about 3,000 species, most of which 
are woody, are separated into 21 families. Here belong the sumacs 
(Rhus), hollies (Ilex), maples (Acer), buckeyes (Aesculus), balsams 



SPERM A TOPH VTA 403 

(Impatiens), etc. The flowers are regular or irregular, mostly hypogy- 
iious, and syncarpous. The perianth is mostly pentamerous but the 
stamens are usually reduced to eight. Most memljers are isocarpic. 
This order has developed parallel with the Geraniales, being distinguished 
from it chiefly by certain obscure ovule characters. 

Rhamnales. This order includes about 1,100 species of woody plants 
belonging to 2 families, the Rhamnaceae, represented by the buckthorns 
(Rhamnus) and Ceanothus, and the Vitaceae, including the grapes (Vitis) 
and Virginia creeper (Parthenocissus) . The flowers are regular, mostly 
hypogynous, syncarpous, and tetracyclic. The perianth is trimerous or 
tetramerous. The Rhamnales have developed parallel with the Gera- 
niales and Sapindales, differing from them chiefly in having the stamens 
opposite the petals instead of alternate with them. 

Malvales. This is an order of 8 familes and about 2,300 species of 
herbs and woody plants. The best-known families are the Tiliaceae, 
represented by the basswood (Tilia), and the Malvaceae, to which belong 
the mallows (Malva), hollyhock (Althaea), cotton (Gossijpium), etc. The 
flowers are regular, hypogynous, syncarpous, and have a pentamerous 
perianth. The stamens are usually indefinitely numerous (rarely five) 
and more or less united. The placentation is axial. 

Parietales. The Parietales, containing 30 families and about 5,000 spe- 
cies of herbs and woody plants, is an order representing an extremely con- 
fused classification. The most familiar forms are the violets (Viola). 
The order is characterized by parietal placentation, its other characters 
being rather inconstant. The flowers are regular or irregular; hypogy- 
nous, perigynous, or epigynous; and mostly syncarpous. They are typ- 
ically pentamerous, but the stamens may be 3, 5, 10 or indefinitely 
numerous. 

Opuntiales. Here belongs a single family, the Cactaceae, with about 
1,100 species indigenous to America. The flowers are regular, epigy- 
nous, and syncarpous; they are peculiar in being spiral and polymerous. 
Thus the group represents a combination of primitive and advanced 
features. 

Myrtales. This is a large tropical order of herbs and woody plants. 
It contains about 7,500 species grouped into 19 families, of which the 
Myrtaceae and Onagraceae are well known. The myrtles (Myrtiis), 
Eugenia, and Eucalyptus belong to the Myrtaceae, while the fireweeds 
[Epilohium), evening primroses (Oenothera), and Fuchsia are familiar mem- 
bers of the Onagraceae. The flowers are mostly regular, perigynous or 
epigynous, and syncarpous. The perianth is mostly pentamerous and 
the stamens often indefinitely numerous. The carpels vary from two to 
many. This order resembles the Rosales in many ways, but here the car- 
pels are never free. 



404 PLANT MORPHOLOGY 

Umbellales. The Umbellales constitute the highest order of the Archi- 
chlamydeae. They include 3 famihes and about 3,000 species, nearly all 
herbaceous. Of these, 2,500 species belong to the Umbelliferae, where 
are found such familiar forms as carrot (Daucus), celery (Apium), pars- 
nip (Pastinaca) , and dill (Anethum). The order is characterized by reg- 
ular, epigynous flowers having a tetramerous or pentamerous perianth, a 
reduced calyx, a single whorl of stamens, and usually a bicarpellary, biloc- 
ular ovary with a single ovule in each locule. The floral formula of the 
Umbelliferae, 5-5-5-2, is an advanced one. 

2. Metachlamydeae 

The Metachlamydeae are characterized chiefly by their sympetalous 
corollas, and so are often called the Sympetalae. The entire group has 
reached a condition of definite numbers for all the floral sets, and therefore 
is constantly cyclic. The stamens are generally attached to the corolla. 

Ericales. The Ericales comprise about 2,000 species of woody plants, 
mostly shrubs. Of its 6 families, the Ericaceae, with about 1,500 species, 
is by far the largest. It includes such well-known genera as azalea 
(Rhododendron), wintergreen {Gaultheria), heather (Calluna), heath 
(Erica), and blueberry (Vaccinium). The flowers of the Ericales are 
regular or nearly so, tetramerous or pentamerous, pentacyclic or some- 
times tetracyclic, mostly isocarpic, hypogynous or often epigynous, and 
syncarpous. In tetracyclic flowers the stamens are opposite the petals. 
The ovary is multilocular. Some of the Ericaceae are choripetalous and 
in nearly all the stamens are free from the corolla. Thus this order serves 
to connect the Archichlamydeae and Metachlamydeae. 

Primulales. The Primulales constitute an order of about 1,100 species 
of herbs grouped into 4 families, of which the Primulaceae is of chief 
interest. The representative genus is primrose (Primula). The flowers 
are regular, pentamerous, tetracyclic, isocarpic, mostly hypogynous, and 
syncarpous. The single whorl of stamens stands opposite the petals and 
not, as in the higher tetracyclic orders, alternate with them. The outer 
whorl of stamens is often vestigial. The ovary is unilocular and has 
free-central placentation. In some respects this order resembles the 
Centrospermales. 

Ebenales. This is an order of about 1,000 species of tropical trees and 
shrubs. There are 4 small families, the characteristic one being the 
Ebenaceae, of which the ebony and persimmon (Diospyros) are examples. 
The flowers are regular, tetramerous or pentamerous, pentacyclic, iso- 
carpic, mostly hypogynous, and syncarpous. The ovary is multilocular. 
The floral parts show some variation in number, with an occasional 
increase in stamens and carpels. 

The preceding orders constitute the pentacyclic isocarpic series of 



SPERMATOPHYTA 405 

Metachlamydeae, in which the floral formula is typically 5-5-10-5. They 
are more primitive than the other sympetalous orders and more closely 
related to the Archichlamydeae. They are, with few exceptions, regular 
and hypogynous. 

The following orders comprise tlie tetracyclic anisocarpic series of 
Metachlamydeae, in which the floral formula is generally 5-5-5-2. Here 
belong three hypogynous and two epigynous orders. 

Gentianales. This is a genetic order. It comprises about 5,000 spe- 
cies of herbs and woody plants separated into 5 families, the principal 
ones being the Oleaceae, Gentianaceae, and Asclepiadaceae. The Olea- 
ceae is represented by the olive (Olea), ashes (Fraxinus), lilacs (Syringa), 
and privets (Ligustrum); the Gentianaceae by the gentians {Gentiana); 
and the Asclepiadaceae by the milkweeds (Asclepias). The flowers are 
regular, tetramerous or pentamerous, tetracyclic, anisocarpic, hypogy- 
nous, and apocarpous or syncarpous. They have two carpels. The 
Oleaceae have the peculiar floral formula of 4-4-2-2. 

Tubiflorales. This great central order of Metachlamydeae is closely 
related to the Gentianales and difficult to separate from it. It includes 
20 families and about 16,000 species, most of which are herbs. The eight 
principal families, with several representative genera, are as follows : Con- 
volvulaceae — bindweed (Convolvulus), morning-glory (Ipomoea), and 
dodder (Cuscuta); Polemoniaceae^ — Polemonium, Phlox, and Gilia; Hydro- 
phyllaceae — Hydrophyllum, Nemophila, and Phacelia; Boraginaceae — 
forget-me-not (M yosotis) , bluebells (Mertensia), and Heliotr opium; Verbe- 
naceae — Verbena and Lantana; Labiatae — sage (Salvia) and mint (Men- 
tha); Solanaceae — nightshade (Solarium), tobacco (Nicotiana), and 
Petunia; Scrophulariaceae — mullein (Verbascum), foxglove (Digitalis), 
and snapdragon (Antirrhinum). 

The flowers of the Tubiflorales are regular or irregular, mostly pentam- 
erous, tetracyclic, anisocarpic, hypogynous, and syncarpous. They have 
two carpels (three in Polemoniaceae). The corolla is mostly regular in all 
families except the Verbenaceae, Labiatae, and Scrophulariaceae, where it 
is almost always irregular. These three families have only four stamens, 
or sometimes only two, while the others have five. The ovary is uniloc- 
ular or bilocular in the Hydrophyllaceae ; mostly bilocular in the Convol- 
vulaceae, Solanaceae, and Scrophulariaceae ; mostly trilocular in the Pol- 
emoniaceae; and mostly tetralocular in the Boraginaceae, Verbenaceae, 
and Labiatae. 

Plantaginales. These are the plantains, comprising one family of 
about 200 species of herbs. The principal genus is Plantago. The 
flowers are regular, tetramerous, tetracyclic, anisocarpic, hypogynous, 
and syncarpous. They have two carpels. The corolla is dry and mem- 
branaceous. This order is related to the Tubiflorales. 



406 PLANT MORPHOLOGY 

Rubiales. This order consists of over 5,000 species of herbs and woody- 
plants distributed among 5 families, by far the largest being the Rubia- 
ceae, to which belong coffee iCoffea) and Cinchona. The Caprifoliaceae, 
another family, is represented by the honeysuckles (Lonicera), Viburnum, 
and the elders (Samhucus) . The flowers are regular or irregular, tetram- 
erous or pentamerous, tetracyclic, anisocarpic, epigynous, and syncar- 
pous. They have usually two or three carpels. The calyx is reduced. 
This order shows a resemblance to the Umbellales. 

Campanulales. This is the culminating order of Metachlamydeae. 
It includes 8 families and 16,000 species, about 13,000 of which belong to 
the Compositae, the largest family of dicotyledons. A few well-known 
genera are goldenrod (Solidago), Aster, sunflower (Helianthus), Chrysan- 
themum, thistle (Cirsium), dandelion (Taraxacum) , lettuce (Lactuca), etc. 
Other familiar families are the Cucurbitaceae, Campanulaceae, and Lobe- 
liaceae. The flowers are regular or irregular, pentamerous, tetracyclic, 
anisocarpic, epigynous, and syncarpous. They have two or three carpels. 
The calyx is reduced. A special feature is the tendency of the five 
stamens to be united in various ways. The flowers of the Compositae are 
organized to form a compact head surrounded by an involucre of many 
bracts. The head is usually composed of peripheral ray flowers and cen- 
tral disk flowers. 

3. MONOCOTYLEDONEAE 

The monocotyledons are generally regarded as having been derived 
from the Ranales region of the Archichlamydeae. In the lower orders the 
floral parts fluctuate in number and are more or less spiral in arrangement, 
while in the higher orders they are constant and cyclic. 

Pandanales. This is a very primitive order of about 450 species 
grouped into 3 families. Many are hydrophytes. The screw pine (Pan- 
danus) of the tropics is the representative form, but the most familiar 
member in temperate regions is the cattail (Typha). The flower clusters 
are usually surrounded, when young, by a conspicuous sheathing bract. 
The flowers are imperfect and mostly naked, but sometimes a simple 
bract-like perianth is present. The stamens and carpels show great varia- 
tion in number and are mostly spiral. Both hypogyny and apocarpy are 
features of the order. 

Helobiales. Here belong about 300 species of primitive aquatic and 
marsh plants comprising 6 families. Familiar genera are Potamogeton, 
Sagittaria, Elodea, and Alisma. The Helobiales are a genetic group, 
showing several lines of descent. A sheathing bract surrounds the inflo- 
rescence, as in the Pandanales, but the flowers are usually perfect. The 
flowers may be naked, apetalous, or choripetalous, are usually hypogy- 



SPERM A TOPHYTA 407 

nous but, in one family, are epigynous. The stamens range from indef- 
inite to definite in number, the pistils from apocarpy to syncarpy. The 
numerous stamens and apocarpous pistils suggest a relationship to the 
Ranales. 

Glumales. The Glumales include the Cyperaceae, or sedges, and the 
Gramineae, or grasses, together numbering about 8,000 species. The 
flowers are surrounded by scale-like bracts, called glumes, the perianth 
being either wanting or represented by minute scales. The flowers, which 
are mainly perfect, have six stamens or less (mostly three), and a uniloc- 
ular ovary with a single ovule. The pistil, commonly said to consist of a 
single carpel, really is formed of three completely united carpels. All 
members are hypogynous. There is much evidence indicating that the 
Glumales are not primitive but reduced from lily-like ancestors. 

Palmales. This order includes only the Palmaceae, with about 1,200 
species, mostly of tropical and subtropical trees. The flowers are small, 
mostly imperfect, and borne in a massive inflorescence at first surrounded 
by a large sheathing bract. The presence of a perianth is a constant fea- 
ture of the group ; it is inconspicuous and composed of two similar whorls 
of three members each. The flowers are mostly trimerous, hypogynous, 
and mostly syncarpous. They commonly have six stamens (often more) 
and three carpels. 

Arales. This order, of about 1,500 species, comprises 2 families of 
herbs, the principal one being the Araceae. Most of its members are 
tropical, some familiar ones being jack-in-the-pulpit (Arisaema), calla lily 
(Z anted eschia), and elephant's-ear {Calocasia). The flowers, which are 
small and inconspicuous, are borne on a fleshy axis, the spadix, surrounded 
by a conspicuous bract, the spathe. They are perfect or imperfect, hypog- 
ynous, and syncarpous. A simple scale-like perianth is sometimes pres- 
ent, its absence in most forms being a result of reduction. The number of 
stamens and carpels, although variable, is small. 

The preceding orders constitute the spiral series of Monocotyledoneae. 
They are characterized, for the most part, by fluctuating numbers of floral 
parts, a simple perianth or none, apocarpy, and the development of 
sheathing bracts. In the Glumales the bracts appear in connection 
with the individual flowers, in the other orders, in connection with the 
inflorescence. 

The following orders constitute the cyclic series of Monocotyledoneae. 
Here the flowers are typicaUy trimerous, pentacyclic, and syncarpous. 
The perianth consists of a distinct calyx and corolla. In a relatively few 
members the perianth parts are more or less united; otherwise they are 
separate. 

Farinales. The Farinales comprise an order of about 2,500 species of 
mostly grass-like herbs distributed into 13 families. The most familiar 



408 PLANT MORPHOLOGY 

member is the spiderwort (Tradescantia) , but the pineapple (Ananas) 
belongs here and also the "long moss" {Tillandsia) of the Southern 
United States. The flowers are regular or nearly so, mostly trimerous 
and pentacyclic, hypogynous or sometimes epigynous, and syncarpous. 
The perianth may or may not be differentiated into a calyx and corolla 
and the latter may be bract-like or petaloid. A special feature is the 
presence of mealy (farinose) endosperm. 

Liliales. The Liliales are the great central order of monocotyledons. 
They number about 5,000 species, most of which are herbs. They are 
grouped into 9 families, the principal ones being the Juncaceae, Liliaceae, 
Amaryllidaceae, and Iridaceae. The Juncaceae include the rushes {Jun- 
cus). The Liliaceae contain such well-known genera as Trillium, Ery- 
thronium, lily (Lilium), tulip (Tulipa), hyacinth (Hyacinthus), onion 
(Allium), and Yucca. The Amaryllidaceae are represented by the tube- 
rose (Polianthes), Agave, and Narcissus. The Iridaceae comprise the 
flags (Iris), Crocus, Gladiolus, etc. 

The flowers are mostly regvdar and have a perianth consisting of two 
trimerous whorls that are nearly always alike, being bract-like in the 
Juncaceae and petaloid in the other families. They have mostly six 
stamens (three in the Iridaceae) and a tricarpellary pistil. The flowers 
are either hypogynous (Juncaceae and Liliaceae) or epigynous (Amaryl- 
lidaceae and Iridaceae). 

Scitaminales. Here belong 4 families and about 1,000 species of mostly 
tropical herbs, including Canna, banana (Musa), and ginger (Zingiber). 
The perianth, displaying a special type of irregularity, is composed of two 
whorls that are often entirely petaloid. All members are epigynous. 
Generally only one fertile stamen is present. The pistil is tricarpellary 
and the ovary trilocular. 

Orchidales. This is the highest order of monocotyledons as well as the 
largest. It comprises 2 families and about 15,000 species of herbs, nearly 
all of which belong to the Orchidaceae. The orchids reach their greatest 
display in the tropics, where most of them are epiphytes. The flowers of 
the Orchidaceae are irregular, epigynous, and syncarpous. The irregu- 
larity is of a special type. The perianth consists of two trimerous whorls, 
one petal being strikingly different from the others. The stamens are 
reduced to three, but these do not belong to the same whorl ; usually only 
one stamen is fertile. The pistil is tricarpellary and the ovary unilocular. 
The seeds are without endosperm. 

COMPARISON OF GYMNOSPERMS AND ANGIOSPERMS 

The most important distinguishing characters of the Gymnospermae 
and Angiospermae are as follows: 



SPERM ATOPH VTA 



409 



Gymnospermae 
Plants woody 

Wood without vessels (except in Gnetales) 
Sporophylls borne in strol)ili (except in 

Cycadofilicales) ; perianth absent 
Ovules freely exposed, not in a closed ovary 

Female gametophyte with abundant vege- 
tative tissue and with archegonia (ex- 
cept in Welwitschia and Gnetum) 

Pollen coming in direct contact with ovules 

Male gametophyte usually with one or 
more prothallial cells; generative cell 
producing a stalk cell and a body cell, 
the latter giving rise to two male cells 
that may or may not be organized as 
ciliated sperms 

Endosperm formed from vegetative tissue 
of female gametophyte 

Development of embryo initiated by free- 
nuclear division (except in Welwitschia 
and Gnetum) 

Cotyledons two to many 



Angiospermab 

Plants woody or herbaceous 

Wood almost always with vessels 

Sporophylls borne in flowers; perianth 
generally present 

Ovules in a closed ovary formed by one or 
more megasporophylls 

Female gametophyte with little or no 
vegetative tissue, consisting typically 
of an eight-nucleate embryo sac ; arche- 
gonia absent 

Pollen not coming in direct contact with 
ovules 

Male gametophyte without prothallial 
cells; generative cell directly producing 
two male cells that are never ciliated 



Endosperm arising after fertilization, gen- 
erally from a triple fusion of nuclei 

Development of embryo without a free- 
nuclear stage 



Cotyledons one or two 



GENERAL CONCLUSIONS 

The two most outstanding features of the spermatophytes are the 
presence of seeds and the development of the flower. They excel the 
pteridophytes in the complexity of their vegetative organs, while the 
gametophyte is subordinated to the sporophyte to such an extent that it is 
always dependent upon it. The microsporangia and megasporangia are 
borne by sporophylls that, with few exceptions, are considerably less leaf- 
like than the sporophylls of most pteridophytes. Almost invariably the 
megasporangium (nucellus of the ovule) produces only one functional 
megaspore, which always gives rise to the female gametophyte without 
being shed. In all spermatophytes the development of the sporangia is 
eusporangiate, as among the lower pteridophytes, but the initials are 
hypodermal rather than epidermal in origin. 

In gymnosperms the archegonia are more reduced than in pterido- 
phytes, while in angiosperms they are eliminated. The male gameto- 
phyte produces only two sperms, or male cells, and true antheridia are not 
present. Among existing groups, ciliated sperms occur only in the Cyca- 
dales and Ginkgo. Fertilization, accomplished with the aid of a pollen 
tube, results in an embryo that develops inside the ovule. 

The Seed. The adaptation to life on land is more nearly perfect in the 
spermatophytes than in any other group of plants. This has come about 



410 PLANT MORPHOLOGY 

partly through the greater complexity of their vegetative organs, but 
chiefly by the development of the seed. The development of the seed is a 
resvilt of three conditions — heterospory, retention of the megaspore within 
the megasporangium, and formation by the zygote of a dormant embryo. 

Both the bryophytes and pteridophytes are handicapped in their 
adjustment to the land habit by two basic requirements: (1) In order to 
reach the egg and effect fertilization, the sperm must swim through water, 
a handicap persisting from aquatic ancestors. (2) After fertilization has 
taken place, the embryo must continue its development whether external 
conditions are favorable or not. 

The seed overcomes both these handicaps. The necessity for external 
water as a means by which the sperm may reach the egg is obviated by 
the transportation, through air, of the male gametophyte, inside a pollen 
grain, to the vicinity of the female gametophyte and the development of a 
pollen tube through which the male gamete can pass. In this way fer- 
tilization is made more certain. Following fertilization, the passing of 
the embryo into a state of dormancy, as well as the formation of a protec- 
tive seed coat, enables the embryo to live until conditions become favor- 
able for its continued gro\vth. 

The Flower. The flower is difficult to define because every possible 
transition exists between a typical strobilus and a typical flower. It is 
apparent, therefore, that the strobilus is the forerunner of the flower and 
that the changes involved in passing from the one to the other represent 
an important evolutionary advance. The perianth probably arose as a 
protective envelope for the sporophylls, but later came to have additional 
functions, such as the attraction of insects. In gymnosperms the transfer 
of pollen is precarious, depending upon the vagaries of the wind. To 
increase the chances of success, an enormous excess of pollen must be pro- 
duced, thus involving a tremendous waste. The transfer of pollen by 
insects, to which the flowers of most angiosperms are highly adapted, is 
more certain and consequently much less w^asteful than wind-pollination. 
Probably the most important factor responsible for the evolutionary prog- 
ress made by the angiosperms has been the development of the flower in 
adaptation to insect pollination. 

Associated with the development of the flower and its specialization for 
insect pollination has been the enclosure of the ovules. Originally the 
carpels of seed plants must have been leaf-like structures bearing marginal 
ovules, a condition preserved in such existing gymnosperms as Cycas 
revoluta. The change from the open carpel with its ovules freely exposed 
to the closed carpel with its ovules inside a cavity marks another great 
advance which the angiosperms have made over the gymnosperms. This 
change led to the development of the fruit. 

Interrelationships. The fossil record gives no evidence as to which of 
the two great Paleozoic groups of gymnosperms — the Cycadofilicales and 



SFERMATOPHYTA 411 

the Cordaitales^ — is the older. Except for the presence of seeds, however, 
the Cycadofilicales are so fern-like as to leave little doubt that they have 
been derived from some ancient fern stock. The Cordaitales, less fern- 
like, may have branched off from the Cycadofilicales early in the Paleo- 
zoic, although it seems more likely that both groups have had a common 
origin. The Cj^cadofilicales very probably gave rise to two divergent 
lines of descent, one represented by the Bennettitales of the Mesozoic, a 
specialized offshoot that became extinct, the other leading to the Cyca- 
dales, a group that still survives. 

There is strong evidence that the Cordaitales were ancestral to both the 
Ginkgoales and the Coniferales, orders that reached a climax in the Meso- 
zoic. Except for Ginkgo hiloha, the Ginkgoales are extinct, while the 
Coniferales are still so abundant as to constitute the largest order of living 
gymnosperms. The Gnetales are a group of obscure origin. They may 
represent a specialized offshoot from the Coniferales, but any relationship 
to the angiosperms is very doubtful. 

The oldest undoubted angiosperms appear in the deposits of the Lower 
Cretaceous. Their characters are so distinct that they must have orig- 
inated at a much earlier time, but it is not known when or from what 
older group they arose. There is a possibility that the angiosperms orig- 
inated independently from the pteridophytes, but this is remote in view 
of the many common features existing between gymnosperms and angio- 
sperms. There is no convincing evidence, however, indicating from what 
group of gymnosperms the angiosperms may have sprung. 

One theory holds that the angiosperms have been derived from the 
Gnetales. Their compound strobili resemble the inflorescences of certain 
angiosperms with monosporangiate (imperfect) flowers and a simple 
undifferentiated perianth. The presence of vessels in the secondary wood 
is a character shared by both groups. If the resemblances between the 
Gnetales and angiosperms are a result of parallel evolution, there can be 
no direct relationship between them, although they may have come from 
a common ancestry. 

Another theory claims the derivation of the angiosperms from the 
Bennettitales. Their strobilus, which is bisporangiate, somewhat resem- 
bles the flower of a magnolia, but the stamens, and particularly the car- 
pels, are very different in the two groups. On the whole, the cycadeoid 
strobilus is so specialized that it is very unlikely that an angiosperm 
flower could have evolved from it. It seems more reasonable to suppose 
that the resemblance between them is a result of parallel development and 
does not denote any direct relationship. The fossil record has not pro- 
duced transitional forms connecting the gymnosperms and angiosperms. 
In their absence, speculation concerning the ancestry of the angiosperms 
seems futile. 



CHAPTER X 
EVOLUTION OF THE PLANT KINGDOM 

The doctrine of evolution states that all forms of life, living and extinct, 
have been derived from preexisting forms by a process of gradual change. 
This principle of "descent with modification," supported by an over- 
whelming mass of evidence, has been fully established as a fundamental 
axiom of biology. The method by which evolution has taken place, how- 
ever, is imperfectly understood and considerable uncertainty exists as to 
the relative importance of the various factors involved. 

These factors are of two kinds : primary or causative and secondary or 
directive. Causative factors give rise to heritable variations, which are 
the raw^ materials of evolution. These are built up into new species under 
the influence of directive factors, which determine the course of evolution. 
Heritable variations arise both from new combinations of genes in fer- 
tilization and by mutation, a process involving changes in the chromo- 
somes of reproductive cells, the causes of which are largely unknown. 
The addition or loss of one or more chromosomes by irregularities in 
meiosis may cause changes affecting several or many characters simulta- 
neously. Much more common and of greater importance are changes 
involving individual genes, these giving rise to innumerable small varia- 
tions that supply most of the raw^ material upon which the directive 
factors of evolution operate. Thus evolution is mainly dependent upon 
the appearance of mutations, especially gene mutations. 

The general trend of evolution is toward greater fitness to the conditions 
of existence. Some of the variations arising by recombination of genes 
or by mutation are adaptive, while others are not. By natural selection, 
favorable variations having survival value are preserved and accumulated 
through successive generations, thus bringing about greater adaptation to 
the environment. Natural selection determines which individuals among 
a diversified population shall survive in the "struggle for existence." It 
is generally regarded as the most important directive factor yet 
discovered. 

Theories dealing with the causes of evolution are concerned chiefly with 
the origin of species. One of the great problems of morphology is the 
determination of the origin and phylogenetic development of the larger 
plant groups. The fossil record demonstrates that groups once dominant 
on the earth have been replaced by others more advanced, but it seldom 

412 



EVOLUTION OF THE PLANT KINGDOM 413 

indicates from which older group a younger one has been derived. Inter- 
relationships among the larger groups must be inferred principally from 
evidence based on studies in comparative morphology. The "lower" 
groups of plants are merely those which have undergone relatively little 
modification, the "higher" groups, a vastly greater amount. Existing 
groups are usually represented as twigs on a phylogenetic tree. The 
larger branches denote divergent lines of descent. These are often 
obscure and difficult to trace, but become clear as knowledge advances. 

Specialization. The general trend of evolution toward greater adapta- 
tion to the environment has resulted in the development of specialized 
forms from generalized forms. Structural complexity is ahvays a result 
of evolution from a simpler condition of organization, but simplicity does 
not always represent a primitive state. Often it denotes reduction from 
a more highly developed ancestry. Sometimes there is structural evi- 
dence of such reduction, especially in ontogeny, but usually this evidence 
is obscure or wanting. Consequently it is often difficult to ascertain 
whether structural simplicity is a primary or a derived condition. 

Higher types have arisen from generalized members of lower groups, 
not from speciahzed members. Highly specialized groups, like the red 
algae and the mosses, represent blindly ending lines of descent. They 
may change in the direction of greater specialization, but cannot revert to 
a generalized condition and then become specialized in another direction. 

In many groups evolutionary advance has not affected all parts of the 
plant to the same extent and, as a consequence, advanced features are 
often combined with primitive ones. For example, although the cycads 
are seed plants, they have retained swimming sperms, an extremely 
ancient character, the phylogenetic continuity of which can be traced back 
to the algae. Sometimes the development of one character is associated 
with the suppression of another. Thus, in the Compositae, the formation 
of an involucre has resulted in a reduction of the calyces of the individual 
flowers in the head. Similarly the strong development of mechanical tis- 
sues in the stems of many large herbaceous angiosperms is related to the 
weak development of xylem and might be regarded as a compensation 
for it. Such instances of compensation are common throughout the 
plant kingdom. 

Parallel Development. The same evolutionary tendency, acting inde- 
pendently in different groups of plants, may bring about similar changes, 
thus resulting in parallel development or homoplasy. For example, 
heterogamy has arisen independently in a number of widely separated 
algal groups and heterospory in various groups of vascular plants. Epig- 
yny has developed independently in many different families of angio- 
sperms. Structural similarity resulting from parallel development is no 
indication of phylogenetic relationship. It merely signifies that evolution 



414 PLANT MORPHOLOGY 

in two or more different groups has occurred in the same general direction 
and often in response to the same influence. Instances of parallel devel- 
opment are numerous. Unless they are recognized, false conclusions 
regarding relationships may be drawn. 

Real relationships among different kinds of plants are shown by the 
presence of similar characters derived from a common ancestry. The 
members of every natural group, despite superficial differences, are built 
according to the same basic pattern. This is expressed by the characters 
that distinguish it from other groups. Related plants display many 
structural resemblances because of a common origin, while their differ- 
ences are a result of divergent evolutionary tendencies. The greater the 
degree of basic resemblance between any two kinds of plants, the closer 
is their relationship and the less remote their common ancestry. 

Homologous Structures. In any natural group the various members 
possess certain structures that are considered as homologous, or morpho- 
logically equivalent. Such structures may display considerable diversity 
in form or function, but have a similar ontogeny and so bear the same 
relation to the plant as a whole. In liverworts, for example, spores and 
elaters are homologous, since they develop from the same mass of undiffer- 
entiated sporogenous tissue. In angiosperms stamens and carpels are 
homologous with leaves, as well as with the sporophylls of other vascular 
plants. Tendrils and thorns may be homologous either with stems or 
leaves, depending on their place of origin on the plant. Cladophylls are 
homologous with stems, while bracts and scales are homologous with 
leaves. 

Recapitulation. Developmental stages not only reveal homology 
between different kinds of structures, but frequently furnish other evi- 
dence of evolutionary changes. In the early development of many 
plants, stages appear that correspond to adult stages in less highly spe- 
cialized plants. This suggests that such developmental stages may repre- 
sent ancestral conditions. The theory that "ontogeny recapitulates 
phylogeny" cannot be regarded as a principle of broad application, how- 
ever, for in many plants embryonic or juvenile stages have no apparent 
evolutionary significance. On the other hand, the theory receives sup- 
port from many sources. For example, among kelps Laminaria is a gen- 
eralized type, the simplest species consisting of a holdfast, stipe, and an 
undivided blade. Most of the other kelps are more highly differentiated 
when mature, but in early development pass through a Lamiwana-like 
stage. This indicates that Laminaria represents the ancestral condition. 

The occurrence of a protonemal stage in the life history of a moss recalls 
an algal stage in the ancestry. The presence of needle-like leaves on the 
seedlings of certain conifers and of scale-like leaves on older plants sug- 
gests that the juvenile foliage represents the ancestral type. The seed- 



EVOLUTION OF THE PLANT KINGDOM 415 

lings of some species of Acacia have bipinnate leaves that are soon 
replaced by phyllodia, the adult form of foliage. In such species tran- 
sitional stages are common. In the development of the common bracken 
fern {Pteridium aquilinum), the young plant passes through a protostelic 
and then a siphonostelic stage before the dictyostelic or permanent condi- 
tion is reached. These stages represent a phylogenetic series. In the 
seedlings of most monocotyledons the stem is at first a siphonostele, grad- 
ually becoming a dictyostele with scattered vascular bundles. This 
indicates that the siphonostelic condition is the more primitive one. 

PROMINENT EVOLUTIONARY TENDENCIES 

Comparative morphology furnishes abundant evidence of evolution 
along determinate lines. Changes that have taken place in definite direc- 
tions are apparent throughout the plant kingdom. Because some mem- 
bers are more advanced than others, it is possible, within a group, to 
construct a series of forms showing various degrees of modification. Such 
a series may indicate either an advance or a decline, depending on whether 
evolution has been progressive or retrogressive. The species of Lycopo- 
dium display various stages in the organization of a strobilus, while the 
genera of Fucales show a reduction series with respect to the number of 
eggs produced in an oogonium. Often an advance in one direction has 
been accompanied by a decline in another. The development of an 
irregular corolla in certain families of angiosperms, such as the Labiatae 
and Scrophulariaceae, has resulted in a reduction in the number of sta- 
mens from five to four or two. Evolutionary tendencies, whether pro- 
gressive or retrogressive, usually continue as long as the group displaying 
them persists. Some of the more conspicuous evolutionary tendencies 
seen in the major groups of green plants will be briefly summarized. 

Algae. Among the algae vegetative advance has been marked by the 
organization of single cells into colonies and the development of multicel- 
lular bodies into filamentous, plate-like, and massive types. Progress is 
also shown by the beginning of cellular differentiation, resulting in spe- 
cialization of different parts of the body for particular functions. 

From a condition where reproduction is wholly asexual, an advance is 
seen in the establishment of sexual reproduction and in its change from 
isogamy to heterogamy. The tendency to interpose a vegetative growth 
phase between gametic union and meiosis has resulted in the establish- 
ment of an alternation of haploid and diploid generations. Some algae 
show a tendency to develop the sporophyte at the expense of the gameto- 
phyte. The algae are a polyphyletic group representing a number of 
parallel evolutionary lines whose connections are very uncertain. Most 
groups seem to have arisen independently from a flagellate ancestry. 



416 PLANT MORPHOLOGY 

Bryophytes. The contributions of the bryophytes to the evolution of 
the plant kingdom include the establishment of the land habit, the appear- 
ance of archegonia and multicellular antheridia, and the development of a 
distinct alternation of unlike generations as a constant feature of the 
group. Prominent evolutionary trends include a differentiation of the 
gametophyte in internal structure, the development of a leafy gameto- 
phyte from a thallus, and a tendency of the sporophyte to become par- 
tially independent of the gametophyte. From a sporophyte almost 
wholly sporogenous, progress has been made by ever-increasing steriliza- 
tion of tissue and its diversion to other functions. This is seen in the 
development of a foot and seta, elaters, and a dehiscence mechanism, in 
the formation of a columella, and in the development of green tissue. 

Relationships between the bryophytes and green algae are mainly con- 
jectural, there being no direct fossil connection between the two groups. 
However, the bryophytes seem clearly to have been derived from aquatic 
ancestors, their structural advances being correlated with the establish- 
ment of the land habit. These include a compact plant body, absorptive 
rhizoids, jacketed sex organs, heavy-walled aerial spores, etc. 

Pteridophytes. The advance of the pteridophytes over the lower 
groups is shown by the establishment of an independent sporophyte, 
evolution of a vascular system, organization of a strobilus, and appear- 
ance of heterospory. With few exceptions, the sporophyte consists of 
roots, stem, and leaves. In one line of descent the leaves have remained 
small, undivided, and single- veined ; in the other line they have become 
large, divided, and many- veined. In the evolution of the vascular system 
the trend has been from exarch to mesarch xylem and then to endarch, 
also from a protostelic to a siphonostelic condition and then to a dictyo- 
stelic one. The presence of leaf gaps in the ferns is regarded as an 
advanced feature, their absence in other pteridophytes being primitive. 

Lycopods show an advance from those with every leaf a sporophyll to 
those with a compact strobilus. Both lycopods and ferns show a trend 
from homospory to heterospory. Heterospory has accelerated the reduc- 
tion of the gametophyte generation and sex organs. Among homospo- 
rous forms, subterranean gametophytes have been derived from aerial 
ones. Ferns show evolutionary tendencies affecting the sporangia, such 
as a change from the eusporangiate to the leptosporangiate type, reduc- 
tion in the thickness of the wall and in the output of spores, development 
of an annulus, etc. 

The origin of pteridophytes is uncertain, a direct connection with either 
the algae or the bryophytes being hypothetical. Some paleontological 
evidence indicates that the major groups have been derived independently 
from the Psilophytales. 



EVOLUTION OF THE PLANT KINGDOM 417 

Spermatophytes. The spermatophytes reach the culmination of evolu- 
tion in the development of the seed, development of pollen tubes, ultimate 
enclosure of the seed by the carpel (angiospermy) , ultimate elimination of 
swimming sperm.s and later of archegonia, a great reduction in the game- 
tophyte generation, and the evolution of the flower. 

The abandonment of ciliated sperms by the conifers resulted in a change 
in the pollen tube from a branched haustorial organ to a carrier of the 
male cells. The male gametophyte has undergone a reduction in the 
number of male cells to tw^o and of prothallial cells to two or one and 
finall}^ to none. Throughout gymnosperms the female gametophyte 
exhibits various stages in reduction that reach an extreme in angiosperms. 
The tendency for eggs to mature earlier and earlier in the development of 
the gametophyte has finally resulted in the elimination of archegonia. 

A marked feature of evolution in the seed plants has been the develop- 
ment of the strobilus into a flower. Among angiosperms floral evolution 
has been marked by a number of evolutionary tendencies, among which 
are the following: floral parts numerous and spiral to few and cyclic, pen- 
tacyclic to tetracyclic, free to united; perianth undifferentiated to differen- 
tiated into a distinct calyx and corolla; corolla regular to irregular; flowers 
hypogynous to perigynous and epigynous; ovules with two integuments 
to only one; embryo dicotyledonous to monocotyledonous; endosperm 
abundant to little or none; fruit simple to aggregate and multiple. Angi- 
osperms also show trends from a woody to an herbaceous habit, from erect 
types to vines, from perennials to annuals, from a spiral to a cychc 
arrangement of leaves, from simple to divided leaves, and from net-veined 
to parallel-veined leaves. 

It is clearly evident that the gymnosperms have been derived from the 
pteridophytes and are a much older group than the angiosperms, whose 
origin is obscure. If the angiosperms have not come from the gymno- 
sperms, the presence of seeds, pollen tubes, and many other common fea- 
tures are a result of parallel development. 

EVOLUTION OF SEX 

There are two primary types of reproduction in the plant kingdom, 
sexual and asexual. The distinctive feature of sexual reproduction is the 
fusion of two cells to form a zygote. In asexual or vegetative reproduc- 
tion no such fusion occurs. 

Asexual Reproduction. Cell division is the simplest and oldest method 
of reproduction and in many unicellular plants it is the only method. In 
multicellular plants cell division does not result in reproduction but in 
growth. To make reproduction possible, a cell or group of cells must 
become detached from the parent plant. A spore shares with ordinary 



418 PLANT MORPHOLOGY 

vegetative cells its ability to divide, but does so only after being liberated. 
It differs from other cells not in power but in opportunity. Asexual 
reproduction by means of spores is a feature of nearly all the green algae 
and of most fungi, occurring even in some of the unicellular forms. 

It should be understood that the formation of vegetative spores never 
involves a reduction of chromosomes. In fact, with few exceptions, they 
are borne on a haploid plant body. Vegetative spores always give rise to 
the same kind of plant body as the one that produced them. Spores 
formed by meiosis from a diploid cell, like tetraspores in the red algae, 
ascospores and basidiospores in the fungi, and all spores in the higher 
plants, are not vegetative spores but meiospores and belong to the sexual 
life cycle. 

In plants above the thallophyte level, asexual or vegetative repro- 
duction is carried on by various means, such as gemmae, bulbils, bulbs, 
tubers, runners, isolated branches, etc. Each of these consists of a group 
or mass of vegetative cells isolated from the parent and capable of 
reproducing it. 

Origin of Sex. Most algae producing vegetative zoospores, whether 
unicellular or multicellular, also bear gametes on the same kind of vegeta- 
tive body. Gametes not only resemble zoospores structurally but, in 
isogamous forms, commonly intergrade with them. This indicates that 
gametes have been derived from zoospores. It is not known what 
induced reproductive cells to first unite in pairs, but the tendency soon 
became a fixed habit. 

The fact that the zygote becomes a thick-walled resting cell in most 
green algae suggests that the original function of sexual reproduction was 
protection over a period of unfavorable conditions. In fact, experiments 
have shown that the advent of such conditions induces gamete formation. 
When conditions for vegetative growth are at their best, reproduction 
does not occur. When they become somewhat less favorable, vegetative 
activity begins to wane and spores are formed. As conditions become 
more severe and the plant approaches the end of its growing season, 
gametes appear. The conditions favoring gamete production in the green 
algae are those that inhibit germination of the zygote and result in its 
dormancy. When it germinates at the beginning of the next season, con- 
ditions are usually not conducive to maximum vegetative activity. It is 
possible that the formation of spores by the zygote, a feature of so many 
green algae, is merely a response to these conditions. 

Differentiation of Gametes. In isogamy all the gametes are alike in 
appearance and behavior. The fact that they pair, however, implies a 
mutual attraction and indicates that a difference exists between them. 
Each of the pairing gametes must represent an opposite sex. In heterog- 



EVOLUTION OF THE PLANT KINGDOM 419 

amy the differences between the two kinds of gametes merely become 
apparent, so that they are recognizably male and female. 

Isogamy represents the original condition of gametic union. It has 
been retained by many green and brown algae. These groups display 
various degrees of heterogamy, however, indicating that this condition 
has been derived from isogamy by a differentiation of gametes into sperms 
and eggs. In isogamy, as a rule, both kinds of gametes are motile and 
equally small, neither containing much food. In heterogamy there is a 
division of labor, the sperm providing motility and remaining small, the 
egg providing food and becoming large. Its advantage lies in the greater 
supply of food available for the zygote and the young plant that develops 
from it. In the red algae, where heterogamy is universal, the sperm is 
nonmotile but much smaller than the egg. Heterogamy is estabhshed in 
all the higher plants, with swimming sperms occurring in all bryophytes, 
all pteridophytes, and a few gymnosperms. 

Evolution of Sex Organs. The production of gametes in ordinary 
vegetative cells is characteristic of most of the green algae. In isogamous 
forms these cells remain unchanged, while in nearly all heterogamous 
forms they become modified in size and shape. Thus there are not only 
two kinds of gametes but two kinds of gametangia, the sperms arising in 
antheridia and the eggs in oogonia. A differentiation of gametes has been 
accompanied by a differentiation of sex organs, but the gametes develop 
from the protoplasts of vegetative cells. 

In a few green algae, such as Vaucheria, in the Charophyceae, and in 
r. early all the brown and red algae a more advanced condition has been 
reached. Here the gametes are borne in sex organs that have never been 
a part of the vegetative body, but arise as special reproductive branches. 
A differentiation has taken place between cells that remain entirely veg- 
etative in function and those that are strictly reproductive. Although 
this condition is found mainly among heterogamous algae, it occurs in a 
few isogamous members of the Phaeophyceae, such as Ectocarpns, where 
gametangia are developed on special branches. 

Thus among the algae three stages may be recognized in the evolution 
of sex organs, depending upon whether gametes are produced in (1) an 
unmodified vegetative cell, (2) a transformed vegetative cell, or (3) a 
special reproductive cell distinct from the rest of the body. The first 
stage is characteristic of isogamous forms, the second and third of het- 
erogamous forms. 

The sex organs of the bryophytes and pteridophytes are more highly 
developed than those of the algae in that both kinds are multicellular and 
have an outer jacket of sterile cells usually forming a single layer. The 
sterile jacket, which protects the gametes from drying out, was probably 



420 PLANT MORPHOLOGY 

developed as a response to air exposure. Although the bryophytes are 
thought to have arisen from chlorophycean ancestors, existing green algae 
have unicellular sex organs. Therefore it is necessary to assume that the 
ancestral forms had multicellular gametangia of the Ectocarpus type. 
The bryophyte antheridium could readily have been derived from such a 
gametangium by sterilization of the outer layer of cells. The archego- 
nium, having diverged more widely from its original condition, went 
through several stages in its evolution. At first it may have resembled 
the antheridium, consisting of a group of fertile cells enclosed by a sterile 
jacket. Further progress may have been marked by reduction of the fer- 
tile cells to a single row and then by sterilization of all of these except the 
lowest one, the other cells in the row becoming canal cells. Evidence for 
this theory comes from the occasional appearance, in both liverworts and 
mosses, of reversionary archegonia with multiple eggs, with two rows of 
canal cells, or with some of the canal cells replaced by spermatogenous 
cells. 

The sex organs of bryophytes and pteridophytes perform accessory 
functions related to gametic union and embryo development. The sterile 
jacket of the antheridium not only protects the developing sperms but 
facilitates their dispersal. Frequently dehiscence occurs suddenly and 
the sperms are discharged with considerable force. The neck of the 
archegonium serves as a passageway for the entrance of sperms, the canal 
cells breaking down to form mucilage through which the sperms swim. 
The venter of the archegonium enlarges after fertilization, protecting the 
embryo and aiding in the transfer of food to it. 

Antheridia with a large number of spermatogenous cells and archego- 
nia with many neck canal cells are generally regarded as primitive. 
Throughout the bryophytes and pteridophytes the tendency to reduce the 
number of these cells reaches an extreme in the heterosporous pterido- 
phytes, where the antheridium may produce only four sperms, as in 
Isoetes, and the archegonium has only one neck canal cell. These trends 
are continued into the spermatophytes, w^here antheridia are not organ- 
ized and only two sperms or their equivalent are formed and where arche- 
gonia, without any neck canal cells, are present only in the gymnosperms. 

The embryo sac of angiosperms may have evolved from the typical 
female gametophyte of gymnosperms, but, except for the formation of free 
eggs in the Gnetales, intermediate stages are lacking. Although most 
angiosperms possess an eight-nucleate embryo sac that develops in a 
characteristic way, many deviations from the typical pattern occur. 
These reveal several trends, such as the participation of more than a single 
megaspore nucleus in the formation of the embryo sac and a reduction in 
the number of nuclear divisions that intervene between the formation of 
the megaspore nuclei and the egg nucleus. 



EVOLUTION OF THE PLANT KINGDOM 421 

Further Expressions of Sexuality. In the evokition of sex the differen- 
tiation of gametes was soon followed by a differentiation of sex organs. A 
further stage was the differentiation of structures bearing the sex organs, 
while a final stage was a sexual differentiation of entire individuals. 

In most liverworts belonging to the Marehantiales the antheridia are 
borne on male receptacles and the archegonia on female receptacles. 
These show a marked structural differentiation. In some members, such 
as Marchantia, there is also a differentiation of individuals, the male 
plants bearing antheridial receptacles and the female plants archegonial 
receptacles. In Sphaerocarpus, belonging to another group of liverworts, 
the male plants are much smaller than the female. Certain species of 
Oedogonium have dwarf male filaments consisting of only a few cells. 

Although Spirogyra has not reached the level of heterogamy, some spe- 
cies show a differentiation of sexual individuals. This expresses itself 
only in the behavior of the gametes, those of one member of a pair of con- 
jugating filaments being active, while those of the other are passive. The 
occurrence of distinct male and female individuals is a feature of many 
heterogamous algae, such as Cutleria, Dictyota, Polysiphonia, and many 

others. 

In plants with an alternation of generations the gametophyte is com- 
monly called the sexual generation and the sporophyte the asexual one. 
This misconception arises from failure to regard fertilization and meiosis 
as complementary processes, both of which are integral parts of a complete 
sexual life cycle. Vegetative spores are asexual but meiospores are not. 
Where the zygote directly gives rise to four meiospores, as in Oedogonium, 
it is easy to associate their formation with fertilization. Where meio- 
spores are borne on a sporophyte, the time interval between fertilization 
and meiosis is longer, but the relation between them is the same. 

In the bryophytes and homosporous pteridophytes the sporophyte does 
not express any sexual characters. But, with the establishment of het- 
erospory, sexual differentiation becomes extended from the gametophyte 
to the sporophyte. The significance of heterospory lies in the production 
of a male gametophyte by the microspore and a female gametophyte by 
the megaspore. The occurrence of two kinds of gametophytes is reflected 
in a visible differentiation of the spores. This differentiation may be 
extended to the sporangia, sporophylls, strobili, and even to the entire 
sporophyte. In Selagindla the visible consequences of heterospory are 
not as far-reaching as in seed plants, where the organs associated with the 
production of microspores (stamens and pollen sacs) and of megaspores 
(carpels and ovules) are as highly differentiated as the sex organs of bryo- 
phytes and pteridophytes. Stamens and carpels are not sex organs, but 
their differences are associated with a sexual differentiation that has been 
extended to them from the gametes. 



422 PLANT MORPHOLOGY 

Among the gymnosperms a separation of microspore-bearing and 
megaspore-bearing structures to two kinds of cones is another expression 
of sexual differentiation. In the cycads, Ginkgo, and some conifers this is 
extended to the entire sporophyte, so that there are male and female 
plants as well as male and female cones. Most angiosperms bear flowers 
having both stamens and carpels, but some have two kinds of flowers, one 
with stamens and the other with carpels. The two kinds may occur on 
the same plant or on separate plants, depending on the species. 

Significance of Sex. The most important feature of sexual reproduc- 
tion is the union of the two gametic nuclei. This brings together two 
haploid sets of chromosomes to form a diploid zygote nucleus. Each set 
consists of innumerable genes that determine hereditary characters and 
each ordinarily represents a somewhat different assortment of genes. 
Meiosis, sooner or later following gametic union, provides a means of 
reshuffling the paternal and maternal chromosomes brought together 
in the previous act of fertilization, thus resulting in many new gene 
combinations. 

It is evident that sexual reproduction, through fertilization and meiosis, 
creates great variation among individuals related by descent and so fur- 
nishes raw material for evolutionary processes to work upon. Asexual 
reproduction results in organic similarity; sexual reproduction results in 
diversity. The significance of sexual reproduction is not primarily the 
multiplication of individuals, but the production of heritable variations 
that accelerate the process of evolution. It is in this feature that its 
great advantage lies. In plants first reaching the level of sexuality, this 
advantage would tend to be perpetuated through natural selection and to 
become established as a permanent part of the life history. 

ALTERNATION OF GENERATIONS 

Many algae and fungi, as well as all plants above the thallophyte level, 
are characterized by an alternation of generations, in which the life cycle 
consists of two kinds of individuals that follow each other in alternate 
sequence. One of these, the gametophyte, is haploid and produces 
gametes, while the other, the sporophyte, is diploid and produces spores. 
The diploid condition arises in the zygote, produced by gametic vuiion. 
The zygote develops into a sporophyte, meiosis taking place when spores 
are formed. The spore gives rise to the gametophyte. The zygote is the 
first cell of the sporophyte generation, and the spore is the first cell of the 
gametophyte generation. 

Types of Life Cycles. Fertilization and meiosis are the two cardinal 
events in every life cycle involving sexual reproduction and each is a 
necessary consequence of the other. Thus every plant with sexual repro- 
duction displays both a haploid and a diploid phase, but in many thallo- 



EVOLUTION OF THE PLANT KINGDOM 423 

phytes the life cycle includes only one kind of individual. In most green 
algae the vegetative body is haploid and meiosis occurs in connection with 
the germination of the zygote. Here a prolonged haploid growth phase 
alternates with a single diploid cell, the zygote. Although the zygote 
usually gives rise to four spores, it could hardly be regarded as a sporo- 
phyte, and so there is no true alternation of generations. 

In many diatoms, Siphonocladiales, Siphonales, and in all the Fucales 
the vegetative body is diploid, meiosis occurring when gametes are formed, 
as in animals. Here a prolonged diploid growth phase alternates with a 
few haploid cells, the gametes, and again there is no true alternation of 
generations. Thus, where the life cycle includes only one vegetative 
phase, this may be either haploid, meiosis directly following fertilization, 
or diploid, meiosis immediately preceding fertilization. 

An alternation of generations occurs wherever a diploid growth phase 
intervenes between fertilization and meiosis, and a haploid growth phase 
between meiosis and fertilization. Such a condition is displayed by Ulva 
and a few other green algae, by most brown and red algae, and by all 
bryophytes and pteridophytes. It is also characteristic of the spermato- 
phytes, although in angiosperms the haploid phase (gametophyte) con- 
sists of only a very few cells or nuclei. Obviously this is a result of reduc- 
tion, while in Fucus, for example, where a somewhat similar condition 
prevails, reduction may have taken place but evidence is lacking. 

In Fucus, where spores are absent from the life cycle, a diploid plant 
body produces gametes. In Ulothrix and Oedogonium, on the other hand, 
a haploid body may produce either gametes or spores. Such spores are 
vegetative spores and are not homologous with the four spores derived 
from the zygote or with the spores borne by the sporophyte of the higher 
plants, both of which are meiospores and belong to the sexual life cycle. 
Either a haploid or a diploid plant body may be propagated vegetatively 
by various means without fertilization and meiosis being involved. Veg- 
etative reproduction is always asexual. 

Origin of Alternation of Generations. Regarding the origin of alterna- 
tions, two different theories have been advanced, the homologous theory 
and the antithetic theory. The homologous theory was based originally 
on conditions in those algae in which a single plant can produce either 
spores or gametes. It assumes that these functions later became sep- 
arated into two distinct individuals, sporophyte and gametophyte, alter- 
nating regularly in the life cycle. The theory furnishes a more adequate 
explanation of alternation where the two generations are alike vegeta- 
tively than w^here they are unlike. Against it may be urged the fact that 
vegetative spores produced by a haploid plant body and meiospores pro- 
duced by a zygote or a sporophyte are not morphologically equivalent. 
Also, the homologous theory fails to account for the difference in chromo- 



424 PLANT MORPHOLOGY 

some numbers that exists between the gametophyte and sporophyte. 
The phenomena of apogamy and apospory have been cited as evidence in 
favor of this theory, but they are merely digressions from the normal life 
cycle. 

The antithetic theory seems to be more in accord with actual conditions 
in the plant kingdom. It contends that the gametophyte is the original 
generation and the sporophyte a later one interpolated in the life history 
between fertilization and meiosis. It holds that the sporophyte has 
evolved from the zygote, an initial stage occurring in such algae as Oedo- 
gonium, where the zygote gives rise to four meiospores. A second stage 
might be represented by Coleochacte, where the zygote, after undergoing 
meiosis, forms a small group of spore-producing cells (up to 32). In a 
third stage, as seen in Riccia, the zygote develops into a very simple spo- 
rophyte in which meiosis is delayed until spores are formed. In the evo- 
lution of the plant kingdom, such a delay may have taken place in a single 
step by mutation. 

Where the zygote becomes a thick-walled resting cell, its nucleus divides 
reductionally upon germination. This is a feature only of fresh-water 
green algae. Where the zygote germinates at once, it gives rise to a 
diploid vegetative body. This occurs in the marine green algae, in the 
brown algae, and in most of the red algae. ^ In most algae with a diploid 
vegetative phase, there is a distinct alternation of generations, exceptions 
being such forms as Acetabularia, Codium, Bryopsis, and the Fucales. 

It is apparent that the behavior of the zygote is related to environ- 
mental conditions. Seasonal variations are more pronounced in bodies 
of fresh water than in the ocean. A resting zygote, usually formed near 
the close of the growing season, carries the plant over a period of unfavor- 
able conditions. Algae with a resting zygote display no diploid vegeta- 
tive phase. Thus the origin of alternation of generations may be sought 
in a determination of the factors that induce prompt germination of the 
zygote with an accompanying postponement of meiosis. Perhaps these 
factors have been responsible for a mutation that has resulted in the 
establishment of a diploid generation. 

Alternation of generations, once established in a group of plants, con- 
ferred such advantages that it would tend to be retained as a permanent 
feature. These advantages are: (1) An increase in the number of individ- 
uals produced as a result of a single gametic union, thereby conferring on 
them any beneficial results of such a union which, in many of the lower 
plants, sometimes occurs with a great deal of rarity. Instead of produc- 

1 In the red algae the zygote always germinates promptly, but subsequent stages are 
variable and complicated by the formation of carpospores. These are haploid in the 
two lowest orders, where the zygote is the only diploid cell in the life cycle, and diploid 
in the other orders, where they give rise to sporophytes. 



EVOLUTION OF THE PLANT KINGDOM 425 

ing only one new plant, the zygote now indirectly produces, through the 
development of the sporophyte, a large number of new plants. (2) An 
increase in the possible range of variation. Where the zygote undergoes 
meiosis only two genetically different lines of descent are possible, since 
the segregation of genes takes place during the first reduction division. 
The development of a sporophyte results in meiosis in many spore mother 
cells, all descended from the same zygote, and so makes possible a great- 
many new chromosome combinations. The advantages of alternation of 
generations are proportional to the size and length of life attained by the 
sporophyte and account, at least in part, for its dominance over the game- 
tophyte in vascular plants. 

The evolution of the sporophyte in the higher plants has been marked by 
a prolongation, through vegetative growth, of the interval between fer- 
tilization and meiosis. The sporophyte of Riccia represents a primitive 
condition in that its growth period is short and nearly all its cells produce 
spores. Throughout bryophytes and pteridophytes progressive steriliza- 
tion of potentially sporogenous tissue has resulted in an elaboration of 
vegetative structures. At the same time, not only is relatively less and 
less tissue devoted to spore production, but it appears later and later in 
the life history. 

As the sporophyte has achieved independence and become the dom- 
inant generation in all vascular plants, the gametophyte has undergone a 
progressive decline. This has been accentuated by the development of 
heterospory, first seen in the pteridophytes. In spermatophytes the 
gametophyte has become not only greatly reduced structurally, but actu- 
ally is parasitic on the sporophyte, thus reversing conditions in the bryo- 
phytes, where the sporophyte is parasitic on the gametophyte throughout 
its entire existence. 



I 



i 



SELECTED REFERENCES 
General 

Arnold, C. A., Introduction to Paleobotany. New York, 1947. 

Brown, W. H., The Plant Kingdom. Boston, 1935. 

Campbell, D. H., The Evolution of the Land Plants. Stanford University, 1940. 

Darrah, W. C, Textbook of Paleobotany. New York, 1939. 

Engler, A., and L. Diels, Syllabus der Pflanzenfamilien. Berlin, 1936. 

Jeffrey, E. C, The Anatomy of Woody Plants. Chicago, 1917. 

McLean, R. C, and W. R. Ivimey-Cook, Textbook of Theoretical Botany, vol. I. 

London, 1951. 
Sharp, L. W., Fundamentals of Cytology. New York, 1943. 
Walton, J., An Introduction to the Study of Fossil Plants. London, 1940. 
Wettstein, R., Handbuch der systematischen Botanik, 2 vols. Leipzig and Vienna, 

1933. 

Algae 

Cavers, F., The Interrelationships of Flagellata and Primitive Algae. New PhytoL, 

Reprint 7. 1913. 
Chapman, V. J., An Introduction to the Study of Algae. Cambridge, 1941. 
Fritsch, F. E., The Structure and Reproduction of the Algae, 2 vols. Cambridge, 

1935 and 1945. 

, Present-day Classification of Algae. Bot. Rev. 10:233-277. 1944. 

Geitler, L., Reproduction and Life History in Diatoms. Bot. Rev. 1:149-161. 1935. 
Kniep, H., Die Sexualitat der niederen Pflanzen. Jena, 1928. 
Oltmanns, F., Morphologic und Biologic der Algen, 3 vols. Jena, 1922-1923. 
Setchell, W. A., and N. L. Gardner, The Marine Algae of the Pacific Coast of North 

America. I. Myxophyceae. Univ. Calif. Pubs. Bot. 8:1-138. 1919. II. 

Chlorophyceae, ibid. 8:139-374. 1920. III. Melanophyceae, ibid. 8:383-898. 

1925. 
Smith, G. M., The Fresh-water Algae of the United States. New York, 1950. 

, Cryptogamic Botany, vol. I. Algae and Fungi. New York, 1938. 

-, Nuclear Phases and Alternation of Generations in the Chlorophyceae. Bot. 

^et;. 4:132-139. 1938. 
, Marine Algae of the Monterey Peninsula, California. Stanford University, 



1944. 
Taylor, W. R., Marine Algae of Florida. Carnegie Inst. Wash. Pub. 379:1-219. 1922. 
, Marine Algae of the Northeastern Coast of North America. Univ. Mich. 

Studies Sci. Ser. 13:1-427. 1937. 
Tilden, Josephine E., The Algae and Their Life Relations. Minneapolis, 1935. 

Fungi 

Alexopoulos, C. J., Introductory Mycology. New York, 1952. 
Bessey, E. A., Morphology and Taxonomy of Fungi. Philadelphia, 1950. 
Fitzpatrick, H. M., The Lower Fungi — Phycomycetes. New York, 1930. 

427 



428 PLANT MORPHOLOGY 

Gaumann, E. A., and C. W. Dodge, Comparative Morphology of Fungi. New York, 

1928. 
Gwynne-Vaughan, H. C. I., and B. Barnes, The Structure and Development of the 

Fungi. Cambridge, 1937. 
Kniep, H., Die Sexualitat der niederen Pflanzen. Jena, 1928. 
Martin, G. W., The Myxomycetes. 5o<. /???;. 6 :356-388. 1940. 
Smith, G. M., Cryptogamic Botany, vol. I. Algae and Fungi. New York, 1938. 
Wolf, F. A., and F. T. Wolf, The Fungi, vol. I. New York, 1947. 

Bryophytes 

Bower, F. O., Primitive Land Plants. London, 1935. 

Campbell, D. H., Structure and Development of Mosses and Ferns. New York, 1918. 

, The Relationships of the Hepaticae. Bot. Rev. 2:53-66. 1936. 

Cavers, F., The Interrelationships of the Bryophyta. New PhytoL, Reprint 4. 191 1 . 
Smith, G. M., Cryptogamic Botany, vol. II. Bryophytes and Pteridophytes. New 
York, 1938. 

Pteridophytes 

Bower, F. O., Ferns (Filicales), 2 vols. Cambridge, 1923-1926. 

, Primitive Land Plants. London, 1935. 

Campbell, D. H., Structure and Development of Mosses and Ferns. New York, 1918. 
Eames, A. J., Morphology of Vascular Plants. Lower Groups. New York, 1936. 
Smith, G. M., Cryptogamic Botany, vol. II. Bryophytes and Pteridophytes. New 
York, 1938. 

Gymnosperms 

Chamberlain, C. J., Gymnosperms, Structure and Evolution. Chicago, 1935. 
Coulter, J. M., and C. J. Chamberlain, Morphology of Gymnosperms. Chicago, 1917. 
Johansen, D. A., Plant Embryology. Waltham, Mass., 1950. 
Schnarf, K., Embryologie der Gymnospermen. Berlin, 1933. 

Angiosperms 

Coulter, J. M., and C. J. Chamberlain, Morphology of Angiosperms. New York, 

1903. 
Eames, A. J., and L. H. MacDaniels, Introduction to Plant Anatomy. New York, 

1947. 
Esau, Katherine, Origin and Development of Primary Vascular Tissues in Seed Plants. 

Bot. Rev. d:l25-20G. 1943. 
Foster, A. S., Problems of Structure, Growth, and Evolution in the Shoot Apex of 

Seed Plants. Bot. Rev. B:4:54:-470. 1939. 
Johansen, D. A., A Critical Review of the Present Status of Plant Embryology. Bot. 

Rev. 11:87-107. 1945. 

, Plant Embryology, Waltham, Mass., 1950. 

Maheshwari, P., The Angiosperm Embryo Sac. Bot. Rev. 14:1-56. 1948. 

, The Male Gametophyte of Angiosperms. Bot. Rev. 15:1-75. 1949. 

, An Introduction to the Embryology of Angiosperms. New York, 1950. 

Schnarf, K., Embryologie der Angiospermen. Berlin, 1929. 

, Vergleichende Embryologie der Angiospermen. Berlin, 1931. 

Wilson, C. L., and T. Just, The Morphology of the Flower. Bot. Rev. 5:97-131. 

1939. 



SELECTED REFERENCES 429 



Evolution 

Coulter, J. M., The Evolution of Sex Plants. Chicago, 1914. 

Svedelius, N., Alternation of Generations in Relation to Reduction Division. Bot. 

Gaz. 83 :3r)2-384. 1927. 
Turrill. W. B., Taxonomy and Phylogeny. Bot. Rev. 8:473-532; 655-707. 1942. 
Wahl. H. A., Alternation of Generations and Classification with Special Reference to 

the Teaching of Elementary Botany. Torreya ^5:1-12. 1945. 



GLOSSARY 

Abaxial. Situated on the side away from the axis or stem. 

Accessory fruit. A fruit consisting of a ripened ovary and other ripened parts, as the 

calyx or receptacle. 
Acropetal. An order of development in which the youngest structures are at the apex 

and the oldest at the base. 
Actinomorphic. Having flowers with radial symmetry, the parts extending outward 

from a common center like the spokes of a wheel. 
Adaxial. Situated on the side toward the axis or stem. 

Adventitious. Arising sporadically without order or out of the usual place, as a bud. 
Aeciospore. In various rust fungi, one of the spores produced in an aecium. 
Aecium. A cup-like structure in the life cycle of a typical rust, producing spores in 

chains. 
Aerobic. Living or active only in the presence of free oxygen. 
Aggregate fruit. A fruit consisting of a group of consolidated ripened ovaries derived 

from a single flower. 
Akinete. In certain algae, a nonmotile, thick-walled resting spore derived from an 

entire vegetative protoplast whose wall becomes the wall of the spore. 
Alternation of generations. The presence in the same life cycle of two distinct plant 

bodies or individuals that succeed each other, a haploid body (gametophyte) pro- 
ducing gametes and a diploid body (sporophyte) producing spore^^. 
Ament. A catkin, or scaly spike. 

Amphicribral. A vascular bundle with phloem surrounding the xylem. 
Amphigastrium. One of the reduced ventral leaves in a leafy liverwort. 
Amphiphloic. A siphonostele with phloem both external and internal to the xylem. 
Amphithecium. In bryophytes, the outer layer of cells developed in the young 

sporophyte. 
Amphivasal. A vascular bundle with xylem surrounding the phloem. 
Anaerobic. Living or active in the absence of free oxygen. 

Anatropous. Having the ovule inverted and straight, with the micropyle bent down- 
ward to the funiculus, to which the body of the ovule is united. 
Androspore. In such algae as Oedogonium, a zoospore that produces a dwarf male 

filament. 
Anisocarpic. Flowers in which the number of carpels is not equal to that of each of 

the other cycles. 
Annulus. A ring or ring-like part, as in a mushroom, a moss capsule, or a fern 

sporangium. 
Anther. The pollen-bearing part of a stamen. 
Antheridium. The sperm-producing organ in thallophytes, bryophytes, and pterido- 

phytes. 
Anticlinal. Inclined in an opposite direction; a cell wall perpendicular to the outside 

surface. 
Antipodal. Pertaining to the opposite end, as the cells or nuclei in an embryo sac at 

the end opposite the egg. 
Apetalous. Without petals but with sepals. 

431 



432 PLANT MORPHOLOGY 

Apical cell. A single cell at the tip of a structure from the segments of which all its 

cells are derived. 
Aplanospore. In various algae, a nonmotile spore with a wall not derived from the 

wall of flio cell in which it is formed. 
Apocarpous. With s(>parate carpels. 
Apogamy. Development of an embryo from a cell of the gametophyte other than 

the egg. 
Apomixis. A condition in which sexual reproduction is replaced by some form of 

asexual reproduction. 
Apophysis. An enlargement at the base of the capsule of some mosses. 
Apothecium. A cup-like or disk-like ascocarp. 
Archegonium. The female sex organ of bryophytes, pteridophytes, and most gymno- 

sperms. 
Ascocarp. In most ascomycetes, a fruiting body producing asci. 
Ascogenous hyphae. Hyphae that bear asci. 
Ascogonium. The female sex organ of ascomycetes. 
Ascospore. One of the spores borne in an ascus. 

Ascus. In ascomycetes, a sac-like cell in which ascospores are produced. 
Autoecious. Passing through all stages in the life cycle on the same host, as certain 

rust fungi. 
Autotrophic. Self-nourishing; capable of making its own food, as a green plant. 
Auxospore. In diatoms, a reproductive cell formed by the union of two cells or 

asexually. 
Axil. The upper angle between a leaf or branch and the stem from which it arises. 

Bacillus. A straight, rod-shaped bacterium. 

Basidiocarp. In most basidiomycetes, a fruiting body producing basidia. 

Basidiospore. One of the spores borne on a basidium. 

Basidium. In basidiomycetes, a club-shaped structure that produces basidiospores. 

Basipetal. An order of development in which the youngest structures are at the base 
and the oldest at the apex. 

Biciliate. Having two cilia. 

Bilocular. Having two locules or cavities. 

Bisporangiate. With microsporangia and megasporangia borne in the same strobilus 
or flower. 

Bract. A scale borne on a floral axis, especially one subtending a flower or inflores- 
cence. 

Calyptra. In bryophytes and pteridophytes, a covering developed from the venter 

of the archegonium and surrounding the sporophyte, at least when young, and in 

some mosses later carried on top of the capsule as a hood. 
Calyptrogen. That part of the embryonic tissue of a root tip from which the rootcap 

is developed. 
Calyx. The sepals of a flower, collectively; the outer whorl of perianth parts. 
Cambium. A lateral meristem consisting of a layer of cells giving rise to secondary 

tissues, particularly xylem and phloem, in many roots and stems. 
Campylotropous. Having the ovule turned so that the base and apex are close 

together. 
Capillitium. A network of delicate threads, as in the sporangium of a myxomycete. 
Capsule. In bryophytes, any closed vessel containing spores; in angiosperms, a dry, 

dehiscent, many-seeded fruit derived from a compound pistil. 



GLOSSARY 433 

Carinal canal. In the Equisetinae, one of the canals in the stele of the stem Ij'ing 
beneath a ridge on the surface. 

Carotin. Any of a group of mainly deep yellow to orange-red pigments found in var- 
ious plants, eitlier alone or associated with others, particularly chlorophyll; they 
differ from the xanthophylls in containing only carbon and hydrogen. 

Carotinoid. Any of a group of pigments that includes the carotins and xantliophylls. 

Carpel. The megasporophyll or ovule-bearing organ of seed plants; in angiosperms, 
forming a simple pistil or part of a compound pistil. 

Carpogonium. In the red algae, the basal portion of the procarp where the egg is 
formed, or sometimes comprising the entire female sex organ. 

Carpospore. In the red algae, one of a group of nonmotile spores produced either by 
the zygote directly or budded off from the tips of short filaments arising from the 
carpogonium after fertilization. 

Cauline. Growing on, or belonging to, a stem. 

Cellulose. A carbohydrate constituting the chief substance in the cell wall of plants. 

Chalaza. The portion of an ovule below where the integuments are united to the 
nucellus. 

Chalazogamy. Fertilization of an egg by means of a pollen tube entering the ovule 
through the chalaza. 

Chlamydospore. A thick-walled resting spore produced in certain fungi, particularly 
the smuts, and representing a transformed vegetative cell. 

Chlorophyll. The green coloring matter of plants, occurring in two associated forms, 
chlorophyll a and chlorophyll b. 

Chloroplast. A plastid containing chlorophyll. 

Choripetalous. Having separate petals. 

Chromatin. A deeply staining, granular, protoplasmic material occurring in the 
nucleus of cells. 

Chromosome. One of the organized bodies, of definite number, into which the 
chromatin of a nucleus resolves itself in connection with mitotic division. 

Cilium. A hair-like protoplasmic process capable of vibratory or lashing movement. 

Circinate. Coiled ; rolled up on the axis with the apex at the center. 

Cleistothecium. A closed ascocarp. 

Coccus. A spherical bacterium. 

Coenocyte. A plant body, as in some algae and fungi, the protoplasm of which is con- 
tinuous and multinucleate and not divided into separate cells. 

Coleorhiza. In some seed plants, a sheath covering the root tip of an embryo. 

Collateral. Side by side, as the arrangement of xylem and phloem in the vein of a leaf. 

Colony. A group of unicellular plants of the same kind held together by a common 
investment or stalk. 

Columella. A sterile central portion of the sporangium of certain molds; a sterile cen- 
tral axis in the capsule of certain liverw^orts and mosses. 

Companion cell. A cell associated with a sieve tube in the phloem of angiosperms and 
of common origin with it. 

Conceptacle. In certain algae, as in the Fucales, a cavity with an external opening 
containing reproductive cells. 

Conidiophore. In certain fungi, a specialized hypha that produces conidia by 
abstriction. 

Conidium. An aerial spore produced by abstriction from the tip of a conidiophore. 

Conjugation. The fusion of two similar gametes. 

Corolla. The petals of a flower, collectively; the inner whorl of perianth parts. 

Corpus. The central growth zone in a stem tip. 



434 PLANT MORPHOLOGY 

Cortex. Tho region of a root or stem that lies between the epidermis and the stele. 

Cotyledon. The first leaf, or one of the first pair or whorl of leaves, formed on the 
embryo of seed plants. 

Cuneate. Wedge-shaped, as an apical cell with four cutting faces. 

Cupule. A small cup-shaped structure, especially in certain liverworts, in which 
gemmae are produced. 

Cystocarp. A kind of sporocarp, in the red algae, produced after fertilization and con- 
sisting of the carpogonium, gonimoblasts, carpospores, and often other associated 
structures. 

Cytoplasm. The protoplasm of a cell exclusive of the nucleus. 

Deciduous. Falling off at the end of the growing season, as some leaves; said of plants 

having leaves of this type. 
Dehiscence. Bursting open, at maturity, in some regular manner, of a sporangium 

to discharge its spores or of a fruit to liberate its seeds. 
Dermatogen. A layer of embryonic cells, in a root tip or stem tip, that gives rise to 

the epidermis. 
Dichotomy. A type of branching in which the main axis forks repeatedly into two 

branches of equal length. 
Dicotyledonous. Having two cotyledons. 
Dictyostele. A dissected stele ; a stele consisting of a wide-meshed network of vascular 

strands. 
Dimorphic. Occurring in two distinct forms upon the same plant or upon other plants 

of the same species. 
Dioecious. Having the male organs on one plant and the female on another; having 

staminate and pistillate flowers on separate plants. 
Diploid. Having twice the basic or haploid number of chromosomes. 
Dolabrate. Hatchet-shaped, as an apical cell with two cutting faces. 
Dorsal. Pertaining to the back or outer side of an organ; designating the surface 

turned away from the axis, as the underside of a leaf; in liverworts and ferns, 

pertaining to the upper side of the prothallium. 
Dorsiventral. Having distinct dorsal and ventral surfaces. 

Ectophloic. A siphonostele with phloem external but not internal to the xylem. 

Egg. A female gamete. 

Egg apparatus. In angiosperms, a group of three cells at the micropylar end of the 

embryo sac, consisting of the egg and two synergids. 
Elater. A filament or filamentous appendage for dispersing spores, as in the capsule 

of a liverwort. 
Elaterophore. A structure to which a group of elaters is attached. 
Embryo. A young sporophyte, ordinarily derived from a fertilized egg. 
Embryo sac. In angiosperms, a large cell within the nucellus of the ovule in which 

the egg is produced and, following fertilization, the embryo develops; the female 

gametophyte of angiosperms. 
Endarch. Development of primary xylem in a centrifugal direction. 
Endodermis. The innermost layer of cortical cells in a root or stem. 
Endogenous. Arising from within; growing from or on the inside. 
Endophyte. A plant that grows inside another plant of a different species, but not 

parasitically. 
Endosperm. Nutritive tissue in a young or mature seed, formed within the embryo 

sac and lying outside the embryo. 



GLOSSARY 435 

Endothecium. In bryophytes, the central mass of cells developed in the young sporo- 
phyte; in an anther, a specialized layer of cells lying beneath the epidermis and 
assisting in dehiscence. 

Epidermis. A superficial layer of cells on leaves, young stems and roots, etc. 

Epigynous. Said of flowers whose ovary is sunken in the receptacle so that the peri- 
anth seems to arise from its summit. 

Epiphyte, k plant that grows upon another plant, but not parasitically. 

Eusporangiate. A type of sporangial development occurring in spermatophytes and 
most pteridophytes in which the sporogenous tissue arises from the inner segment 
of the initial cell. 

Exarch. Development of primary xylem in a centripetal direction. 

Excurrent. Having a prolonged main stem extending to the top, as in some trees. 

Exine. The outer layer of the cell wall of a pollen grain. 

Eyespot. An eye-like spot of pigment in certain motile unicellular algae and repro- 
ductive cells. 

Fertilization. The fusion of a sperm and an egg or of a male nucleus and a female 
nucleus to form a new individual (zygote). 

Fiber. A thread or thread-like structure; a long, slender, thick-walled cell, as in 
sclerenchyma. 

Filament. A thread-like series of cells or a very long, cylindrical, single cell, as in cer- 
tain algae and fungi; the part of the stamen that supports the anther. 

Fission. Reproduction of a unicellular plant by division into two equal cells. 

Flagellum. A long, whip-like, protoplasmic process or appendage of a cell capable of 
lashing movement. 

Flower. In angiosperms, a group of sporophylls usually surrounded by a perianth. 

Foliar. Pertaining to, or consisting of, leaves. 

Foot. The basal portion of the sporophyte in mosses and most liverworts; a part of 
the embryo of pteridophytes that is embedded in the gametophyte and acts as an 
absorbing organ. 

Fruit. In angiosperms, a ripened ovary with any external parts that ripen in associa- 
tion with it. 

Fucoxanthin. A brown pigment associated with chlorophyll in the brown algae. 

Funiculus. The stalk of an ovule. 

Fusion nucleus. The nucleus, in an embryo sac, typically formed by the union of two 
polar nuclei. 

Gametangium. A cell or organ in which gametes are formed; a sex organ. 
Gamete. A mature sex cell; a cell capable of uniting with another of opposite sex to 

form a zygote. 
Gametophore. A branch bearing sex organs or gametangia, as in mosses. 
Gametophyte. In plants with an alternation of generations, the individual that bears 

gametes and has the haploid number of chromosomes. 
Gemma. A detachable bud or bud-like body capable of developing into a new plant, 

as in bryophytes and pteridophytes. 
Generative cell. In gymnosperms, the cell in the pollen grain that gives rise to the 

stalk and body cells, in angiosperms to the two male cells. 
Gill. One of the plates on the under surface of the pileus of a mushroom. 
Gleba. The inner portion of the basidiocarp in the Gasteromycetales. 
Glycogen. A carbohydrate related to starch and dextrin, very common in animal 

tissues. 



436 PLANT MORPHOLOGY 

Gonimoblast. In many red algae, one of the many short filaments arising from the 

fertilized carpogoniiim and giving rise to carpospores. 
Gullet. A tube leading into the interior of the cell in such organisms as Euglena. 

Haematochrome. An orange-red pigment in the cell sap of certain green algae, such 

as Sphaerdla. 
Haploid. Having the single or basic chromosome number. 
Haustorium. In parasitic plants, a specialized outgrowth that serves to absorb food 

from the host. 
Heterocyst. In certain blue-green algae, an enlarged cell differing from the other cells 

in a filament in being clear, colorless, and thick-walled. 
Heteroecious. Requiring two different hosts to complete the life cycle, as certain 

rust fungi. 
Heterogamous. Having a union of unlike gametes, one (the egg) considerably larger 

than the other (the sperm). 
Heteromorphic. Of unlike form ; a type of alternation of generations in which the 

gametophyte and sporophyte are dissimilar vegetatively. 
Heterosporous. Producing two kinds of spores, usually of different size, the small 

ones (microspores) producing male and the large ones (megaspores) female 

gametophytes. 
Heterothallic. Having two kinds of mycelia, distinct physiologically and represent- 
ing opposite sexes. 
Heterotrophic. Obtaining nourishment from organic matter, either living or dead, 

as all plants lacking chlorophyll. 
Histogen. A group of embryonic cells that gives rise to a particular kind of perma- 
nent tissue. 
Homosporous. Producing spores of only one kind. 
Homothallic. Having two kinds of structures representing opposite sexes borne on 

the same mycelium. 
Hormogonium. In the blue-green algae, a portion of a filament, usually marked off 

by heterocysts, that may become detached and produce a new filament. 
Host. The organism from which a parasite secures its food. 

Hymenium. In the higher fungi, a layer of cells from which asci or basidia arise. 
Hypha. One of the filaments comprising the mycelium of a fungus. 
Hypocotyl. In seed plants, the portion of the stem below the cotyledons in an embryo 

or a seedling. 
Hypodermal. Situated immediately beneath the epidermis. 
Hypogynous. With the perianth and stamens attached to the receptacle below the 

ovary and free from it. 

Imbricate. Overlapping in regular order, like shingles on a roof. 

Imperfect flower. Having either stamens or a pistil but not both, these being in sep- 
arate flowers. 

Indusium. In many ferns, a membrane that covers or invests a sorus. 

Inflorescence. A flower cluster. 

Integument. The covering of an ovule. 

Intercalary. Inserted or occurring between cells or regions of a different kind ; growth 
occurring between the apex and ba.se. 

Inline. The inner layer of the cell wall of a pollen grain. 

Involucre. In some bryophytes, an envelope partially or completely enclosing a 
sporophyte and arising from the surrounding tissue of the gametophyte. In 
angiosperms, a whorl or set of bracts surrounding a flower or flower cluster. 



GLOSSARY 437 

Irregular flower. One showing lack of uniformity among members of the same whorl 

of perianth parts. 
Isocarpic. Flowers in which the number of carpels is equal to that of each of the other 

cj'cles. 
Isogamete. A gamete without apparent sexual differentiation. 
Isogamous. Characterized by the union of gametes of equal size. 
Isomorphic. Of like form ; a type of alternation of generations in which the gameto- 

phyte and sporophyte are similar vegetatively. 
Isthmus. A contracted part or passage connecting two similar structures or cavities, 

as in desmids. 

Lamarin. A dextrin-like carbohydrate constituting a form of reserve food in many 
brown algae. 

Leaf gap. A break or interruption in the continuity of a siphonostele caused by the 
departure of a leaf trace from it. 

Leaf trace. A strand of vascular tissue passing through the cortex of a stem and con- 
necting the stele with a leaf base. 

Leaflet. One of the parts of a divided ("compound") leaf 

Lenticel. A pore in the corky bark of a woody stem through which gases pass. 

Leptosporangiate. A type of sporangial development, occurring in the higher ferns, 
in which the sporogenous tissue arises from the outer segment of the initial cell. 

Leucosin. A protein-like substance constituting a form of reserve food in certain 
algae. 

Lignin. A substance related to cellulose and with it forming the cell walls of woody 
tissue, stone cells, and most fibers. 

Ligule. A single scale-like appendage near the base of some leaves on the adaxial sur- 
face, as in Selaginella and Isoefes. 

Linear. Long and narrow with parallel sides. 

Locule. A compartment or cavity, as in a sporangium or ovary. 

Megaphyllous. Having large leaves. 

Megasporangium. A sporangium producing only megaspores. 

Megaspore. In heterosporous plants, a spore that gives rise to a female gametophyte 
and generally is larger than a microspore. 

Megasporophyll. A sporophyll that bears only megasporangia. 

Meiosis. The two nuclear divisions that result in a reduction in chromosome num- 
ber from the diploid to the haploid condition. 

Meiospore. A spore formed as a result of meiosis. 

Meristem. Embryonic tissue, the cells of which are capable of active division. 

Mesarch. Development of primary xylem in both centripetal and centrifugal 
directions. 

Mesophyll. Green parenchyma occurring between the epidermal layers of a leaf and 
forming the internal ground tissue. 

Metaxylem. Primary xylem formed later than the protoxylem. 

Microphyllous. Having small leaves. 

Micropyle. A minute opening in the integument of an ovule through which the pollen 
tube ordinarily enters. 

Microsporangium. A sporangium producing only microspores. 

Microspore. In heterosporous plants, a spore that gives rise to a male gametophyte 
and is generally smaller than a megaspore. 

Microsporophyll. A sporophyll that bears only microsporangia. 



438 PLANT MORPHOLOGY 

Mitosis. The series of complex changes through which a nucleus ordinarily passes 
when it divides. 

Monocotyledonous. Having a single cotyledon. 

Monoecious. Having the male and female organs on the same plant; having stamens 
and pistils in separate flowers on the same plant. 

Monophylletic. Developed from a single stock or from a common ancestry. 

Monopodium. A main stem that continues its original line of growth, giving off lat- 
eral branches. 

Monosiphonous. Consisting of a single filament. 

Monosporangiate. With microsporangia and megasporangia borne in separate 
strobili or flowers. 

Multicellular. Many-celled. 

Multiciliate. Having many cilia. 

Multilocular. Having many locules or cavities. 

Mycelium. The mass of hyphae forming the vegetative body of most fungi. 

Myxamoeba. A uninucleate, amoeboid, reproductive cell in the myxomycetes. 

Naked flower. A flower without a perianth. 

Nucellus. The main body of an ovule, constituting the megasporangium, and sur- 
rounded by the integument or integuments. 

Nucleus. A rounded protoplasmic body, enclosed by a membrane, embedded in the 
cytoplasm of nearly all cells, and controlling their metabolism, growth, reproduc- 
tion, and inheritance. 

Ontogeny. The development of an individual organism throughout its successive 

growth stages. 
Oogonium. The female sex organ of thallophytes. 
Operculum. A lid formed at the top of a capsule, as in mosses. 
Orbicular. Flat and circular, or nearly circular, in outline; disk-shaped. 
Orthotropous. Having the ovule erect, with the micropyle and chalaza at opposite 

ends. 
Ovary. In angiosperms, the part of the pistil that contains the ovules. 
Ovule. The megasporangium of a seed plant with its integument or integuments; an 

immature seed. 

Palisade tissue. The portion of the mesophyll that is composed of elongated cells 

lying directly below and at right angles to the upper epidermis of a leaf. 
Palmate. Having lobes, divisions, or leaflets radiating from a common center. 
Papilla. A small protuberance or projection. 
Paramylon. A starch-like carbohydrate constituting a form of reserve food in the 

Euglenophyceae. 
Paraphysis. One of the sterile filaments commonly accompanying the sporangia or 

gametangia in many thallophytes and bryophytes. 
Parasite. An organism living on or in another organism and getting its food at the 

other's expense. 
Parenchyma. A soft, loose tissue composed of living, thin-walled cells that are not 

greatly differentiated. 
Parietal. Pertaining or belonging to the walls of a part or cavity. 
Parthenocarpy. The development of a fruit without fertilization. 
Parthenogenesis. The development of an embryo from an unfertilized egg. 
Pellicle. A thin, firm, outer membrane on certain unicellular organisms that lack a 

cell wall, such as Euglena. 



GLOSSARY 439 

Peltate. Shield-shaped, with the support attached to the lower surface instead of at 

the base or margin. 
Pentacyclic. Having five cycles or whorls of floral parts. 
Pentamerous. Having the floral parts in sets of five or multiples of five. 
Perfect flower. Having both stamens and pistil in the same flower. 
Perianth. The floral envelope; the calyx and corolla collectively. In the leafy liver- 
worts, a group of specialized united leaves surrounding the archegonia. 
Periblem. A group of embryonic cells, in a root tip or stem tip, that gives rise to the 

cortex. 
Pericarp. The ripened wall of an ovary. 
Perichaetium. A sheath or rosette of modified leaves surrounding the sex organs of 

many mosses. 
Periclinal. Parallel to the outer surface, as a cell wall. 
Pericycle. One or more layers of cells forming the outermost part of the stele in most 

vascular plants. 
Peridium. The outer covering of the basidiocarp in the Gasteromycetales. 
Perigynous. With the perianth and stamens borne on a disk or cup formed by the 

receptacle. 
Perisperm. Nutritive tissue in a seed formed outside the embryo sac, mainly in the 

nucellus. 
Peristome. A ring of teeth surrounding the open rim of a moss capsule. 
Perithecium. A spherical or flask-shaped ascocarp opening by a small terminal pore. 
Petal. One of the leaf-like parts of the corolla. 
Petiole. A leafstalk. 

Phellogen. The cambium that produces cork tissue. 
Phloem. In vascular plants, a complex tissue consisting of sieve tubes and often also 

of companion cells, parenchyma, and fibers, and serving for the conduction of 

food. 
Phycocyanin. A blue pigment associated with chlorophyll in the blue-green algae. 
Phycoerythrin. A red pigment associated with chlorophyll in the red algae. 
Phylogeny. The evolutionary development of the race or group to which an organism 

belongs. 
Pileus. The expanded or cap-like part of a mushroom. 
Pinna. A leaflet or primary division of a pinnate leaf. 
Pinnate. Feather-like ; with the leaflets or primary divisions arranged on each side of 

a common axis or rachis. 
Pinnule. One of the ultimate divisions of a bipinnate leaf. 
Pistil. In angiosperms, a carpel or an organization of two or more carpels. 
Pith. The central portion of a siphonostelic stem, generally consisting of parenchyma. 
Placenta. The place within an ovary to which the ovules are attached. 
Plasma membrane. A thin membrane that encloses the cytoplasm of all living cells. 
Plasmodium. A mass of naked, multinucleate protoplasm usuallj^ showing amoeboid 

movement. 
Plastid. An organized protoplasmic body, other than the nucleus, occurring in some 

plant cells and concerned with some special metabolic activity. 
Plerome. A group of embryonic cells, in a root tip or stem tip, that gives rise to the 

stele. 
Plumule. The primary bud of the embryo of a seed plant. 
Polar nucleus. One of two free nuclei in an embryo sac, one coming from each pole, 

that eventually unite to form the fusion nucleus. 
Pollen chamber. A cavity developed at the apex of the nucellus, as in cycads. 
Pollen sac. One of the pollen-containing cavities in an anther. 



440 PLANT MORPHOLOGY 

Pollen tube. The tube produced by a pollen grain when it germinates. 

Pollination. In angiosperms, the transfer of pollen from a stamen to a pistil or, in 
gymnosperms, to an ovule. 

PoUinium. A mass of coherent pollen grains, as in milkweeds and orchids. 

Polycotyledonous. Having many cotyledons, at least more than two. 

Polyembryonous. Having several embryos in the same seed. 

Polymerous. Having many floral parts in each set. 

Polyphyletic. Derived from more than one ancestral stock. 

Polyploid. Having several or many times the basic or haploid chromosome number. 

Polysiphonous. Consisting of several or many united filaments, as the plant body of 
certain brown and red algae. 

Primary xylem. Wood differentiated directly from a terminal meristem and consist- 
ing of protoxylem and metaxylem. 

Primordium. A rudiment or first-formed part of an organ or member. 

Procarp. The female sex organ of the red algae, consisting of a carpogonium and a 
trichogyne. 

Proembryo. In pteridophytes and spermatophytes, a group of cells derived from the 
fertilized egg and later differentiating into a suspensor and embryo. 

Prothallium. The reduced, thalloid gametophyte of the pteridophytes. 

Protonema. The primary or filamentous stage in the development of a moss gameto- 
phyte. 

Protostele. A solid stele, without a central pith. 

Protoxylem. The first xylem to be formed in an organ. 

Pseudopodium. A temporary foot-like protrusion of an amoeboid cell, used in loco- 
motion and in the procuring of food. In Sphagnum and Andreaea, a leafless 
stalk developed from the stem of the gametophyte in which the foot of the sporo- 
phyte is embedded. 

Pyrenoid. A starch-forming body in the chloroplast of many algae. 

Pyriform. Pear-shaped. 

Rachis. In divided ("compound") leaves, the extension of the petiole bearing the 
leaflets. 

Radial. Occupying alternate radii, as the arrangement of primary xylem and phloem 
in a root. 

Radicle. The rudimentary root in the embryo of a seed plant. 

Ramentum. A woolly covering of scales on the stem, as in many ferns. 

Raphe. In anatropous ovules, the portion of the funiculus that is united to the integu- 
ment, forming a ridge along the body of the ovule. In diatoms, a longitudinal 
slit extending along the median line of a valve. 

Receptacle. In liverworts, a special branch or portion of the thallus that bears the 
sex organs. In angiosperms, the portion of the axis that bears the floral parts. 

Regular flower. One in which the members of each whorl of perianth parts are 
uniform in size and form. 

Reticulate. Forming or resembling a network. 

Rhizoid. In the lower plants, one of the root-like filaments that attaches the game- 
tophyte to the substratum. 

Rhizome. An elongated underground stem; a rootstock. 

Rhizophore. A special root-bearing organ. 

Rootcap. A cellular sheath at the end of a root tip. 

Saprophyte. A plant living on dead organic matter. 

Scalariform. Ladder-like; having bars or markings like the rungs of a ladder. 



GLOSSARY 441 

Sclerenchyma. A hard tissue with thickened, generally lignified cell walls and non- 
conductive in function. 

Sclerotium. In myxomycetes, a resting stage in which the Plasmodium becomes a 
hard waxy mass. In certain higher fungi, a hard compact mass of mycelium con- 
taining reserve food material. 

Secondaiy xylem. Wood formed by a cambium. 

Sepal. One of the leaf-like parts of the calyx. 

Septate. Divided by walls or partitions. 

Sessile. Without a stalk; attached directly by the base. 

Seta. A slender, bristle-like organ or part, especially the stalk that supports the cap- 
sule of bryophytes. 

Sieve tube. An elongated living cell in which at least the end walls are perforated, 
characteristic of phloem tissue. 

Siphonostele. A tubular stele containing a central core of pith. 

Soredium. A special reproductive body of lichens, consisting of a few algal cells sur- 
rounded by fungal hyphae. 

Sorus. A group or cluster of sporangia, as in ferns. 

Spatulate. Spoon-shaped; gradually narrowed downward from a rounded apex. 

Sperm. A male gamete, generally motile by means of cilia. 

Spermatium. In red algae, a naked, nonmotile male gamete; in certain fungi and 
lichens, a cell apparently functioning as a male gamete. 

Spermatogenous. Sperm-producing. 

Spermogonium. In certain fungi and lichens, a flask-shaped, sunken receptacle in 
which spermatia are produced. 

Spike. An inflorescence consisting of sessile flowers borne on a common elongated 
axis. 

Spirillum. A bacterium that is curved or spiral in form. 

Spongy tissue. The loose parenchyma in a leaf, forming all the mesophyll except the 
palisade tissue. 

Sporangiophore. A stalk or branch bearing one or more sporangia. 

Sporangium. A cell or organ containing spores. 

Spore. A reproductive body, typically unicellular, capable of direct development into 
a new individual. 

Sporocarp. A special structure enclosing the sporangia in the water ferns. 

Sporogenous. Capable of producing spores. 

Sporophyll. A spore-bearing leaf, usually more or less modified in form and structure. 

Sporophyte. In plants with an alternation of generations, the individual that bears 
spores and has the diploid number of chromosomes. 

Stamen. The microsporophyll or pollen-bearing organ of seed plants. 

Stele. The central cylinder in the roots and stems of vascular plants, containing the 
vascular tissues. 

Sterigma. A slender tip arising from a basidium and bearing a basidiospore. 

Stigma. The portion of a pistil that receives pollen grains and on which they ger- 
minate. 

Stipule. One of a pair of appendages borne at the base of some leaves. 

Stoma. A small mouth-like opening in the epidermis of a leaf or young stem, bounded 
by a pair of guard cells, and through which gases pass. 

Strobilus. A compact group of sporophylls borne on a more or less elongated axis; 

a cone. 
Stroma. A compact, cushion-like mass of mycelium on or in which perithecia are 

borne. 
Style. The usually attenuated portion of a pistil situated above the ovary. 



442 PLANT MORPHOLOGY 

Suspensor. In many pteri(lo})hytes and spermatophytes, an appendage to the 
embryo, both derived from the zygote. 

Symbiosis. The living together, in intimate association, of two dissimihir organisms 
to their nnitiial advantage. 

Sympetalous. Having petals more or less united. 

Sympodium. An apparent main axis composed of successive secondary axes, each rep- 
resenting one fork of a dichotomy, the other being weaker or entirely suppressed. 

Synangium. A group of sporangia developed as a single structure, as in some ferns. 

Syncarpous. With united carpels. 

Synergid. In angiosperms, one of a pair of cells accompanying the egg and with it 
forming the egg apparatus. 

Tapetum. In pteridophytes and spermatophytes, one or several layers of nutritive 
cells investing the sporogenous tissue in a young sporangium. 

Teliospore. In the rust fungi, a thick-walled spore, commonly bilocular and stalked, 
that produces a basidium when it germinates, generally after a period of dor- 
mancy. 

Telium. A group of teliospores. 

Tetracyclic. Having four cycles or whorls of floral parts. 

Tetrad. A group of four cells formed by two successive divisions of a spore mother 
cell. 

Tetramerous. Having the floral parts in sets of four or multiples of four. 

Tetraspore. In certain red algae, one of the nonmotile spores produced in a group of 
four in a sporangium. 

Thallus. A vegetative body without differentiation into true roots, stems, and leaves. 

Trabecula. A plate of cells bridging an intercellular space or extending partially or 
completely across the cavity of a sporangium. 

Tracheid. A slender, elongated, nonliving, water-conducting cell with tapered, closed 
ends and thickened, lignified walls. 

Trichogyne. In the red algae, a thread-like extension of the carpogonium; in the 
ascomycetes, a tubular outgrowth of the ascogonium. 

Trilocular. Having three locules or cavities. 

Trimerous. Having the floral parts in sets of three or multiples of three. 

Triploid. Having three times the basic or haploid chromosome ninnber. 

Tunica. The outer growth zone in a stem tip, consisting of one or several superficial 
layers of embryonic cells. 

Undulate. Having a wavy surface or margin. 
Unicellular. One-celled. 
Uniciliate. Having a single cilium. 
Unilocular. Having a single locule or cavity. 
Uredinium. A group of uredospores. 

Uredospore. In the rust fungi, a unicellular spore, commonly borne singly on a stalk, 
and generally capable of producing a mycelium at once. 

Vacuole. A cavity or vesicle in the protoplasm of a cell, containing a watery fluid, the 

cell sap. 
Vallecular canal. In the Equisetinae, one of the canals in the cortex of the stem and 

lying beneath a groove on the surface. 
Vascular bundle. A strand composed primarily of vascular tissue traversing some 

part of the plant. 



GLOSSARY 443 

Velum. A nienil)ranou.s partition or oovoring resembling a veil or curtain, especially 

the membrane covering the gills in an immature mushroom. 
Venation. The arrangement or system of veins or vascular bundles in a leaf blade. 
Venter. The enlarged basal portion of an archegonium enclosing the egg. 
Ventral. Pertaining to the front or inner side of an organ; designating the surface 

toward the axis, as the upper side of a leaf; in liverworts and ferns, pertaining 

to the lower side of the prothallium. 
Vermiform. Worm-like. 

Vernation. The arrangement of foliage leaves within a bud. 
Vessel. A water-conveying duct composed of a row of thick-walled, nonliving cells 

that have lost their end walls; one of the cells forming such a duct. 
Volva. A cup at the base of the stipe in certain mushrooms. 

Whorl. A circle of similar parts about the same point on an axis. 

Xanthophyll. Any of a group of mainly light yellow pigments found in various plants, 
eitlier alone or associated with others, particularly chlorophyll; they differ from 
carotins in containing oxygen as well as carbon and hydrogen. 

Xylem. In vascular plants, a complex tissue consisting of tracheids, vessels, or both, 
and often also of wood fibers and parenchyma, and serving for the conduction of 
water and for mechanical support; woody tissue. 

Zoospore. A naked spore with cilia, by the vibration of which it moves. 
Zygomorphic. Having bilateral symmetry, the parts arranged symmetrically on 

either side of a median longitudinal axis. 
Zygote. A cell formed by the fusion of two gametes. 



INDEX 



Numbers in boldface type indicate pages on which illustrations appear 



Abietaceae, 340 
Acetabularin, 57 
Achlya, 113, 114 
Acorus, vascular bundle, 372 
Acrasieae, 107 
Adaea, root, 366 
Actinostele, 220 
Adianfum, leaflet, 285 
Adoxa, embryo sac, 388, 389 
Aeciospore, 144 
Aeeium, 144 
Aethalivun, 105 
Agapanthus, embryo, 396 
Agaricaceae, 149 
Agaricus, 148, 149 
Air chamber, 162, 163 
Air pore, 162, 163 
Akinete, 12, 19 
Albugo, 114, 115 
Algae, 7 

alternation of generations, 98 

asexual reproduction, 96 

bkie-green, 8 

brown, 66 

classes, 7 

comparison, 93 

evolutionary tendencies, 415 

general conclusions, 94 

golden-brown, 16, 17 

green, 25 

interrelationships, 98 

multicellular bodies, 95 

red, 85 

sexual reproduction, 96 

yellow-green, 18 
Allium, embryo sac, 387, 388 

root tip, 365 
Alternation of generations, 42, 422 

Algae, 98 

Bryophyta, 160 

Cladophora, 55 

Cutleria, 70 

Dictyota, 72 

Ectocarpus, 68 



Alternation of generations, heteromor- 
phic, 70 

isomorphic, 42 

Laminaria, 76 

origin, 423 

Polysiphonia, 92 

Pteridophyta, 209 

Ulva, 42 
Amanita, 149 
Amphigastrium, 183, 184 
Amphipleura, 22 
Amphithecium, 169, 171, 181, 189, 196, 

202 
Anabaena, 10, 12 
Andreaea, 198 
Andreaeales, 197 

gametophyte, 197 

sex organs, 197 

sporophyte, 197 

summary, 198 
Androspore, 47 
Anemia, sporangium, 287 
Anemone, development of embryo sac, 
386, 386 

megasporogenesis, 385 

ovule, 385, 386 
Angiopteris, 271 

antheridium, 274 

archegonium, 275 

leaflet, 272 

sporangium, 272 
Angiospermae, 362 

apomixis, 397 

chief orders, 399 

embryo, 394 

endosperm, 393 

female gametophyte, 384 

fertilization, 391 

flower, 372 

male gametophyte, 389 

subclasses, 362 

summary, 409 

vegetative organs, 363 
Annulus, Agaricus, 148 

fern sporangium, 284, 286, 287 

moss capsule, 204 



445 



446 

Anther, 378 
lorhronta, 380 
Liiiuin, 379 
Trillium, 373 
Anthcridium, Albugo, 115 
Angiopteris, 274 
AnthoceroH, 188 
Azolla, 303 
Batrachospermuni. 90 
Botrychium, 268, 269 
Bryales, 201 
Bryophyta, 160 
C/iora, 65, 66 
Claviceps, 135 
Coleochaete, 43 
Cutleria, 71 
Didyota, 74 
Equisetum, 256 
Filicales, 289 
Fwras, 82, 83 
Isoetes, 245 
Laminaria, 77 
Lycopodium, 226 228 
Marchantin, 165, 167 
Marsilea, 297 
Milium, 201 
Monoblepharis, 111 
Nemalion, 88 
Nephrolepis, 290 
Oedogonium, 47 
Ophioglossum, 268, 269 
Pe?/m, 180 
Perisporiales, 128 
Pohjsiphonia, 91 
PoreMa, 184 
Porphyra, 86 
Psilotum, 216 
Pyronema, 130 
Saprolegnia, 113 
Selaginella, 236 
Sphaerocarpus, 175 
Sphaeroplea, 56 
Sphaerotheca, 129 
Sphagnum, 193 
Vaucheria, 58, 59 
Zonaria, 75 
Anthoceros, 187 
antheridivim, 188 
archegonium, 188 
embryo, 189 
gametophyte, 187 
plan of, 207 
sporophyte, 187, 190 
Anthocerotales, 186 
gametophyte, 186 
sex organs, 187 
sporophyte, 189 



PLANT MORPHOLOGY 



Anthocerotales, summary, 190 
Antibiotics, 125 
Antipodal cell, 386, 390 
Apical cell, Anthocerotales, 187 
Bryales, 199 
Chara, 64 
Didyota, 73 
Equisetum, 252 
Filicales, 278 
Fossombronia, 178 
Fucus, 18 
Jungermanniales, acrogynae, 183 

anacrogynae, 178 
Marchantiales, 162 
Ophioglossales, 263 
Porella, 184 
Psilotales, 215 
Pfem, 279 
Reboulia, 162 
Selaginella, 231 
Sphacelaria, 70 
Sphaerocarpales, 174 
Sphagnales, 194 
Aplanospore, 19 
Apogamy, 398 
Apomixis, 397 
Apophysis, 203, 204 
Apothecium, 130 
Helvetia, 133 
Morchella, 133 
Peziza, 132 
Physcia, 156 
Sderotinia, 133 
Apple scab, 137 
Arales, 407 
Aravicariaceae, 340 
Archaeocalamites, 258, 259 
Archegoniatae, 2 
Archegonium, Angiopteris, 276 
Anthoceros, 188 
AzoWo, 303 
Bryales, 201 
Bryophyta, 161 
Z)7oon, 327 
Dryopteris, 291 
Ephedra, 359 
Equisetum. 256 
Filicales, 289 
Ginkgo, 338 
Isoetes, 246 

Lycopodium, 226, 227, 229 
Marchantia, 166, 168 
Marsilea, 298 
Mnium, 202 
Ophioglossum, 269 
Pe/Zm, 181 
FwMS, 347 



INDEX 



447 



Archegonium, Porella, 184 

PsilotHw. 216 

Selaginella. 237 

Sphaerocarpus. 176 

Sphagniivi. 195 

Zamin, 326 
Archichlamydeae, 362, 399 
Arcyria, 105 
Aristolochiales. 400 
Armillaria, 150 
Ascocarp, 121 

Aspergillus, 126 

Claviceps. 136 

Helvella, 133 

Mtcrosphacra. 128 

Morchella, 133 

A'ectria, 135 

Pezi'za. 132 

Physcia, 156 

Plowrightia, 138 

Pyronema, 130 

Sclerotinia, 133 

Ti/ber, 134 

Veniuria, 138 

Xylarm, 138 
Ascogonium, Claviceps, 135 

Laboulbeniales, 140 

Lichenes, 155 

Neurospora, 139 

Pyronema, 130 

Sphaerotheca. 129 

Fe«/;*/-m, 138 
Ascogynous hyphae, Claviceps, 136 

Lichenes, 155 

Plectascales, 126 

Pyronema, 130 

Feniwrm, 137 
Ascomycetes, orders, 121 

summary, 157 
Ascospore, 121 

Aspergillus, 126 

Claviceps, 135 

Microsphaera. 128 

Neurospora, 139 

Peziza, 132 

Pyronema, 130 

Schizosaccharomyces, 123 

Taphrina, 124 

Venturia, 138 

Xylaria, 139 
Ascus, 121 

Aspergillus, 126 

Claviceps, 136 

Microsphaera, 128 

Nectria, 135 

origin, 130, 131 

Peziza, 132 



Asciis, Physcia, 156 

Plowrightia, 138 

Pyronema, 130 

Sphaerolheca, 129 

Taphrina, 124 

Venturia, 138 

Xylaria, 139 
Aspergillus, 125, 126 
Asterella, 163, 165, 166, 168, 169 
Asteroxylon, 212, 213 
Aulocodiscus, 22 
Aurirularia, 146 
Auriculariales, 146 
Autotrophic, definition, 7, 103 
Auxospore, 24 
A2o//a, 299 

antheridium, 303 

gametophytes, 303 

massula, 303 

sporangia, 301 

sporocarps, 301 
Azotobacter, 102 

B 

Bacillariophyceae, 21, 22 

cell structure, 23 

orders, 23 

reproduction, 24 

summary, 94 
Bacillus, 101 
Bacteria, 100 

aerobic, 102 

anaerobic, 102 

autotrophic, 103 

denitrifying, 103 

nitrifying, 103 

nitrogen-fixing, 102 
Basidiocarp, 140 

Agaricus, 148 

Auriculariales, 146 

Clavaria, 147 

Gasteromycetales, 151 

Hvmenomvcetales, 147 

Phallus, 152 

Tremellales, 146 
Basidiomj^cetes, orders, 140 

summary, 157 
Basidiospore, 140 

Coprinus, 149 

Gasteromycetales, 151 

Hymenomycetales, 147 

Puccinia, 144 

Ustilago, 141 
Basidium, 140 

Auriculariales, 146 

Coprinus, 149 



•448 



FLANT MORPHOLOGY 



Basidiuni, dovclopmont, 160 

ExobasidiaU's, 1 47 

Gasteromycetales, 151 

Ilymonoinycotalcs, 147 

I'uccinia, 144 

Tremellales, 146 

l^stilafp, 141 
Batrachospcrtmun, 89, 90 
Beggiatoa, 101, 103 
Bennottitales, 3 1 5 

sporophyte, 315 

strobilus, 316 

summary, 318 
Black knot, 136. 137 
Botrychium, 262, 263 

antheridium, 268, 269 

archegonium, 268, 269 

embryo, 269 

gametophyte, 268 

petiole, 266 

rhizome, 266 

root, 264 
Botrydium, 19, 20 
Branch trace, 212, 222 
Bread mold, 117 
Bryales, 198 

gametophyte, 199 

sex organs, 200 

sporophyte, 201 

summary, 204 
Bryophyta, classes, 160 
comparison, 205 

evolutionary tendencies, 416 

gametophyte, 206 

general conclusions, 205 

sporophyte, 206 
Bryopsis, 60 
Budding, 122 
Bulbochaeie, 48 

C 

Calamitales, cones, 258 

Calamites, 267 

Calamophyton, 248 

Calyptra, 161, 168, 171, 172, 290, 292 

Calyptrogen, 364, 366 

Calyx, 373 

Campanulales, 406 

Capillitium, 105, 106, 152 

Capsella, development of embryo, 394, 

396 
floral development, 377 
Capsule, 161, 169, 170, 172, 177, 182, 186, 

194, 203, 204 
Carboniferous swamp forest, 239, 257, 

312 
Carinal canal, 252, 253 



Carotin, 19 
Carpel, 309, 381 
Carpogonium, 87 

Batrachosprnn ii tn , 90 

Netnalion, 88 

Polysiphonia, 92 

Porphyra, 86 
Carpospore, 87 

Batrachospenmim, 90 

Netnalion, 88 

Polysiphonia, 91 

Porphyra, 86 
Caulerpa, 60, 61 
Cell division, Anabaena, 10 

Oedogonium, 43, 44 

Surirella, 24 
Centrospermales, 400 
Ceratium, 18 
Chaetophora, 39 
Chalaza, 383 
Chamaesiphon, 12 
Chara, 64, 65, 66 
Charophyceae, 63 

reproduction, 64 

summary, 66, 94 

vegetative body, 63 
Chlamydomonas, 26, 27 
Chlamydospore, 141 
Chlorella, 32 
Chlorochromonas, 19 
Chlorococcales, 32 

summary, 38 
Chlorococcum, 32, 33 
Chlorophyceae, 25 

orders, 26 

summary, 61, 94 
Chromulina, 16, 17 
Chroococcus, 9 
Chrysamoeba, 16, 17 
Chrysophyceae, 16, 17 

summary, 93 
Chytridiales, 108 

summary, 1 1 1 
Chytridium, 108, 109 
Cilium, 14 
Cladophora, 54, 56 
Cladothrix, 101 
Classification, Embryophyta, 2-4 

plants, 1 

Thallophyta, 2, 3 
Clavaria, 147 
Clavariaceae, 147 
Claviceps, 135, 136 
Cleistothecium, Aspergillus, 126 

]\I icrosphaera, 128 
Closterium, 48, 49 
Clostridium, 102 



INDEX 



449 



Cocconei.'^. 25 
C odium, 59 
Codonothera, 313 
Coelosphaeriiini, 9 
Coenocyte, 20, 34, 8(), 54, 57 
Coenoptoridalcs, 260 
Colaciiim, 15 
Coleochaete, 42, 43, 44 
Coleus, stem tij), 368 
Columella, Anthoceros, 189 

Funaria, 203 

Rhizopns, 117, 118, 119 

Sphayniini, 190 
Conceptacle, 82 
Conducting tissues, 369 
Cone (see Strobilus) 
Conidiophore, 125 

Aspergillus, 125, 126 

Claviceps, 135 

Erysiphe, 127 

Neciria, 135 

Penicillium, 125, 126 

Plowrightia, 136 

Sclerotinia, 132 

Venturia, 137, 138 

Xylaria, 139 
Conidium, 124, 125 

Aspergillus, 125 

Claviceps, 135 

Erysiphe, 127 

Neciria, 135 

Neurospora, 139 

Penicillium, 125 

Plowrightia, 136 

Sclerotinia, 132 

Taphrina, 124 

Venturia, 138 

Xylaria, 139 
Coniferales, 340 

embryo, 349 

families, 340 

female gametophyte, 346 

male gametophyte, 348 

ovulate strobilus, 345 

sporophyte, 342 

staminate strobilus, 345 

summary, 353 
Conjugales, 48 

summary, 54 
Conjugation, 27 

Closterium. 49 

Cocconeis, 25 

Mougeotia, 50 

Rhizopus, 118 

Schizosaccharomyces, 123 

Spirogyra. 51, 53 

Zygnema, 54 



Conorephahim, 103, 105, 100, 109 
Contractile vacuole, 15 
Coprinus, 149 
Corddianihus, 333 
Cordaital(>s, 330 

gametophytes, 332 

sporophyte, 330 

strobili, 331 

summary, 333 
Cordaites, 332 
Cormophyta, 2 
Corn smut, 141 
Corolla, 373 
Corpus, 307 
Cortex, 211 
Cosmarium, 50 
Cotyledon, 327, 353 
Craterutn, 105 
Crossotheca, 313 
Cryptogam, 1 
Cryptogamia, 1 
Cryptomitrium, embryo, 170 
Cryptophyceae, 17 
Cupressaceae, 340 
Cupressus, 343 
Cutleria, 70, 71 
Cutleriales, 70 

summary, 72 
Cyanophyceae, 8 

branching, 12 

cell structure, 10 

distribution and habitat, 8 

false branching, 13 

orders, 8 

plant body, 9 

reproduction, 11 

resting spores, 12 

simple colonial, 9 

summary, 13, 93 
Cyanophycin, 10 
Cyatheaceae, 277 
Cycadales, 319 

embryo, 326 

female gametophyte, 323 

male gametophyte, 325 

ovulate strobilus, 323 

sporophyte, 319 

staminate strobilus, 321 

summary, 328 
Cycadeoidea, stem, 315 

strobilus, 316-318 
Cycadofilicales, 311, 312 

gametophytes, 313 

megasporangium, 311 

microsporangium, 311, 313 

sporophyte, 311 

summary, 314 



450 



PLANT MORPHOLOGY 



Cycas, female cone, 322 

mega.sporophvlls, 325 

microsporopliylls, 323 

vernation, 320 
Cyrtomium, sporangium, 288 
Cyst, 15, 16, 57 
Cystocarp, 88 

Nemalion, 88 

Polysiphonia, 91 

D 

Danaea, 272-274 

Dendroceros, 186, 187, 190 
Dennstaedtia, stele, 281 
Dermatogen, 864, 365, 394, 395 
Dermocarpa, 12 
Desmids, 48, 49, 50 
Diachea, 105 
Dianthus, pistil, 382 
Diatoms {see Bacillariophyceae) 
Dicksonia, 277 
Dicksoniaceae, 277 
Dicotyledoneae, 362, 399 
Dictyostele, 211, 283 
Dictyostelium. 107 
Diciyota, 72, 73, 74 
Dictyotales, 72 

summary, 76 
Didymium, 104 
Dinobryon, 16, 17 
Dinodadium, 18 
Dinoflagellate, 18 
Dinophyceae, 18 

summary, 93 
Dinothrix, 18 
Dioecious, 30 
Dioon, 319 

archegonium, 327 

male gametophyte, 328 

ovule, 329 
Docidiuin, 50 
Dorycordaites, 331 
Dothideales, 139 
Draparnaldia, 39, 40 
Dryopteris, archegonium, 291 

leaflet, 285 

sperm, 290 



E 



Ebenales, 404 

Ectocarpales, 67 
summary, 68 

Eclocarpus, 67 

Egg, 29 

Anemone, 386 
Angiopteris, 276 



Egg, Anthoceros, 188 
Chara, 66 
Coleorhaete, 43, 44 
Cutlcria, 71 
Didyota, 74 
Dioon, 329 

DryopUris, 291 

Equisetum, 266 

Eiidorind, 30 
Fritilhiria, 390, 392 
Fucus, 83 

IsoeU'x, 246 

Laminurid, 77 

Lycopodiutn, 229 

]\Iarchariti(i. 168 

Marnilea. 298 

Mnium, 202 

Monohlephari-s, 111 

Oedoyonium, 47 

Ophioglossiuii. 269 

Pe//m, 181 

PiAiws, 347, 350 

Pore/to, 184 

Saprolegnia, 113 

Sphaerocarpus, 176 

Sphaeroplea, 56 

Sphagnum. 196 

Vaucheria, 58 

Volvox, 31 
Egregia, 79 
Elater, 171, 172, 190 
Elaterophore, 181, 182 
Embryo, Angiospermae, 394 

Anthoceros, 189 

Botrychiuni. 269 

Capsella, 395 

Coniferales, 349 

Cryptomitrium, 170 

Cycadales, 326 

Equisetales, 256 

Filicales, 290 

Fossomhronin, 182 

Funarid, 203 

Gm^-£/o, 339 

Ginkgoales, 339 

Gnetales, 360 

Hydropteridales, 297, 302 

Isoetales, 247 

Isoetes, 246 

Lycopodiales, 229 

Lycopodium, 227, 230 

^iarattiales, 274 

Marchantiu, 166, 171 

Marsilea, 298 

Megaceros, 189 

Ophioglossales, 269 

Pmws, 351, 353 



INDEX 



451 



Embryo, Polypodiaceae, 292 

Psilotales, 217 

Riccia, 169 

Sagittaria, 397 

Sekujinella, 238 

Selaginellales, 237 

Sphagnjuu. 196 

Zamia, 330 
Embryo sac, 384 

ylnemone, 385, 386 

Fritillaria, 390 

ordinary type, 388 

variation in development, 386, 388 
Embrj'ophyta, 2 

classification, 2-4 
Empusa, 120, 121 
Endarch, 210 
Endodermis, 21 1 
Endosperm, 327, 353, 393 
Endospore, 12, 102 
Endothecium, 169, 171, 181, 189, 196, 

202, 379, 380, 381 
Entomophthorales, 120 
Ephedra, 354, 355 

archegonium, 359 

female gametophyte, 359 

male gametophyte, 360 

ovule, 359 

proemhryo, 361 

strobili, 355 
Epidermis, 211 
Epigyny, 375, 376 
Equisetales, 250 

gametophyte, 254 

sporangium, 253 

sporophyte, 250 

summary, 256 

vascular anatomy, 252 
Equisetinae, orders, 247 

summar}', 304 
Equisetum, 250, 251 

embryo, 256 

gametophyte, 256 

sporangium, 254, 255 

stem structure, 253 
Ergot, 135 
Ergotine, 135 
Ericales, 404 

Erigeron, floral development, 378 
Erysiphe, 127 
Etapteris, 261 
Eudorina, 28, 29 
Euglena, 14 
Euglenophyceae, cell structure, 14 

relationshifjs, 15 

reproduction, 15 

summary, 16, 93 



Eusporangiato, 226 
Evolution, i)lant kingdom, 412 

sex, 417 
organs, 419 
Evolutionary^ tendencies, algae, 415 

bryophytes, 416 

prominent, 415 

pteridophytes, 416 

spermatophytes, 417 
Exarch, 210 
Exohasidiales, 146 
Exohasidium, 146 
Eyespot, 14, 26, 27, 39 



Fagales, 399 

Families, Coniferales, 340 

Filicales, 276 

Gasteromycetales, 151 

Hymenomycetales, 147 
Farinales, 407 
Fertile spike, 265 
Fertilization, 30 

Angiospermae, 391 

Fritillaria, 392 

Finns, 350 
Fiber, 210, 369 
Filicales, embryo, 290 

families, 276 

gametophyte, 287 

sorus, 284 

sporangium, 286 

sporophyte, 278 

summary, 291 

vascular anatomy, 278 
Filicinae, orders, 260 

summary, 304 
Fission, 11 
Flagellates, 14 
Flagellum, 14 
Floral development, 376 

Capsella. 377 

Erigeron, 378 

Ranunculus, 377 
Flower, 372, 410 

actinomorphic, 374 

apetalous, 373 

apocarpous, 375, 381 

carpel, 381 

choripetalous, 375 

establishment of whorls and definite 
numbers, 374 

hypogyny, perigyny, and epigyny, 375 

Magnolia, 374 

naked, 374 

ovule, 383 



452 



PLANT MORPHOLOGY 



Flower, pcntacyclic, 375 

perianth, 373 

stamen, 378 

sj'inpetalous, 375 

syncarpous, 375, 381 

tctraeyclio, 375 

Trillium, 373 

zonal development, 375 

zygomorphic, 374 
Foot, 161, 169, 171, 172, 230, 238, 246, 

292 
Fossomhronia, apical cell, 178 

embryo, 182 
Fritillaria, embryo sac, 388, 389, 390 

endosperm, 393 

fertilization, 392 
Frond, 278 
Fruit, 398 
Fucales, 80 

summary, 84 
Fucoxanthin, 66 
Fucus, 80, 81-83 
Fuligo, 105 
Funaria, 200, 203 
Fungi, 100 

alga-like, 108 

bird's-nest, 152 

classes, 100 

comparison, 156 

club, 140 

coral, 147 

ear, 146 

gametic reproduction, 158 

general conclusions, 157 

gill, 149 

Imperfecti, 153 

pore, 147 

sac, 121 

spore reproduction, 158 

stinkhorn, 152 

tooth, 147 

trembling, 146 

vegetative body, 157 
Funiculus, 383 

G 

Gametangium, Brijopsis, 60 

C odium, 59 

Edocarpus, 67 

Pylaiella, 68 
Gametes, 27 

differentiation, 418 
Gametophore, 199 
Gametophyte, 42 

Andreaea, 198 

Andreaeales, 197 



Gametophyte, Anthorero.s, 187 
Anthocerotales, 186 
Botri/chiujn, 268 
Bryales, 199 
Bryophyta, 206 
Culler id, 71 
Cycadofilicales, 313 
Equisetales, 254 
Equifielum, 256 
female, Angiospermae, 384 

Azolla, 303 

Coniferales, 346 

Cordaitales, 332 

Cycadales, 323 

Ephedra, 369 

Ginkgoales, 338 

Gnetales, 356 

Isoetes, 246 

Marsilea, 298 

Pinus, 347 

Selaginella, 237 

Zamia, 326 
Filicales, 287 
Isoetales, 245 
Jungermanniales, acrogynae, 183 

anacrogynae, 178 
Laminaria, 77 
Lycopodiales, 226 
Lycopodium, 226, 227 
male, Angiospermae, 389 

Azolla, 303 

Coniferales, 348 

Cordaitales, 332 

Cycadales, 325 

Dioon, 328 

Ephedra, 360 

Ginkgoales, 339 

Gnetales, 358 

Isoetes, 245 

Marsilea, 297 

Pinus, 348 

Selaginella, 236 
Marattiales, 273 
Marchantia, 164 
Marchantiales, 162 
Marsileaceae, 296 
Ophioglossales, 267 
Ophioglossum, 268 
Pallavicinia, 179 
Pellia, 179 
Polypodiaceae, 289 
Porella, 184 
Psilotales, 216, 217 
Fsilotum, 216 
Pteridophyta, 306 
Riccia, 162 
Salviniaceae, 301 



INDEX 



453 



Gametophyte, Solagincllales, 234 

Sphaerocarpales, 1 73 

Sphaerocarpiis, 174 

Sphagnalos, 192 

Sphagnum, 193, 194 

Symphyogyna, 179 
Gap, 212 

Gasteromycetales, families, 151 
Gemma, 164, 179, 183, 199 
Gentianales, 405 
Geologic eras, 5, 6 
Geologic periods, later, 6, 310 
Geologic time, division, 5, 6 
Geothallus, 173, 174 
Geraniales, 402 
Gill, 148, 149 
Ginkgo, 333. 334 

archegoniuni, 338 

embryo, 339 

leaf blade, 335 

megaspores, 337 

ovulate strobili, 337 

ovule, 337 

staminate strobili, 336 

stem, 335 
Ginkgoales, 333 

embryo, 339 

female gametophyte, 338 

male gametophyte, 339 

ovulate strobilus, 337 

sporophyte, 334 

staminate strobilus, 336 

summary, 340 
Girdle, 21 
Gleba, 151 
Gleichenia. 276 

sporangium, 287 

stele, 280 
Gleicheniaceae, 277 
Glochidium, 310, 303 
Gloeocapsa, 9 
Gloeodinium, 18 
Gloeotrichia, 12 
Glumales, 407 
Glycogen, 10 
Gnetales, 354 

embryo, 360 

female gametophyte, 356 

male gametophyte, 358 

ovulate strobilus, 356 

sporophyte, 354 

staminate strobilus, 355 

summary, 360 
Gnetiim, 354 

strobili, 357 
Gonimoblast, 88, 90 
Gonium, 28 



Gullet, 15 
Gymnospermae, 301 

geologic distribution, 310 

orders, 31 1 

summary, 409 

H 

Haematochrome, 28 
Haplostele, 220 
Haustorium, Albugo, 115 

Erysiphe, 127 

Rhizopus, 118 
Helminthoslachys, 262, 264-267, 269, 270 
Helobiales, 406 
Helvella, 133, 134 
Helvellales, 133 
Hemitrichia, 105 
Hepaticae, orders, 161 

summary, 205 
Heterocyst, 11, 12 
Heterogamy, 30 
Heterospory, 209, 233, 306 
Heterothallic, 120 
Heterotrophic, 7, 100 
Histogen, 364, 365 
Homologous structures, 414 
Homospory, 161, 209 
Homothallic, 120 
Hormogonium, 11 
Hornea, 214 
Horsetails, 250 
Hydnaceae, 147 
Hydrodictyon, 34, 36 
Hydropteridales, families, 292 

summary, 302 

{See also Marsileaceae; Salviniaceae) 
Hydrurus, 16, 17 
Hyenia, 248 
Hyeniales, 247 
Hymenium, 124 

Agaricaceae, 149 

Auricular iales, 146 

Claviceps, 135 

Gasteromycetales, 151 

Hymenomycetales, 147 

Peziza, 132 

Physcia, 156 

Ploiorightia, 136 

Pyronema, 130 

Taphrina, 124 

Tremellales, 146 

Xylaria, 139 
Hymenogastraceae, 151 
Hymenomycetales, families, 147 
Hymenophyllaceae, 277 
Hynienophyllum, sporangium, 287 



454 



PLANT MORPHOLOGY 



Hypha, 108 
Hypocot.yl, 327, 353 
Hypocrcalos. \'M) 
Hyposyny, 375, 376 
Hypophysis, 395 



Indusium, 284, 285, 295, 301 

false, 285 
Integument, 312, 323, 326, 347, 359, 386 

Interrelationships, Algae, 98 

Pteridophyta, 307 

Spermatophyta, 410 
lochroma, microsporangiiim, 380 
Isoetales, 241 

embryo, 247 

gametophytes, 245 

sporangia, 243 

sporophyte, 241 

summary, 248 

vascular anatomy, 242 
Isoetes, 241 

embryo, 246 

female gametophyte, 246 

male gametophyt(>, 245 

megasporangium, 244 

microsporangium, 244, 245 

sporangia, 244 

stele, 243 

stem tip, 242 
Isogamete, 27 

Acetabularia, 57 

Caulerpa, 61 

Chlamydomonas, 27 

Chlorococcum, 32 

Cladophora, 55 

Desmids, 49 

Edocarpus, 67 

Gonium, 26 

Hijdrodictyon, 36 

Olpidium, 109 

Pandorina, 29 

Pediastrum, 36 

Plasmodiophora, 112 

Protnsiphon, 37 

Rhizopus, 118 

Spirogyra, 51 
Synchytrium, 1 11 
Ulothrix, 38 
Ulva, 41 
Zygnema, 54 
Isogamy, 27 
Isthmia, 22 



Juglandales, 399 
Jungermanniales, 177 



Jungormanniales, Acrogynae, 183 

gametophyte, 183 

sex organs, 184 

sporophyte, 185 
Anacrogynae, 177 

gametophyte, 178, 179 

sex organs, 179 

sporophyte, 180 
summary, 185 



K 



Kelps, 76 
other, 79 



Laboulbeniales, 140 
Labyrinthuleae, 108 
Laminaria, 76, 77 
Laminariales, 76 

summary, 79 
Land habit, establishment, 206 
Leaf, gap, 212, 260 

structure, angiosperms, 363 
Pinus, 343 

trace, 212, 222 
Lepidodendrales, 238 
Lepidodendron, 240 
Lepidostrobus. 239 
Leptosporangiate, 286 
Leucosin, 16 
Lichenes, 153, 154, 155, 156 

crustose, 153, 154 

foliose, 153, 154 

fruticose, 153, 154 
Life cycles, types, 422 
Ligule, Isoetes, 242, 244 

SelagineUa, 231, 233, 234, 238 
Liliales, 408 
Lilium, anther, 379 

pollen grain, 391 
Liverworts, 161 
Lunularia, 164 
Lycogala, 105 
Lycoperdaceae, 152 
Lycopodiales, 218 

embryo, 229 

gametophyte, 226 

sporangium, 223 

sporophyte, 219 

summary, 229 

vascular anatomy, 220 
Lycopodiinae, orders, 218 

summary, 304 
Lycopodium, 218, 223, 224 

anther idium, 228 



INDEX 



455 



Lycopodhim, archegoniuni, 229 

embryo, 230 

gametophyto, 226, 227 

sporangium, 225 

stele, 221, 222 

stem tip, 219 
Lycopsida, 3. 4, 218 
Lyginopteris, 312-314 
Lynybya, 11 

M 

Macrocystis, 78 

Macrozamia, megasporophyll, 326 

Magnolia, flower, 374 

stem, 370 
Malvales, 403 
Marattia, leaflet, 272 

root, 271 
Marattiales, 270 

embryo, 274 

gametophyte, 273 

sporangium, 272 

sporophyte, 270 

summary, 274 

vascular anatomy, 270 
Marchantia, air chamber, 163 

air pore, 163 

antheridium, 165, 167 

archegoniuni, 166, 168 

cupule, 164 

embryo, 166, 171 

female receptacle, 166 

gemma, 164 

male and female plants, 164 

male receptacle, 165 

sperm, 165 

sporophyte. 172 

thallus, 163 
Marchantiales, 161 

gametophyte, 162 

sex organs, 164 

sporophyte, 168 

suinmary, 172 
Marsilea, 293 

female gametophyte, 298 

male gametophyte, 297 

rhizome, 294 

sporangia, 295 

sporocarp, 296 
Marsileaceae, 293 

embryo, 297 

gametophytes, 296 

sporocarp, 294 

sporophyte, 293 
Massula, 301, 303 
Megaceros, 189, 190 



Megasporangium, 233 

Anentoiic, 385 

Azolla, 301 

Cycadolilicalo.s, 311 

Frit ilia rid. 390 

Isoetes. 244 

Marsilca, 295 

Salix, 384 

Salvinia, 302 

Selaginella, 233, 235 
Megaspore, 233 

Anemone, 385 

Azolla, 301 

Ginkgo, 337 

Isoetes, 244 

Marsilea, 298 

Pinus, 348 

Salvinia, 302 

Selaginella, 233, 235 
Megasporocarp, 300 

Azolla, 301 

Salvinia, 302 
Megasporophyll. 233 

Cycadales, 325 

Isoetes, 244 

Selaginella, 233 
Merismopedia, 9 
Mesarch, 210 

Metachlamydeae, 316, 404 
Metaxylem, 210 
Micrasterias, 50 
Micropyle, 312, 323, 326, 386 
Microsphaera, 127, 128 
Microsporangium, 233 

Angiospermae, 378 

Azolla, 301 

Bennettitales, 317 

Cycadofilicales, 311, 313 

Ginkgoales, 336 

Gnetales, 355 

lochroma, 380 

Isoetes. 244 

Lilium, 379 

Marsilea, 295 

Pinus, 345 

SalviTiia, 302 

Selaginella, 233-235 

Zamia, 324 
Microspore, 233 

Azolla. 303 

Dioon, 328 

Isoetes, 244 

Marsilea, 297 

Pinus, 348 

Selaginella, 233, 235 
Microsporocarp, 300 

Azolla, 301 



LIBRaR 

^ ^ V MASS. 



456 



PLANT MORPHOLOGY 



Microsporophyll, 233 

Cycadeoidea, 317 

Cycas, 323 

Ginkgo. 336 

Isoetet;, 244 

Pimis, 345 

SelagineUa, 233 
Mildews, downy, 114 

other, 116 

powdery, 127 
Mnium, 199 

antheridium, 201 

archegonium, 202 

capsule, 204 
Monoblepharidales, 111 
Monoblepharis. Ill 
Monocotyledoneae, 316, 406 
Monosiphonoiis, 67 
Monospore, 89 
Morchella, 133 
Mosses, 192 

plan of, 207 
Mougeotia, 60, 51 
Mucorales, 117 

summary, 120 
Multicellular bodies, development of, 95 
Musci, orders, 192 

summary, 205 
Mushroom, 149 
Mycelium, 108 
Mycophyta, 2, 3, 7 
Myrtales, 403 
Myxamoeba, 106 
Myxobacteria, 103 
Myxomycetes, plant body, 104 

reproduction, 105 

summary, 106, 156 



N 



Navicula, 22 

Nedria, 134, 135 

Nemalion, 87, 88 

Nereocystis, 79 

Neuropteris. 239, 312 

Nenrospora, 139 

Nidulariaceae, 152 

Nitrifying bacteria, 103 

Nitrobacter, 103 

Nitrogen-fixing bacteria, 102 

Nitrosomonos, 113 

Nosioc, 9 

Notoihylas, 187, 189, 190 

Nucellus, 312, 313, 323, 326, 347, 383 

Nucleus, fusion, 384, 386 

generative, 348, 391 

male, 350, 391, 392 



Nucleus, polar, 384, 390 
primary endosperm, 392 
tube, 328, 348, 350, 391 

O 

Oedogoniales, 45 

summary, 48 
Oedogonium, 45, 46-48 
Oenothera, eml)ryo sac, 387, 388 
Olpidium, 109 
Ontogeny, 1, 414 
Oogonium, Albugo, 115 

Chara, 65, 66 

Coleochaete, 44 

Cutleria, 71 

Dictyota, 73 

Fucus, 82, 83 

Laminaria, 77 

Monoblepharis, 111 

Oedogonium, 47, 48 

Saprolegnia, 113 

Sphaeroplea, 55 

Vaucheria, 58, 59 

Zonaria, 75 
Ooplasm, 115, 116 
Operculum, Bryales, 203, 204 
Ophioglossales, 261 

embryo, 269 

gametophyte, 267 

sporangium, 265 

sporophyte, 262 

summary, 270 

vascular anatomy, 263 
Ophioglossum, 262 

antheridium, 268, 269 

archegonium, 269 

gametophyte, 268 

root, 264 

sporangium, 267 
Opuntiales, 403 
Orchidales, 408 
Oscillatoria, 9 
Osmunda, sporangium, 287 

stele, 282 
Osmundaceaea, 276 
Ovary, 362, 373, 381 

inferior, 376 

superior, 376 
Ovule, 309, 312, 383 

anatropous, 383 

Anemone, 385, 386 

campylotropous, 383 

Cycadeoidea, 318 

development, 383 

Dioon, 329 

direction, 383 



INDEX 



457 



Ovule, Ephedra. 359 

Fritillaria, 390 

Lyginopteris, 314 

orthotropoup, 383 

Pinus, 345, 346 

Welwitschia, 358 

Zarnia, 326 
Ovuliferous scale, 345, 346 



Palaeostachya, 258, 259 
Pallavicinia, thallus, 179 
Palmales, 407 
Palmella, 26, 27, 32, 39 
Pandanales, 406 
Pandorina, 28, 29 
Papaverales, 401 
Parallel development, 413 
Paramylon, 15 
Paraphysis, Bryales, 200 
Claviceps, 135, 136 
Cutleria, 72 
Fucus, 82, 83 
Laminaria, 77 
Nedria, 135 
Physcia, 156 
Plowrightia, 136, 138 
Pyronerna, 130 
Zonaria, 76 
Parasite, 100 
Parietales, 403 
Parthenocarpy, 398 
Parthenogenesis, 114, 397 
Pediastrum, 33, 34, 35 
Pelargonium, pistil, 382 
Pellia, antheridium, 180 
archegonium, 181 
capsule, 182 
thallus, 179 
PelUcle, 14 

Penaea, embryo sac, 387, 388 
Penicillium, 124, 125 
Peperomia, embryo sac, 387, 388 
Perianth, 373 

Periblem, 364, 365, 394, 396 
Pericarp, 398 
Perichaetium, 200 
Peridium, 151 
Perigyny, 375, 376 
Periplasm, 116 
Perisperm, 393 
Perisporiales, 127 
Peristome, 203, 204 
Perithecium, 135 
Claviceps, 136 
Laboulbeniales, 140 



Perithecium, A^edria, 135 
Neurospora, 139 
Plowrightia, 138 
Venturia, 138 
Xylaria, 139 
Peronosporales, 114 

summary, 117 
Petal, 373' 
Peziza, 132 
Pezizales, 129 

summary, 133 
Phaeocystis, 16 
Phaeophyceae, 66 
orders, 67 
summary, 85, 94 
Phaeothamnion, 16, 17 
Phallaceae, 152 
Phallus, 152 
Phanerogam, 1 
Phanerogamia, 1 
Phellogen, 369 
Phloem, 210 

elements, 369 
Phycocyanin, 8 
Phycoerythrin, 8, 85 
Phycomycetes, orders, 108 
summary, 157 
Phycophyta, 2, 3, 7 
Phylladinia, 127 
Phylloglossum, 219, 220, 226 
Phylogeny, 1, 144 
Physcia, 155, 156 
Phytophthora, 116 
Pileus, 148, 149 
Pilobohis, 120 
Pilularia, 293, 294 
Pinna, 278 
Pinnidaria, 21 
Pinnule, 278 
Pinus, archegonium, 347 
embryo, 351, 353 
female gametophyte, 347 
fertilization, 350 
leaf, 343 

male gametophyte, 348 
ovulate strobilus, 341 
ovule, 346, 347 
staminate strobilus, 345 
stem, 344 
Piperales, 399 
Pistil, 373, 382 
Placenta, 383 
Plant life of the past, 4 
Plantaginales, 405 
Plasmodiophora, 112 
Plasmodiophorales, 112 
Plasmodium, 104, 254, 267, 287, 295, 381 



458 



PLANT MORPHOLOGY 



Plasnwpnrn, 1 Hi, 117 
Ploc'tasc-alos, 124 
Plectostele, 220 
Plorome, 304, 365, 394, 395 
Plexrosigtua, 22 
Plowrightia, 136, 137, 138 
Plumhagella, embrj'^o sac, 388, 389 
Plumbago, embryo sac, 388, 389 
Plumule, 327, 353 
Podocarpaceae, 340 
Podosphaera, 127, 128 
Pollen, grain, Dioon, 328, 329 
Ephedra. 360 
Li Hum, 371, 391 
Pinus, 348 

sac, 379 

tube, Angiospermae, 391 
Dioon, 328, 329 
Ephedra. 360 
Pinus, 348, 349 
Pollinium, 381 
Polyembryony, 352 
Polygonales, 400 
Polypodiaceae, 278 
Polypodium, leaflet, 285 
Polyporaceae, 147 
Polysiphonia, 89, 91, 92 
Polysiphonoiis, definition, 67 
Polijtrichum. 199, 203 
Porella, 184, 186 
Porphyra. 86 
Porphyridium, 87 
Postelsia, 80 
Primulales, 404 
Procarp, Batrachospermum, 90 

Nemalion, 88 

Polysiphonia, 92 
Proembryo, 326 

Capsella, 395 

Ephedra, 361 

Ginkgo, 339 

Pinus, 351 

Sagittaria, 397 

Zamia, 330 
Propagule, 69 

Prothallium (see Gametophyte) 
Protoascales, 122 
Protococcus, 40, 41 
Protodiscales, 123 
Protonema, 161 

moss, 199 
Protosiphon, 36, 37 
Protostele, 211, 280 
Protoxylem, 210 
Pseudopodium, 196, 198 
Psilophy tales, 212, 213 



Psilophytinae, orders, 212 

siuninary, '.\Q'.\ 
Psilophylon, 212 
Psilopsida, 3, 4, 212 
Psilotales, 214 

embryo, 217 

gametophyte, 217 

sporangium, 216 

sporophyte, 215 

summary, 217 

vascular anatomy, 215 
PsHotum, 214 216, 2 17 
Pteridium, leaflet, 285 

rhizome, 283 

vascular bundle, 284 
Pteridophyta, classes, 209, 260 
comparison, 303 

evolutionary tendencies, 416 

gametophyte, 306 

general conclusions, 304 

interrelationships, 307 

sporophyte, 304 

strobilus, 305 

vascular system, 210 
Pteris, root tip, 279 
Pteropsida, 4, 260 
Puccinia, 142-144 
Puff ban, 152 
Pylaiella, 68 
Pyrenoid, 26 
Pyrenomycetales, 134 

summary, 139 
Pyronema, 129, 130, 131 
Pythium, 116 



R 



Radicle, 327, 353 

Ranales, 401 

Ranunculus, floral development, 377 

Raphe, 23 

Reboulia, 165, 166 

air chambers, 162 

air pore, 162 

apical cell, 162 

embryo, 169 

thallus, 162 
Recapitulation, 414 
Regnellidium, 293 
Reproduction, asexual, 417 
in algae, 96 
in fungi, 158 

sexual, 418 
in algae, 96 
in fungi, 158 
Reservoir, 15 
Rhamnales, 403 



INDEX 



459 



Rhizohium, 102 
Rhizoid, 163 

Rhizophore, 281, 240, 243 
Rhizopus, 117, 118, 119 
Rhodophyceae, orders, 85 

summary, 93, 94 
Rhynia, 213 
Riccardia, capsule, 182 
Riccia, gametophyte, 162 

sporophyte, 170 
Riella, 173, 174 
Rivularia, 12 
Root, mature, 365, 366 

tip, Allium. 365 
Pteris, 279 

trace, 212, 222 
Rootcap, 365 
Resales, 402 
Rubiales, 406 
Rusts, 142 

autoecious, 146 

heteroecious and other, 145 

S 

Saccharomyces, 122 

Sagittaria, development of embryo, 396, 

397 
Salicales, 399 
Salix, megasporangium, 384 

root, 367 
Salvinia, 299 

sporangia, 302 

sporocarp, 302 
Salviniaceae, 299 

embryo, 302 

gametophytes, 301 

sporo carps, 300 

sporophyte, 299 
Santalales, 400 
Sapindales, 402 
SaprohgnJa, 113 
Saprolegniales, 113 

summary, 114 
Saprophyte, 100 
Sarcina, 101 
Sargassum, 84 
Sarraceniales, 402 
Scenedes7niis, 33, 34 
Schizaeaceae, 276 
Schizomycetes, 100 

activities, 102 

structure and reproduction, 100 

summary, 103, 156 
Schizosaccharomyrea, 123 
Scitaminales, 408 
Sclerodermaceae, 152 
Sclerotinia, 132 



Sclerotium, 135 
Claviceps, 136 
Myxomycetes, 105 
Xylaria, 139 
Scytonenia, 12, 13 
Sedum, carpels, 382 
Seed, 353, 409 
Seedling, 398 
Selaginella, 230 
embryo, 238 

female gametophyte, 237 
male gametophyte, 236 
megasporangium, 233, 235 
microsporangium, 233-235 
stele, 232 
stem tip, 231 
Selaginellales, 230 
embryo, 237 
gametophytes, 234 
sporangia, 231 
sporophyte, 231 
summary, 237 
vascular anatomy, 231 
Sepal, 373 
Seta, 161, 169, 172 
Sex, determination, 175 
evolution, 417 
organs, Andreaeales, 197 
Anthocero tales, 187 
Aspergillus, 126 
Bryales, 200 
Claviceps, 135 
evolution, 149 
Jungermanniales, acrogynae, 184 

anacrogynae, 179 
Laboulbeniales, 140 
Lichenes, 155 
Marchantiales, 164 
Perisporiales, 128 
Plectascales, 127 
Pyronema, 130 
Sphaerocarpales, 174 
Sphaerotheca, 129 
Sphagnales, 193 
Venturia, 137 
origin, 418 
significance, 422 
Sexuality, further expressions of, 421 
Sieve tube, 210, 369 
Sigillaria, 239 
Silphium, endosperm, 393 
Siphonales, 57 
summary, 61 
Siphonocladiales, 54 

summary, 57 
Siphonostele, amphiphloic, 211, 281 
ectophloic, 211, 282 



460 



PLANT MORFHOLOGY 



Slimo fungi, other, 106 
Slime molds, 104 
Solanum, pistil, 382 
Soredium, 154 
Sorus, Amjiopteris, 272 

Filicales, 284, 285 

gradate, 28(i 

Marsilea, 295, 296 

mixed, 287 

simple, 285 
Specialization, 413 
Sperm, Chnra, 65 

Coleochnete, 43 

Cutler ia, 71 

Didyota, 74 

Dioon, 329 

Dryoptcris, 290 

Equisetum, 256 

Sudor iv a. 30 

f MCMS, 83 

Isoetes, 246 

Laminar ia, 77 

Lycopodium, 288 

Marchantia, 165 

Marsilea, 297 

Monoblepharis, 111 

Oedogoniiwi, 47 

Ophioglosstnn, 269 

Psilotum, 216 

Selaginella, 236 

Sphaeroplea. 56 

Vaucheria, 58 

Fo/toj, 31 

Zaviia, 328 
Spermatium, Batrachospermum, 90 

Laboulbeniales, 140 

Lichenes, 155 

Nernalion, 88 

Polysiphonia, 91 

Porphyra, 86 

Puccinia, 144 
Spermatophyta, classes, 309 
comparison, 408 

evolutionary tendencies, 417 

flower, 410 

general conclusions, 409 

interrelationships, 410 

seed, 409 
Spermogonium, 143, 144, 155 
Sphacelaria, 69, 70 
Sphacelariales, summary, 69 
Sphaerella, 28 
Sphaeriales, 140 
Sphaerocarpales, gametophyte, 173 

sex determination, 175 

sex organs, 174 

sporophyte, 174 



Sphaerocarpales, summary, 176 
Sphuerocarpua, antlicritlium, 175 

archegoiiium, 176 

sporophyte, 177 
Sphaeroplea, 56 
Sphaerolhera, 127-129 
Sphagnales, 192 

gametophyte, 192 

sex organs, 193 

sporophyte, 194 

summary, 195 
Sphagnum, antheridhim, 193 

archegonium, 195 

embryo, 196 

gametophyte, 193, 194 

leaf, 194 

sporophyte, 194 
Sphenophyllales, 248 
Sphenophyllum, 249, 250, 257 
Sphenopsida, 4, 247 
Spirillum, 101 
Spirogyra, 51-53 
Sporangiophore, Albugo, 115 

Calamitales, 258 

Empusa, 121 

Equisetum, 254 

Hyeniales, 248 

Plasmopara, 117 

Psilotales, 216 

Rhizopus, 118 

Sphenophyllales, 249 
Sporangium, Albugo, 115 

Angiopteris, 273 

Azolla, 301 

Chytridium, 109 

Cladophora, 55 

Cutleria, 71 

Cyrtomium, 288 

Didyota, 74 

Edocarpus, 67 

Einpusa, 121 

Equisetales, 253 

Equisetum, 254, 255 

Filicales, 286 

Hyeniales, 248 

Isoetales, 243 

Laniinaria, 77 

Leptosporangiate ferns, 287 

Lycopodiales, 223 

Lycopodium, 225 

Marattiales, 270 

Marsilea, 295 

Monoblepharis, 111 

Olpidium, 109 

Ophioglossales, 265 

Ophioglossum, 267 

Plasmopara, 117 



INDEX 



461 



Sporangium, Polypodiaceao, 286 
Psilopliy talcs, '21;? 
Psilotalcs, 216 
Psilotum, 214 
Pylaiella, 68 
Rhizopus, 118, 119 
Salvinia, 302 
Saproleynia, 113 
Selagiiiellales, 281 
Splioiiophyllalcs, 248, 249 
Synchytrium, 110 
Tmesipteris, 214 
Vaucheria, 58 
Zonaria, 75 
Spore, mature, Dictyota, 74 

Equiseiuni, 254 

Marchantia, 172 

Polysiphonia, 91 

i2;'rr/a, 170 

Zonaria, 75 
mother cell, 169 

yl«e/HO«e, 385 

Atithoceros, 191 

Cyrtomium, 288 

Fossoinhronia, 182 

Fritillaria, 390 

Isoetes, 245 

Lycopodiutii, 225 

Marchantia, 172 

Ophioglossuni, 267 

PmMS, 346 

Po;e//a, 186 

Riccia, 170 

SelagineUa, 235 

Sphaerocarpus, 177 
tetrad, 169 

Anemone, 385 

Anthoceros, 191 

Ginkgo, 337 

Marchantia, 172 

Riccia, 170 
Sporocarp, 294 
A2o?/a, 301 
Marsilea, 295, 296 
Marsileaceae, 294 
Salviniaceae, 300 
Sporophyll, 223 
Lycopodium, 223 
Sphenophylhon, 250 
Tmesipteris, 214 
Sporophyte, 42 
Andreaea, 198 
Andreaeales, 197 
Angiospermae, 363 
Anthoceros, 187, 191 
Anthooerotales, 189 
Bennettitales, 315 



Sporophyte, Bryalcs, 201 

Bryopliyta, 206 

Coniforales, 342 

Cordaitales, 330 

Cutleria, 71 

Cycadales, 319 

Cycadofilicalcs, 311 

Equisetales, 250 

Filicales, 276 

Funaria, 200 

Ginkgoales, 334 

Gnetales, 354 

Gymnospermae, 309 

independent, 304 

Isoetales, 241 

Jungermanniales, acrogynae, 185 
anacrogynae, 180 

Lycopodiales, 219 

Marattiales, 270 

Marchantia, 172 

Marchantiales, 168 

Marsileaceae, 293 

Ophioglossales, 262 

Pore//a, 184, 186 

Psilotales, 215 

Pteridophyta, 304 

Riccia, 170 

Salviniaceae, 299 

Selaginellales, 231 

Spermatophyta, 309 

Sphaerocarpales, 174 

Sphaerocarpus, 177 

Sphagnales, 192 

Sphagnum, 194 
Sporophytic budding, 398 
Stamen, '309, 373, 378 
Staurastruni, 50 
Stele, 211 

Aciaea, 366 

Botrychium, 264, 265 

Dennstaedtia, 281 

Equisetum, 253 

Ginkgo, 335 

Gleichenia. 280 

/soe/es, 243 

Lepidodendron, 240 

Lycopodium, 221, 222 

Lyginopteris, 313 

Magnolia, 370 

Marattia, 271 

Marsilea, 294 

Ophioglossum, 264 

Osmunda, 282 

Pi?i»s, 344 

Psilotum, 215 

Pteridium, 283 

SelagineUa, 232 



462 



PLANT MORPHOLOGY 



Stele, types. 211. 279, 370 
Zaviia, 321 
Zea. 371 
Stem tip. 367 
Coleus, 368 
Equisetiim. 252 
Isoetes, 242 
Lycopodiam, 219 
Selaginella. 231 
Siemonitis, 105, 106 
Sterigma, 143, 149 
Stigeoclonium, 39 
Stigma, 382 
Stigonema, 13 
Stone\\-orts, &3 
Streptococcus, 101 
Strobilus, 224 

Bennettitales. 315 
Calamitales. 258, 259 
Equisetintu 254, 256 
Hyeniales. 248 
Lycopodium, 224 
ovulate, Coniferales, 345 
Cordaianthus. 333 
Cordaitales. 332 
Cycadales. 323 
Cycas, 322 
Dioon, 319 
Ephedra. 355 
Ginkgo, 337 
Ginkgoales. 337 
Gnetales, 356 
Gnetuin, 357 
PiniiS, 341 
Welivitschia, 358 
Zawn'a, 322 
Phylloglossum, 220 
Pteridophyta. 305 
Selaginella. 233 
Sphenophyllales, 249 
staminate, Coniferales, 345 
Cordaianthus. 333 
Cordaitales, 331 
Cycadales. 321 
Ephedra. 355 
Gj«A-sfo. 336 
Ginkgoales, 336 
Gnetales, 355 
Gnetum. 357 
Pinus, 345 
]re?u'i7.sc/i;'a. 358 
Za»wa, 322 
Stroma, 134 

Claviceps, 135. 136 
.Vedrm, 134, 135 
Plowrightia, 136, 138 
Xylaria, 139 



Style, 373, 382 
Surirella, 22 

cell division. 24 
Suspensor, 229 

Capsella. 395 

Ginkgo, 339 

Lycopodiuiii. 230 

Pinus, 351 

Sagittaria, 397 

Selaginella. 238 

Zamia, 330 
Syniphyogyna. thallus, 179 
Synangium, 273 

Danaea. 272 

Marattia, 272 
Synchytrium. 109, 110 
Synergid, 385. 386, 390 
Synura, 16, 17 
Syrinya, leaf, 363 



Tapetum, 226 

Angiopteris. 273 

Bryales, 203 

Cyrtomium. 288 

Equisetum, 254. 256 

lochroma, 380 

Lilium, 379 

Lycopodium. 225, 226 

3/a/-sz7m, 295 

Salvinia, 302 

Selaginella, 232, 234 

Sphagnum, 196 

Zamm, 324 
Taphrina, 123, 124 
Taxaceae, 341 
Taxodiaceae. 340 
Teliospore. 143 
Telium. 143 
Tetraspore. 91 
Thallophyta, 2, 7 

classification, 2, 3 
Thallus, 7 

Thelephoraceae, 147 
Tmesipteris. 214, 216. 217, 218 
Tolypothrix, 12, 13 
Trabecula. 231, 244 
Trace, 212 

branch, 212, 222 
Tracheid, 210 
Tracheophyta, 2-4 
Tremella, 146 
Tremellales, 146 
Tribonema. 19, 20 
Triceratium, 22, 23 



INDEX 



463 



Trichogj^ne, 87 

Batrachospennum. 90 

Nemalion, 88 

Polysiphonia, 92 

Pyroneina. 130 

Venturia, 137 
Trillium, floral structure, 373 
THber, 134 
Tulierales, 134 
Tubiflorales, 405 
Tunica, 367 

U 

Ulothrix, 38 
Ulotrichales, 38 
summary, 44 
Ulva, 41, 42 
Umbellales, 404 
Uncinula, 127 
Uredinales, 142 
Uredinium, 142, 143 
Uredospore, 142, 143 
Uroglena, 16 
Urticales, 400 
Ustilaginales, 140 
Ustilago, 141 



Vallecular canal, 252, 253 
Vascular anatomy, Angiospermae, 363- 
372 

Coniferales, 344 

Cycadales, 320 

Equisetales, 252 

Filicales, 276 

Ginkgoales, 335 

Gnetales, 354 

Isoetales, 242 

Lycopodiales, 220 

Marattiales, 270 

Ophioglossales, 263 

Psilotales, 215 

Selaginellales, 231 
Vascular bundle, Acorns, 372 

Pteridium, 284 
Vascular system, development of xylem, 
210 

traces and gaps, 212 

types of steles, 211 
Vascular tissues, arrangement, 211 
Vaucheria, 57, 58, 59 
Vegetative body, algae, 95 

fungi, 157 
Vegetative organs (Angiospermae), 363 

leaf structure, 363 



Vegetative organs (Angiospermae), ma- 
ture root, 364, 366 

older stem, 368, 370 

root tip, 364, 365 

stelar types, 370 

stem tip, 367, 368 
Velum, Agarinis, 148, 149 

Isoetes, 243, 244 
Venturia, 137, 138 
Vernation, circinate, 278, 320 
Vessel, 210, 369 
Volva, 149 
Volvocales, 26 

summarv, 32 
Volvox, 30] 31 

W 

Welwitschia, 354. 356 

ovule, 358 

strobili, 358 
Wheat rust, 142 
Woodwardia, leaflet, 285 

X 

Xanthophyceae, 18 

summary, 94 
Xanthophyll, 19 
Xijlaria, 139 
Xylem, development, 210 

elements, 369 

primary, 211 

secondary, 211 



Zamia, embryo, 330 

female cone, 322 

leaflets, 320 

male cones, 322 

megasporophyll, 325 

microsporangium, 324 

ovule, 326 

sperm, 328 

stem, 321 
Zanardinia, 72 
Zea, stem, 371 
Zonal development, 375 
Zonaria, 75 
Zoospore, 19 

Chlamydonionas. 27 

Chlorococcmu, 33 

Chytridium. 108 

Cladophora. 55 

Coleochaete, 43 

Cutleria, 71 



464 



PLANT MORPHOLOGY 



Zoospore, Eciocarpus, 67 
Hydrodictj/on, 36 
Laminaria, 78 
]\fonobIephnns, 111 
Oedogoniion, 47 
Olpidium, 109 
Pediastruui, 35 
Plasmodiophora, 112 
Protosiphon, 37 
Saprolegnia, 113 
Synchytriuni, 111 
Ulothrix, 38 
C/Zwa, 41 
Vaucheria, 58 

Zygnema, 53, 54 



Zygote, Chlamydomonas, 27 

Closterium, 49 

Cocconeis, 25 

Coieochaete, 43, 44 

Hydrodictyon, 36 

Monohlephtin's, 111 

Mougeotia, 50 

Oedogonium, 47 

Pandorina, 29 

Protosiphon, 37 

Rhizopiis, 118 

Spirogyra, 51-53 

Ulothrix, 38 

Fo/z^oj:, 31 

Zygnema, 54 
Zymase, 123