The principles of botany, as exemplified in the phanerogamia

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THE

PRINCIPLES OF BOTANY,

AS EXEMPLIFIED IN THE

PHANEROGAMIA.

BY

HARLAND COULTAS.

Professor of General and Medical Botany in the Penn Medical University of
Philadelphia.

PHILADELPHIA:
KING & BAIRD, PRINTERS, No.9 SANSOM STREET.

1854.

S$

oe, Fo ae

Entered according to the Act of Congress, in the year 1854, by

HARLAND COULTAS,

In the Office of the Clerk of the District Court of the United States, in and for the
Eastern District of Pennsylvania.

IDEDICATED

TO

WILLIAM SCHMOELE.

DOCTOR OF PHILOSOPHY AND MEDICINE,
PROFESSOR OF GENERAL AND SPECIAL PATHOLOGY IN THE

PENN MEDICAL UNIVERSITY OF PHILADELPHIA.

CONTENTS.

Page
Inrropuctory Remarks. Phanerogamous and Cryptogamous plants.
Analogies between the organization of plants and that of animals. 9

PART III.

ON THE ORGANS OF NUTRITION IN PHANEROGAMOUS PLANTS....00000046 17

CHAPTER I.

THE EPIDERMIS AND ITS APPENDAGHS...0cecesscsees cesses cescescescesessees 18
® CHAPTER II.

THE DIFFERENT KINDS OF STEM.s.ccsccseese senses score cnseees dapavasetor sade 265

CHAPTER III.
ON THE ROOT OR SUBTERRANEAN APPENDAGES OF THE AXOPHYTE.... 32

CHAPTER IV.

THE ORGANIZATION OF THE STO Maicrcasancnsecsaccsaraceninceecne cancer neces 46
‘CHAPTER. V.

ON THE DEVELOPMENT OF THE BUDS AND BRANCHES....ceccse0e coseseee 64
CHAPTER VI. i

THE LEAVES .ceseescosenceeete sence coceee ag vakaanenradeat ieiasatauaiienion 72

CHAPTER VII.
ON THE NATURE AND SOURCES OF THE FOOD ASSIMILATED BY PLANTS. 89

6 CONTENTS.
oo

‘

PART IV.

ON THE ORGANS OF REPRODUCTION IN PHANEROGAMOUS PLANTS...... 107

Page

CHAPTER VIII.
GENEBAL CONSIDERATIONS ON THE FLOWER sssesecesssseeccenscoccscsscssees LOT

CHAPTER IX.

THE INFLORESCENCE sscsoesesscssse coscccesasscsececscscece cssserecosesseressees 114
CHAPTER X.
THE FLORAL ENVELOPES cosoicces sities er tssraeeawnereey saveversexcowerseereses 126

CHAPTER XI.
THE ANDRECIUM, OR STAMINAL ORGANS sesscseesccseescsecescsscesscereceee LOD

CHAPTER XII.
THE GYMNGCIUM, OR PISTILLINE ORGANS cossesscessecrscsesessccesessceeeee 148

CHAPTER XIII.
THE PROCESS OF FERTILIZATION OR FECUNDATIONs...0..Tesscscsssseesees LOL

CHAPTER XIV.
ON THE VARIOUS MODIFICATIONS OF THE REPRODUCTIVE ORGANS...... 173

CHAPTER XV.
THE FRUIT, OR MATURE OVARY...... ene Mieterveaevecudorveletareaeen LIZ

CHAPTER XVI.

THE STRUOTURE OF THE SEED... cscsecscecsccscees cesses sesonscossscessessess LOS

CHAPTER XVII.
ON THE DISPERSION AND GERMINATION OF SEED..s.cssscseseceeeeeeeee 220

PREFACE.

——~e—.

Turis volume renders complete the previously published
work of the author entitled, « Principles of Botany as exempli-
fied in the Cryptogamia.” In that work it was shown that
the same laws of nutrition and reproduction operate in the
vital economy of plants composed of a few cells as when
vegetation is constructed on a scale of gigantic magnitude and
grandeur. In this new volume on the Phanerogamia or
flowering plants, the author enters on the investigation of the
anatomy and functions of a more highly organized and complex
vegetation. He has examined with care, the writings of
Schleiden, Lindley, Balfour, Gray, Richard, and other eminent
botanists, but above all the Volume of Creation, of which every
other work is but an imperfect copy. The book is illustrated
with numerous engravings, and such scientific terms as it was
necessary to introduce into the text have been carefully
explained and their etymology given as soon as introduced.

The author takes this opportunity of gratefully acknow-
ledging ‘the kindness and liberality of the physicians of
Philadelphia, who have been and still are his principal

patrons.

8 PREFACE.

He has written this volume on the organography and
physiology of the Phanerogamia, in the hope that it will be
generally useful, but with an especial reference to the wants
of medical students and physicians: life probably exists
under the simplest and least complicated condition in plants.

The author ventures to hope that this volume, (which he
has been encouraged to prepare,) will be found equally as
interesting as the one already published, and contain such an
amount of original and well-selected matter as will render it
worthy of that liberal patronage which has been bestowed on

his previous labors.

INTRODUCTION.

PLANTS exercise, in common with animals, the two princi-
pal functions of organic life—nutrition and reproduction. All
the organs of the most complex, as well as of the most
simple plants, are developed for the purpose of carrying on
one or the other of these two functions.

The tissues which constitute the substance of both animals
and plants are formed from cells, and exhibit a most re-
markable accordance in their vital phenomena. In both,
peculiar secretions are carried on, which are restricted to cer-
tain parts of the organism ; whilst as life advances to the period
of its close, the walls of the fully developed cells become
thickened by the internal deposition of matter in layers.
Ossification in animals exactly corresponds to lignification in
plants.

Plants as well as animals reproduce themselves. Flower-
bearing plants when they arrive at an adult state develope male
and female organs, termed stamens and pistils. These mu-
tually operate in the formation of an embryo or seed, which
contains within its folds, in a rudimentary condition, all the
organs of the fully developed plant. These embryos are
formed in a particular organ termed an ovule, and are de-
veloped in consequence of imbibing the fecundating matter of
certain cells termed pollen. Thus, from the vital actions of

2

x INTRODUCTION.

plants, there may be much instruction derived, which will be
found a valuable contribution to our knowledge of the repro-
ductive function in more highly organized beings.

In the animal organism, the nutritive and reproductive func-
tions are greatly complicated by the presence of a nervous
system. In plants, these two grand functions of organic life
are carried on free from nervous influences, and therefore
under greatly simplified conditions. The careful study of
these functions, thas simplified in plants, ought therefore to
precede the investigation of their higher and more complicated
phenomena as manifested in animals.

It is undeniable that the plant takes precedence of the
animal in nature, being elaborated out of inorganic matter as
material for the subsistence of the animal. It would therefore
seem to be the most natural and philosophical mode of investi-
gating the phenomena of life, first of all, to see to what extent
its functions have been expressed in plants.

All organic matter appears to be only a manifestation of life
in different degrees of development, and a plant may be truly
regarded as the simplest manifestation of its functions.

In the author’s “ Principles of Botany, as exemplified in the
Cryptogamia,” it was shown in sections 50, 51, that the sim-
plest plant in nature is the plant cell, which «constitutes an
entire vegetable without organs, imbibing its food by endosmo-
sis through every part of its surface, which it converts into the
materials of its own enlargement and growth, and finally into
new cells, which constitute its progeny.” But as we advance
in the scale of organization, the cells thus generated do not
separate from the parent plant cell; on the contrary, they
remain united with it, to a greater or less extent, until we find
individual plants composed of a mass of such cells, all mutually

INTRODUCTION. xi

co-operating in carrying on the nutritive and reproductive
functions. It was also there proved (54-57), that “it is not
necessary for cell-development to be carried to any great extent
in order to constitute the fabric of a true and perfect plant ;”
on the contrary, the same laws of nutrition and reproduction
operate in the vital economy of plants composed of a few cells
as when vegetation is constructed on a scale of gigantic mag-
nitude and grandeur. In such plants, it is evident that we
have the phenomena of life existing under extremely simplified
conditions; and if ever ‘‘man, the minister and interpreter of
nature,” is destined to discover those morphological laws
which govern this evolution and endless repetition of the same
definite forms of vegetable and animal life from the same
embryos, it is here that he must commence his investigations.

Hitherto the attention of the student has been directed to
the consideration of cryptogamous vegetation, we are now
about to enter on the examination of vegetable life as unfolded
in the more complex and elaborate organization of the Phanero-
gamia, or flowering plants.

In the lower forms of the Cryptogamia the essential organs
of vegetation, the root, stem and leaves, are blended together
into a flat or filamentous expansion of vegetable matter,
termed a thallus, from whence these plants have received
the name of thallophytes (#aa205 a frond, gvcd» a plant.)
These plants have no vegetable axis or stem, and increase by
additions of matter to their periphery or circumference. They
have a tendency to grow in a horizontal rather than in a ver-
tical plane, their spores germinating indifferently in all direc-
tions from any part of their surface.

The cells which constitute the tissue of thallophytes, in the
lower forms of their development, appear to retain the form,

xii INTRODUCTION.

and to exercise the functions of the parent plant cell, with
which they remain permanently united. Thus, in the nume-
rous tribes of marine Cryptogamia or alge, the endochrome
(%v5ov within, and ypopa a color) is diffused through the entire
substance of the frond, so that the whole plant presents the
same color in all its parts, and the reproductive matter, or
sporules, makes its appearance in many species, indifferently
on any or every part of the plant.

In the Phanerogamia, or flowering plants, on the other hand,
root, stem and leaves are separate, well-defined organs.
From the first commencement of germination there is a stem
more or less manifest, and a tendency to develope in two oppo-
site directions, into the earth and atmosphere, the two grand
sources from whence these plants obtain the materials of their
growth or enlargement. To subserve the purposes of a higher
and more elaborate nutrition, certain cells of the parenchyma
are carried to a much higher degree of development, and
assume the form of woody fibre and spira) vessels.

The pleurenchyma of flowering plants (26) becomes more
distinctly marked in their leaves, as organization advances in
complexity of structure, until at length, in the most highly
organized plants, its fibres form a beautiful anastomosis of
veins, veinlets and capillaries. The leaves of Thalictrum
anemonoides, the rue-leaved anemone, an early and exceed-
ingly abundant spring flower, furnish an admirable illustration
of pleurenchyma thus ramified and attenuated.

But throughout organic nature, a change in the form of any
organ is always associated with a corresponding change in its
function. The secretion of the cells is therefore no longer
uniform, but varied and well defined in its character, certain

INTRODUCTION. xiii

peculiar secretions being restricted or confined to certain por-
tions of the organism.

The analogy between the vegetable and animal tissues is
beautifully apparent in the secretory action of the cells of pha-
nerogamous plants. The same endochrome, or coloring matter,
no longer gives an uniformity of hue to the tissues, but the
leaves which terminate the axis of growth become crowded
together into a beautiful rosette at its summit, and secrete a
variously colored endochrome, which has received the name of
chromule (xpopa color), in contradistinction to chlorophyl
(zarwpds green, gvarov leaf), which is the substance which gives
to the leaves their green hues.

But the possession of a terminal rosette of beautifully
colored leaves, popularly called the flower, ig by no means the
principal characteristic of Phanerogamous vegetation, since
in some flowering plants, as for instance in the grasses, these
colored investments become abortive and rudimentary. Yet
the organs essential to the formation of the embryo are there,
the stamens and pistils, and it is the presence of these bodies
which constitute the true flower.

The difference between phanerogamous and cryptogamous
plants consists in the possession by the former of stamens and
pistils, or true flowers, (of which the latter are wholly deprived,)
by the mutual action of which an embryo or seed is produced,
which is a much more highly organized body than the spore.
The spore from which every eryptogam is developed is commonly
a simple cell filled with organic matter, and the organs which
it developes in germination form themselves as they appear ;
but in the embryo or seed, these organs existed before, and are
only increased by the act of germination. The character of an
embryo in organic beings is, that it contains, in a rudimentary

ox

Xlv INTRODUCTION,

state, all the organs of which the organic being is composed
in its entire developments. Thus the animal embryo con-
sists of the head, the trunk, and the extremities,—in other
words, of all the parts of which the adult animal is composed.
In like manner, the embryo of a phanerogamous plant, of a
bean for example, discloses a plumule or young stem, a pair
of leaves or cotyledons, and a radicle or young root,—in other
words, the entire plant in a rudimentary condition; aud by the
act of germination, analogous in its effects to the commence-
ment of life in infancy, all the parts of the plant develope
themselves into their wonted figure and hues in accordance
with those generic and specific laws to which the plant is sub-
ject; but germination does not increase the number of these
parts, which existed before its influence was exercised on them.

\ *

FART ILL.

ON THE
COMPOUND ORGANS OF PLANTS.

THE PHANEROGAMIA, OR FLOWERING PLANTS.

PART IIT.

ON THE ORGANS OF NUTRITION IN PHANEROGAMOUS PLANTS.

VEGETATION, in the more highly organized and complex
forms which it assumes in flowering plants, consists essentially
of a continuous axis or trunk, which developes in two opposite
directions, and is more or less ramified at its two extremities.
The superior or ascending portion of this vegetable axis is
called the stem, the inferior or descending portion the root,
and the point of departure of either axis the collet or neck.
This neck is usually distinctly visible when the embryo plant
first rises from the ground ; after the cotyledons, or first pair
of young leaves, have developed, it disappears, and becomes a
merely imaginary line of separation between the base of the
stem and the root.

These two extremities of the vegetable axis are beautifully
adapted to the earth and atmosphere, the two grand sources of
all vegetable nutrition. The aerial portion of the plant is
provided with leaves, by which food is taken in from the
atmosphere, and also with flowers, which are the organs of
reproduction ; the subterranean portion is furnished with a
quantity of fibres or smaller roots, which make their appearance
in proportion to the requirements of the plant and the barren
or fertile nature of the soil in which it grows. This vegetable

18 COMPOUND ORGANS OF PLANTS.

axis, with its assemblage of nutritive and reproductive organs
at its two -extremities, has been very properly termed the
axophyte.

Before commencing our exposition of the anatomy and func-
tions of the fundamental organs of flowering plants, it is proper
to examine that peculiar investment which covers them, termed
the epidermis.

CHAPTER I.

ON THE EPIDERMIS AND ITS APPENDAGES.

Every part of a plant, as well as of an animal, with the
exception of the stigma or summit of the pistil and the
extremities of the roots, is covered by a thin membranaceous
investment, termed the epidermis. ~ :

Fig. 1.

Pellicle of cabbage, detached by maceration, covering the hairs, h, and having open-
ings, s, corresponding to the stomata.

The epidermis consists of two parts: Ist, an outward pel-
licle, (Fig. 1,) without appreciable organization called the
cuticle ; 2d, one or more strata of flattened tabular cells, which
are much larger than the cells of the subjacent tissue, consti-

THE EPIDERMIS AND ITS APPENDAGES. 19 ©

tuting the true epidermis or skin. These two superposed
membranes are intimately united and pierced by a number of
apertures, called stomata or pores.

The presence of the cuticle on the exterior surface of the
epidermis, may be detected by a simple chemical process. Ifa
transverse section of the epidermis be treated with a dilute
solution of iodine, the cells of the epidermis will remain colorless,
whilst the cuticle will assume a yellowish or brownish tinge.

Some writers consider the cuticle to be a mere secretion
from the epidermic cells on which it is deposited; but the
recent investigations of M. Gareau, a distinguished French
physiologist, who succeeded in effecting its quantitative analy-
sis, would seem to prove that it ig a distinct organ, formed
from cellulose of a special matter distinct from that which
constitutes the epidermis.

The cuticle is the only part of the epidermis which covers
the surface of the stem and leaves of aquatic plants. It is
developed in the form of a glaucous bloom or vegetable varnish,
which renders the surface of the plant a perfect water shed,
preventing it from obtaining an injurious amount of the fluid
in which it floats.

The epidermis (éi upon, and dépua skin). In flowering
plants, the epidermis may be readily perceived to be a mem-
brane perfectly distinct from the cellular and fibrous tissue
which it covers, on account of the magnitude and peculiar
arrangement of its cells. The epidermic cells contain ordi-
narily no traces of chlorophyl, and therefore the epidermis may
bereadily separated from the parenchymatous tissue, with which
it contracts but a feeble adhesion, as a colorless layer.

The epidermis of plants is clearly intended to guard their
subjacent vasculur and cellular systems from injury, to pro-

20 COMPOUND ORGANS OF PLANTS.

tect those systems with their fluid contents against changes in
the state of the atmosphere, and to control the evaporation
from their cells within proper bounds.

In the Lily and Balsam, which allow of ready evaporation,
the epidermis consists of a single layer of cells; but in plants
which inhabit dry situations, it is so constructed as to retard
evaporation, and either consists of several layers of cells, as in
the Oleander, (Fig. 2,) or else is of considerable thickness, as
in the Aloe and Cactus. By this provision these plants are
enabled to retain their moisture for a greater length of time.

Fig. 2.

Maenified perpendicular section of the leaf of the Oleander, showing the thickness
of the epidermis, which is composed of three layers of cells, and the compact vertical
cells of the upper stratum of parenchyma.

It must be evident that the exhalation of water from the
leaves is to a certain extent necessary, as it is the only means
by which the sap can be concentrated and rendered subservient
to the nutrition of the plant. Now so long as the roots can
absorb as much water as the leaves evaporate, the plant will
appear fresh and green, but the foliage droops (as is often secn
on a hot summer’s day) when the supply at the roots fails, and
there is too much evaporation from the leaves.

To remedy this defect, the epidermal surface of the leaves is
furnished with self-acting valves or openings, called stomata or
pores. These stomata are usually of an oval figure with a slit
in the middle, and are so situated as to open directly into the

THE EPIDERMIS AND ITS APPENDAGES. 21

hollow chambers or air-cavities in the lower stratum of paren-
chyma. The aqueous contents of the cells of the parenchyma,
throughout the whole interior of the leaf, are thus brought into
immediate contact with the external air, whilst at the same
time the evaporation of their contents is controlled and regu-
lated by these foliar apertures.

This is done in the following manner. The slit or perfora-
tion in the epidermal surface lies between two cells, which,
unlike the rest of the cuticular cells, generally contain some
chlorophyll, and in this respect resemble the parenchyma
beneath. These cells are exceedingly hygrometrical, or affected
by moisture. When the atmosphere is damp, these two cells
become swollen and turgid, and by their curvature outwardly,
open the orifice and allow the free escape of the superfluous
water; but when the atmosphere is dry, they straighten and
lie parallel, their sides being brought into close contact, thus
closing the aperture and stopping evaporation the moment
it becomes injurious to the plant. The stomata or pores of
plants are therefore analogous to the governor in machinery,
and are clearly designed to regulate the operation of the
vegetable mechanism, and to promote the healthy passage of
fluids through the system.

The structure of the stomata or pores of plants, may be
readily perceived on the epidermis of the lily, (Fig. 3,) where
they are unusually large. The epidermis must be carefully
removed, and having been freed from all its chlorophyll, or
green matter, it must be placed between two strips of glass
with a drop of water between them so as to give it the neces-
sary degree of transparency. Water ought, for this reason
always to be used whenever objects selected from the tissues
of vegetables are examined microscopically. The epidermis

3

ry

22 CUMPOUND ORGANS OF PLANTS.

thus prepared will exhibit these pores, and the nature and
beauty of their mechanism will be seen and appreciated.

The stomata are generally found on the under surface of the
leaves, the mechanism being too delicate to act well in direct.

Fig. 3.

Epidermis of the lily, showing the stomata sf, composed of two celis with an open-
ing or slit between them.
sunshine. They are invariably absent from the parts of plants
growing beneath the water. The water-lilies (Nuphar and
Nympheea,) and all plants whose leaves float on the water,
have the stomata on the upper surface of their leaves. If the
leaves of plants grow erect, the stomata are equally distributed
on both sides.

Stomata are more or less abundant on the cuticle of all
plants, and as these pores perform the functions of exhalation
in proportion to their number on different plants, it is neces-
sary to supply them with water. The plant called Hydrangea
quercifolia has on one square inch of its surface 160,000 pores,

THE EPIDERMIS AND ITS APPENDAGES. 23

and therefore requires a greater supply of water than plants
possessed of from 70 to 100 pores on the same superficies.

The rapidity with which plants wither and dry when not
watered, is exactly in proportion to the number of their exhaling
pores. Thus when a shower of rain occurs after long drought,
our readers must have witnessed that many plants revive long
before the moisture can have reached their roots. The only
absorbents in this case were the stomata on the epidermis.

Occurring on the surface of many plants are certain minute
expansions of the epidermal cells termed hairs. These consist
either of a single elongated cell, or of several cells, placed end
to end. Those hairs which are not connected with any
peculiar secretion, are termed lymphatic. Those, on the other
hand, which have cellules visibly distended at their base or
apex into receptaeles of some peculiar fluid, are termed
glandular.

Fig. 4.

Magnified view of one of the stinging hairs of the nettle with the gland at its base.

It is from these secreting hairs that the beautiful scent of
the sweet brier is derived, and the sting of the common nettle
(Fig. 4), is produced by an acrid fluid ejected through its
tubular hairs from the glandular receptacles at their base a.
Nettles have been very properly termed the serpents of the

24 COMPOUND ORGANS OF PLANTS.

vegetable world; and not without reason, for there is a
remarkable similarity in structure between the poison teeth of
the latter and the glandular hairs of the former. In both the
apparatus is tubular, and the pressure of the hair or tooth on
the poison gland ejects the poison into the system.

The poison of nettles in temperate climates is not of
much consequence, but as we approach warmer regions sting-
ing nettles become more numerous and deadly. «“ Every
person is acquainted with the sting of the common nettle,
Urtica urens, but no notion can be formed from it of the
torture which its allies, Urtica stimulans, Urtica crenulata,
produce in the East Indies. A gentle touch is sufficient to
make the limb swell up with the most fearful rapidity, and the
suffering lasts for weeks; nay, one species, growing in Timor,
Urtica urentissima, is called by the natives Daun setan,
Devil’s Leaf, because the pain lasts for years, and sometimes
death itself can only be avoided by the amputation of the
injured limb.”*

When the hairs of the epidermis are hardened by deposits,
as in the rose and blackberry, they are called prickles, (aculei).
In their youth, they completely resemble hairs, and are
dispersed without order on the stem and leaves, but with
age they become thickened, elongated and indurated, as may
be seen on the rose, where they present themselves in every
stage of development.

Hairs are sometimes attached to seeds for the purpose of
scattering them, as in the cotton plant. In Rhus cotinus, or
the wig tree, the flower stalks are changed into hairs.

* Dr. Schleiden.

THE DIFFERENT KINDS OF STEM. 25

CHAPTER IL.
THE DIFFERENT KINDS OF STEM.

Tue stem may be regarded as that portion of the axophyte
which is situated between its two extremities, and which catries
the leaves and the flowers.

The stem exists in all flowering plants, but sometimes, as in
Taraxacum dens leonis, the common dandelion, it is hardly
developed at all, so that the leaves and even the floral branches
appear to spring from the root. These plants were formerly
considered to be acaulescent (a without, caul’s a stem); they
have, however, a true stem, but it is so contracted in its growth
as to be hidden in the earth.

The common idea that all the subterranean parts of plants
are roots is quite erroneous. The production of buds and
leaves, and ihe presence of leaf scars, are the distinguishing
characteristics of the stem, and the following roots, so called,
which ‘exhibit these appearances, are only its subterranean
modifications.

The rhizoma, (jiSo, a root). This stem pursues an under-
ground course, growing horizontally at a depth in the soil
which is sufficient to protect the buds on its surface, from
which it sends forth annually herbaceous branches into the
air, which die down to the ground at the close of the flowering
season. In Polygonatum pubescens, (fig. 5,) the annual decay
of the foliage leaves on the rhizoma a broad and conspicuous
scar, which is not unlike the impression of a seal, and for this
reason the plant is commonly called Solomon’s seal.

3%

26 COMPOUND ORGANS OF PLANTS.

Rhizoma of Solomon’s Seal, (Polygonatum pubescens.)

The rhizoma or underground stem grows after the manner
of ordinary aerial stems by the development of both lateral and
terminal buds. In Polygonatum pubescens the development of
the subterranean buds is alone terminal, but in other perennial
herbaceous plants the lateral as well as terminal buds of the
rhizoma are developed, and the subterranean branches, which
are called suckers, send up from the soil aerial stems. These
suckers when severed from the parent stem and planted will
grow into new plants. This mode of multiplying plants is
often resorted to by gardeners, and is called by them, propa-
gating by offshoots.

The bulb. This form of underground stem is much varied.
It may be a scaly bulb, as in the lily, or a tunicated bulb, as
in the onion. These varieties of bulbs are justly regarded by
botanists as subterranean buds or undeveloped stems, being in
every respect similar to the ordinary leaf bud, except that as
they grow beneath the ground, the scales or imperfect leaves
which envelope them are more thick and fleshy. These retain

THE DIFFERENT KINDS OF STEM. 27

their rudimentary character as a protective covering to the
inner leaves, which grow in a tuft from the earth’s surface, the
flower stem rising from their centre.

In the tunicated bulb the scales enclose each other in a con-
centric manner, each scale embracing the entire circumference
of the bulb. The outermost scales are thin and dry, the inner-
most thick and succulent. Tunicated bulbs are restricted to
such plants as have sheathing leaves, and which, consequently,
embrace at their base the entire circumference of the stem. In
the scaly bulb the scales are free from each other and much
smaller, being imbricated, or lying one on the other, like the
tiles of a house. This bulb belongs only to plants, the leaves
of which are sessile, and therefore not connected with the
stem by a sheathing base.

Fig. 6.

Fig. 6.—Bulb of the garlic with a crop of young bulbs.
Fig. 7.—Axillary bulblets, b, of Lilium bulbiferum.

Bulbs being subterranean buds or undeveloped stems, give
birth to new buds or bulbs in the axils of their scales, the
rudimentary leaf-like nature of which is thus rendered apparent.
The young bulbs are called cloves. This mode of increase, is

28 COMPOUND ORGANS OF PLANTS.

exemplified in the common garlic, (fig. 6.) In this respect the
bulb behaves exactly like a leaf-bud after it has been lengthened
into a branch. One or more of these young bulbs or cloves
may develope as flowering stems the next season, and thus the
same bulb survives and blossoms from year to year.

In some plants, as in Lilium bulbiferum, (fig. 7,) bulbs are
produced on the stem in the axils of the leaves, which, when
detached from the stem and placed on the ground, will grow
into independent plants. These bulbs are called bulblets.

The tuber is a subterranean branch which is arrested in its
growth, and becomes remarkably thickened in the place of
being elongated. It is seen in the common garden potatoe,
the eyes of which are true leaf buds. Hence these tubers
when cut into slices, provided the slice contains an eye, will
grow and become independent plants.

In the lower forms of their development stems are so weak
that they trail along the ground, never rising from the earth’s
surface. In other instances these weak stems have a tendency
to grow vertically ; and when this is the case they either twine
in a spiral around the more vigorous herbage in their vicinity,
or the roots of the phytons take a horizontal development and
exhibit themselves all along the side of the axophyte, as in the
Ivy and Virginian creeper. By such aerial or adventitious
roots such plants attach themselves to the surface of rocks
and the bark of trees, and thus elevate themselves to the air
and light.

In some plants, such as the pea and vine, the leaves are
developed as organs of support. By the non-production of the
parenchyma, and the development of the fibro-vascular system,
an organ called a tendril is produced, which has a tendency to
twine round any body with which it may come in contact.

THE DIFFERENT KINDS OF STEM. 29

The tendrils of the vine and pea are well known; let.us
show the beauty of their mechanism. When the plants are
young they are put forth in a straight line, and curved into a
sort of hook at their extremity. In this manner they seem as
if they were reaching forward for the purpose of catching hold
of something on which they can hang for support. If in this
state a young twig or branch be borne by the passing breeze
within reach of their hook, they immediately catch and coil
themselves spirally about it. Now this apparently feeble
organ of self-support is in reality a powerful instrument of self-
defence, and the storm which can overpower the strength of
the forest trees, prostrating them with the earth as it rushes
by in all the wildness of its fury, cannot injure these plants.
It is rendered harmless in its effects by the elastic yielding of
the tendril, which thus secures these weak plants from being
broken off from the object to which they have attached them-
selves, and from sustaining the slightest injury.

In the grasses the stem, which is hollow and fistular, has
received the name of culm (culmusa straw). This ‘structure
also prevails in other plants, and is a beautiful instance of
mechanical contrivance to dispose the limited quantity of
matter in the stem to the greatest possible advantage, so as to
give the greatest strength withthe least expenditure of mate-
rial. By this hollow cylindrical disposition of the matter, an
increase of strength is imparted to the vegetable structure
equivalent to that of a solid stem of the same diameter. The
bones of animals and the feathers of birds are tubes or hollow
trunks, combining strength with lightness, and constructed on
the same principle.

When the philosopher Galileo was confined in the dungeons
of the Inquisition for teaching the heresy of the motion of the

30 COMPOUND ORGANS OF PLANTS.

earth, he was visited by a Catholic priest, who accused him of
Atheism. The persecuted and venerable sage met the accusa-
tion by the following beautiful and sublime, though simple and
affecting appeal. He ‘took from the floor of the dungeon on
which he was lying a wheaten straw, and having explained the
mechanical and scientific principles shown in the structure of
the stem, told the priest that this was evidence to his mind of
the existence of a God. “Tf,” said he, «this wheaten straw,
which supports an ear heavier than its whole stock, were made
of the same quantity of matter disposed in a solid form, it
would make but a poor thin and wiry stem, which would be
snapped with the slightest breeze; its tubular form gives it the
necessary degree of strength, and preserves it from destruc”
tion.”

Not only the strength but the duration of stems depends on'the
degree of their development. A plant is considered to be a
herb if_its stem invariably dies down to the ground each year.
Some herbs are only annuals arriving at their full development
and the term of their existence in one year, the act of repro-
duction exhausting their vital energies. In biennial herbs the
whole of the nutriment assimilated by the vegetative organs
the first year is consumed by the act of reproduction in the
second, and the plant necessafily perishes. In herbaceous
perennials the upper part of the plant only dies, life retrea ts
into the rhizoma, and with the return of light and heat to the
earth in Spring, the plant again makes its appearance above
the ground, and developes into its wonted figure and hues.

The same species may become an annual, biennial, or even a
perennial, according to the treatment which it receives, and
the circumstances in which it is placed. If an annual plant be
deprived of its flowers and preserved from the inclemency of

THE DIFFERENT KINDS OF STEM. 31

winter, it will become a biennial. On, the other hand, tropical
perennial plants, when transported into temperate climates
become annuals. For example; The beautiful climbing vine
so much cultivated called (Cobzea scandens,) and which endures
but for a year in these latitudes, is a perennial in Chili and
Peru its native country ; so also the castor oil plant, (Ricinus
communis), which in Africa forms an elevated tree, is an
annual with us.

In the more highly developed plants, such as shrubs and
forest trees, the act of flovering and fruiting consumes only the
nutriment enclosed in the peduncle and its immediate supports,
but the rest of the plant is not injured. Yonder leafless tree,
whose branches wave in the winter’s wind, loaded with snow,
is still fraught with life. All along its central axis, in its
branches, and innumerable branchlets life exists, dormant
beneath the scales of its numerous buds. The last vegetative
process of plants with ligneous and persistent stems, in autumn,
is, in fact, the formation of the bud, wherein life lies dormant,
yet protected from the severest cold of winter until spring
awakens it to a new existence. The following year these buds
or phytons (uray, a plant,) as they have been correctly named,
develope on the stem or axophyte, and from them ligneous
matter descends, which gives an additional enlargement and
strength to the vegetable axis. In forest trees, therefore, the
stem acquires its greatest development. A forest tree, philoso-
phicaly considered, is not an individual plant, as is commonly
supposed, but a community of individual plants growing
together about a common vegetable axis. These phytons have
‘a downward as well as an upward development, and the stem
or axophyte is formed by the commingling of the ligneous or
fibrous matter which descends from them, and which spreads

82 COMPOUND ORGANS OF PLANTS.

in the earth as in the atmosphere in a form beautifully appro-
priate to the altered condition of the medium.

CHAPTER III.

ON THE ROOT OR SUBTERRANEAN APPENDAGES OF TIE
AXOPHYTE.

Tu rhizoma or the subterranean part of the vegetable axis,
has appendages like the aerial part, organically adapted to the
medium in which they are developed. These appendages,
emitted by the rhizome or its ramifications, are ordinarily
under the form of fibres more or less slender and delicate,
commonly cylindrical, simple or branched, called radicle fibres.
Tt is the assemblage of these fibres which constitute the true
root, that is to say, the organ whose function is to draw from
the soil a part of the elements necessary to the life and
development of the plant.

Each of these fibres is terminated by a blunt and rounded
extremity, which has received the name of spongiole. For a
long time this was considered to be the only part of the root
which absorbed liquids. But it is now ascertained that absorp-
tion takes place throughout the whole extent of the radicle
fibres, the centre of which is occupied by bundles of vessels.

The spongioles or spongelets ought not to be reckoned special
orgaus. Tig. 8 is the extremity of the young root of the
sugar maple, (Acer saccharinum,) highly magnified. Now the
cellular extremity of the root or the spongiole, a, does not con-
sist of the cells most recently formed, which are in reality an

APPENDAGES OF THE AXOPHYTE. 33

older mass of cells, pushed forward by the growth of the cells
at 6, immediately behind them. The cells of the point consist
of older, denser tissues, as inspection plainly shows; and as
these decay and fall away, they are replaced by the layer
beneath. The point of all root fibres is capped in this way.

It would appear from this that absorption does not take place
to any considerable extent at the apex of the root, but princi-
pally through the more recently formed tissues behind it, and
especially by those capillary cells or root hairs with which the
surface of all young and growing roots is usually covered.
These root hairs are in general more abundant and more
developed on plants growing in loose, dry sand. Such plants,
in order to obtain as much moisture as possible from the unfa-
vorable element in which they are placed, shoot forth from
every fibre an incalculable number of them.

Roots produce radicle fibres and root-hairs instead of leaves,
and these organs like leaves are deciduous towards autumn, -
being annually renewed every spring. Hence the best time
for transplanting is in winter, when the fibres are dead or
torpid, or in early spring before they are renewed. Trans-
planting after the season of growth has fully commenced is
always attended with more or less injury to the plant.

4

34 COMPOUND ORGANS OF PLANTS.

In growing, the roots of plants therefore do not elongate
through their entire length, but increase by the addition of
matter to their advancing points, very much like an icicle,
except that the new matter is added from within and not from
without. Growing in a medium of such unequal resistance
as the soil, they elongated through their entire length they
would, when they encountered any obstacle, be thrown into
knotted and contorted forms, which would prevent their acting
as conduits of food from the soil, which is their peculiar office.
But as they only elongate by the formation of fresh tissue at
their extremities, they are thus enabled to accommodate them-
selves to the nature of the soil in which they grow; and,
should any thing impede their progress, they sustain no injury,
but following the surface of the opposing matter, they grow
and extend themselves until they again enter a softer and
more favorable medium. In this manner they penetrate the
soil, as it were, in search of food, insinuating themselves into
the minutest crevices of rocks, and extending themselves from
place to place, as the nutriment in their own immediate
neighborhood is consumed.

Now all newly formed vegetable is extremely hygrometrical,
and hence absorption takes place throughout the whole extent
of the newly formed tissue.

The law which regulates this absorption has been recently
discovered by M. Dutrochet, a distinguished French physiolo-
gist. It is this: if two fluids of unequal densities be separated
by an animal or vegetable membrane, the denser fluid will
draw the lighter through the membrane with a force propor-
tional to the difference of density of the two fluids. A simple
experiment will illustrate this. (Fig. 9).

Take a short tube, and cover one end with a piece of blad-

APPENDAGES OF THE AXOPHYTE. 35

der ; partly fill the tube with a strong solution of sugar, and
immerse it in a vessel containing water. In an hour, or more,
the denser fluid will be found to have attracted the water
through the membrane and to have risen considerably in the
tube. This property is called Endosmosis, (?vdor, inwards, ude,
I seek.)

TAT ATT

Now the cells of the roots and the entire system of the
plant, owing to the evaporation of water from the leaves, always
contain a fluid dense and concentrated. The water in the
earth is therefore attracted into the plant by means of the
denser fluid contained in the cells of the root,—or in other
words, it’ enters the plant by endosmosis.

This simple endosmotic law pervades all vitally active and
newly formed vegetable tissues, and seems to be the only cause
of all the remarkable movements of roots. For example, it is
well known that roots will turn aside from a barren for a
fertile soil, so that to stop their growth in any given direction,

36 COMPOUND ORGANS OF PLANTS.

it is only necessary to interposeatrench of gravel or sand between
them and the premises they are forbidden. How is this to be
accounted for? Are we to suppose, as some have done, a sort
of prescience on the part of the vegetable? On the contrary,
is it not all clearly explicable on the principle of endosmosis ?
There is always, in the forming and vitally active cellg at the
extremities of the roots, a thicker fluid than the fluid in earth ;
the fluid in the earth is attracted by endosmosis through the
cell walls into the system of the plant, and becoming assimi-
lated, the newly formed cells of the roots necessarily take the
direction of the most fertile and favorable soil.

Roots developing in the soil have a natural tendency to an
avoidance of the light, whilst the stem and leaves seem to seek
for the same. Hence hyacinth bulbs will grow much better
in water-glasses which are of a dark color, than in white
uncolored ones. So also when Dutrochet caused a misseltoe
seed to germinate on the inside of a window pane, it sent its
roots inwards towards the apartment ; when on the outside of
the pane it did the same. Hence when seed is sown by
nature or the hand of art, however the seed may fall, yet in
germination the radicle so bends itself as to sink perpendicu-
larly into the soil, whilst the stem rises perpendicularly
from it.

The force with which the radicle or root descends is very
considerable, and many attempts have been made to change its
obstinate tendency to burrow in the ground, but without effect.
We know not yet the cause of this invincible tendency of the
radicle towards the earth’s centre. It has been thought that
the humidity which exists in greater abundance in the soil
exercises a sort of attraction on the radicle, but Duhamel has
shown that it is not so. He caused seeds to germinate between

.

APPENDAGES OF THE AXOPHYTE. 37

two sponges, saturated with water, brought near each other,
and suspended in the air by means of a double thread. When
germination was sufficiently advanced, the radicles instead of
bearing to the right and left towards the water in. the sponges,
glided between them, so that they ultimately hung below them
into the atmosphere. It is not then the humidity in the soil
which causes the radicles to penetrate its surface. It would
rather seem that this tendency ought to be attributed to a
particular force, which developes a sor$ of polarity at the
period of germination, which produces an opposition of growth
in the two extremities of the vegetable embryo, causing the
plumule to rise towards the zenith, and the radicle to move in
the direction of the earth’s centre.

The radicle fibres generally spring from the subterranean
portions of the axophyte, but the aerial portions of that organ
are equally capable of emitting them. When this is the case
they are designated under the name of aerial or adventitious
roots. Some woody vines, as the Bignonia or Trumpet-creeper,
the Rhus toxicodendron or Poison ivy, and the Hedera helix
or European ivy, climb by aerial rootlets, in which way they
reach the summits of the tallest trees, and loftiest buildings,
giving beauty even to the mouldering ruin. Such plants,
however, derive their nutriment from their ordinary roots
embedded in the soil, their copious aerial rootlets merely serv-
ing them for mechanical support. The tenacity with which
these aerial rootlets adhere to trees, rocks, and even to the
hardest flint, is truly astonishing ; and the height to which the
plants themselves will ascend, seems to cease only because
they can find nothing higher on which they can support them-
selves. In warm climates these twining plants (/ianas) take
a much higher degree of development; their stems are ligneous,

Ax

1

38 COMPOUND ORGANS OF PLANTS. *

persistent, and sometimes very thick, whilst with us they are
very slender and herbaceous and perish annually. Heat and
humidity are powerful agents in promoting vegetation, and
hence its superior activity in the tropics.

The roots emitted by the aerial portion of the axophyte
sometimes remain free and floating in the atmosphere, and
sometimes they descend as far as the soil, which they penetrate
in order to draw from it additional nourishment. These pecu-
liarities are observable in many of the vegetables of the warm
and sunny South. A great many palms, figs, and orchideous
plants develope these roots.

When the roots continue aerial, the plants are termed
epiphytes (x: upon, and gurév plant). They are so called
because they grow to other plants as mere points of attach-
ment, dropping their roots into the atmosphere from which
they derive all their food, and which always continue aerial
and greenish, and to-distinguish them from parasites, which
obtain their nutriment from the plant on which they are found.
These plants abound in the tropical forests of South America,
which they enrich and beautify by their gorgeous and fragrant
flowers. That the trees on which they grow are mere points
of attachment, and not sources of food, is evident from the fact
that they may be attached to any substance whatever, as for
instance to the rafters of the stove or hot house, where they
will grow with an equal amount of vigor and luxuriance.

The roots of the epiphytes or air-plants as naturally avoid the
ground and darkness as the roots of other plants seck for the
same; they require air and light, and may be seen searching
for it in the warm, moist atmosphere of the conservatories,
through the crevices of the baskets filled with chips and char-
coal in which they are generally kept. In other instances,

APPENDAGES OF THE AXOPHYTE. 39

these aerial roots emitted from the stem into the open air,
descend to the ground, and establish themselves in the soil.
Many plants of tropical climates present this phenomena.
Amongst which we may mention the Ficus religiosa, or

Fig. 10.

An Epiphytic orchid (Maxillaria) of warm climates.

Banyan tree of British India. This tree drops from its hori-
zontal branches, roots into the air, which, swinging in the
breeze like pendant cords, do finally reach the soil, into which
they penetrate, when they become metamorphosed or changed
into stems, and increasing in diameter, give nutriment and a
natural support or prop to the heavy branches from which
they originally descended, so that those branches can extend
laterally still farther from their parent trunk. By numerous
growths of this kind, one tree ultimately becomes the centre of
a family forest, their united branches and foliage spreading
over a considerable extent of ground.

The Pandanus or Screw pine may be cited as another
instance. In this case, when the tree is much exposed to the
powerful winds of the tropics, strong roots are emitted from
the lower part of the main trunk, which, striking into the soil,

40 COMPOUND ORGANS OF PLANTS.

act as props to the stem, giving the tree the appearance of
having been raised from the ground. If, however, the tree is
under shelter, or cultivated in a stove or hot-house, those
thick, strong roots or props, provided by nature, do not
develope, but are still seen as protuberances on the surface of
the stem. The same phenomena is perceptible on a small
seale in the stem of the Zea mays, or Indian corn, the lower
joints of which give forth aerial rootlets, which reach or do
not reach the soil, according to the amount of support required
by the plant.

In general it may be remarked, that these adventitious roots
are developed from those parts of the stem where the nutritive
sap encounters some obstacle to its free circulation, and in par-
ticular at the nodes or accidental nodosities which exist on the
stem or its branches.

We are able, whenever we please, to produce these adventi-
tious roots on the young branches of most ligneous vegetables.
It is only necessary to surround the young branch with humid
earth contained in any kind of pot or vase. At the end of a
definite period, varying according to the species, the roots will
develope themselves, and the young branch can be separated
and will form another plant. This is a mode of multiplication
very useful in horticulture. ‘

We see the same results produced continually in nature
under similar circumstances. Most creeping stems produce
roots at every leaf node, that is to say, when there are the
suitable conditions, moisture, a certain amount of shade, and
immediate contact with the earth; and the branches of such
stems as are vertical, if bent to the ground and covered with
earth, almost always take root. This is sometimes done by
gardeners, who bury the limbs of shrubs by bending down the

APPENDAGES OF THR AXOPHYTE. 41

body of the tree, after which each limb, being severed from the
parent, forms a new tree.

Separate pieces of young stems containing a bud, and called
by gardeners cuttings, will also take root if due care be taken
with them. For a tree is not an individual, as is commonly
supposed, but a collection of individuals, an elongation of indi-
vidual buds, which, in their development into branches, live
on their parent stem, into which they send down roots just as
that parent stem itself sends its roots into the soil. Tor this
reason the bud of one plant may be transferred to the stem of
a similar or nearly related species, to which, if it be carefully
fitted, it will soon become rooted and develope into a branch,
being sustained by the stem, into which it has been engrafted
equally with the natural branches of the tree.

The observations of Mohl and Unger, both eminent physio-
logists, have proved that adventitious or aerial roots are all
formed in avery similar manner. They show themselves at
first under the form of a little conical excrescence or tubercle,
the base of which rests on the wood. As they increase, they
turn aside the cells of the tuber and cortical parenchyma which
they traverse, and finally form a slight prominence under the
epidermis. A little later, the epidermis is torn in a direction
parallel to the axis or stem, and the root shows itself at the
exterior and directs itself towards the soil.

The roots of plants generally bury themselves in the soil,
but some plants are parasites (xapa beside, sivos food), or
derive their nutriment from the plants on which they grow
and into which they fix their roots, and cannot therefore be
cultivated on the ground. Some parasites grow on the roots
of trees, as the Epiphegus Virginiana, or beech drops, which is
found beneath the shade of beech trees, and on the roots of

42 COMPOUND ORGANS OF PLANTS.

which it is parasitic; others attach themselves to the stem and
branches of trees, as the cuscuta or dodder and Viscum fla-
vescens or mistletoe.

The Indian pipe (Monotropa uniflora) is one of the most
remarkable of our native parasites. This plant may be found
occasionally in the deep, rich woods of North America, during
the summer months. It is a singularly pallid and fungous-
looking plant, to which order it seems to approximate not only
in appearance, but also in the exercise of its functions. It is
fleshy, scentless, and snow-white throughout, and rises to a
height of from four to eight inches above the ground, bearing
at its summit a solitary terminal flower, which is at first
drooping, and in this state the plant looks not unlike a pipe in
appearance, but afterwards becomes erect. The whole of the
plant turns black in drying.

The roots of many species of plants are not fixed to any sub-
stance whatever, the plant possessing a sort of locomotive
power. This is the case with several kinds of aquatic plants,
as the Lemna, or duckmeat, a little frond-like plant, which
covers the surface of stagnant pools with its scum-like vegeta-
tion, and drops its little filiform roots into the water, on the
surface of which it floats. So also the Fucus natans, a species
of marine alge, is found in the Gulf of Florida and other
parts of the ocean, floating many hundreds of miles away
from land. This plant has no distinct root, and is of course
found only within certain latitudes.

Functions of roots—The principal function of roots, con-
sists in drawing from the earth, or from any other medium
in the midst of which they are plunged, those substances which
serve for the nutrition of the plant. Their organization and
vital phenomena prove this.

APPENDAGES OF THE AXOPHYTE. 48

M. -Macaire has proved that plants possess the power of
excreting by their roots such injurious matters as they may
occasionally necessarily meet with in the soil, and absorb
from it in the progress of their development under ground.
This gentleman took a fibrous-rooted plant, and having sepa-
rated its fibres into two sets, he placed one set in a glass con-
taining distilled water, and the other in.a solution of acetate
of lead. After a few days he found that the fibres dipping into
the solution of acetate of lead, had taken up that poison into
the plant, but that the same poison had been excreted or
thrown out by the other set of fibres into the glass containing
distilled water. For on applying sulphuretted hydrogen, the
test for the acetate, he found the distilled water was impreg-
nated with it.

In this experiment we see that the poison was forced into
the circulatory system of the plant, which induced a self-pre-
servative effort on its part analogous to that made in the
higher forms of life. All the experiments made on plants
with narcotics and other poisons prove that they possess a
principle of life analogous to that of animals.

The roots of plants when developed in the soil, are also
clearly designed to fix them in an upright position, so as to
prevent them from being overturned by animals, by the force
of the winds, or by any other cause. Hence it is that the
roots of a tree are always most numerous and strong to wind-
ward, or in the direction of the prevailing winds. When the
tree is sheltered on every side, there is little lateral extensiun
of its roots, and they naturally develope downwards into the
earth. So also the roots of trees growing on the sides of roads
or the banks of rivers, will curve into the embankment, and
thus prevent the tree from being undermined or washed away.

44 COMPOUND ORGANS OF PLANTS.

The roots of rock plants adhere to their surface and crevices
with the most astonishing tenacity; as for example, the beau-
tiful wild columbine, (Aquilegia Canadensis,) one of the early
spring flowers of the northern States.

The roots of plants, particularly, the fibrous and matted
roots of the sedge and grass tribe, bind together the loose soil
on the sea-shore, and prevent it from drifting inland. On
many coasts, the inward drift of the sand by the strong sea
breezes which prevail, produces hills of sand called dunes.
The safety of these shores is greatly promoted by a species of
grass called the Arundo arenaria, whose thick and matted roots
bind together the loose sand and prevent its desolating effects.

That disintegration and destruction of rocks mechanically and
chemically, which is continually going forward in nature, is
also prevented from being carried forward to an injurious
extent by the fibrous roots of grasses and other plants.

Tt is a fact well known to practical geologists, that when
rocks rise above the surface of the earth in cliffs and ridges,
they become exposed to the mechanical and chemical action of
the atmosphere, and their surface gradually shivers off, crumbles
down, and wears away. Hence loose matter collects at the
bottom of the escarpment, forming in the course of ages, a
slope of disintegrated material, called by geologists a talus.
The process of disintegration continues until the talus of fallen .
fragments has accumulated to the very summit of the escarp-
ment, so as to hide it altogether. Now so long as the face of
the escarpment is exposed, and the fall of the detached frag-
ments continues, vegetation will not seize on the slope; but
when the disintegrated material has acquired that degree of
sloping, which is called by geologists, the angle of repose, or
has accumulated to the very summit of the escarpment, no

APPENDAGES OF THE AXOPHYTE. 45

more fragments will roll down, and vegetation will cover the
slope. Vegetation appears on no soil but what is in a state of
rest, and when it is once established in any place, it is not only
a sure indication that the soil is at rest, but a means of keeping
it so. It is by operations of this kind, not performed in a day,
but in ages, that rugged peaks and abrupt precipices are
gradually transformed into rounded summits, gentle slopes,
and habitable surfaces. On precisely the same principle, the
sloping sides of railways are secured from disintegration and
destruction, by being sown with grass seeds or covered with
grass sods.

The lower orders of the Cryptogamia, or flowerless plants,
such as lichens and mosses, appear to derive their nutriment
mainly from the atmosphere. Mosses appear to take in their
nutriment from the air by their whole expanded surface,
although doubtless the delicate root hairs below that surface
perform their part in absorption. Hence some species are only
found growing on the bark of trees, others on rocks and
boulders, whilst numerous genera cover the surface of the
ground. The roots of lichens, when they have any, are mere
holdfasts, the plant being developed wholly from the atmo-
sphere. Some species, however, would seem to attach them-
selves to stones of a calcareous, whilst others form a beautiful
plaiting on the surface of whins, sandstones and granites.
These atmospheric cryptogamia, are the first plants which
clothe the surface of barren rocks, and by their decay form
a humus or foothold for a more highly organized vegetation.

46 COMPOUND ORGANS OF PLANTS.

CHAPTER IV.
ON THE ORGANIZATION OF THE STEM.

WHEN we examine anatomically the stems of phanerogamous
plants, we find them to be remarkably similar in their internal
structure. The stems of forest trees, ligneous and persistent
for centuries, differ only from the stems of the herbaceous and
humble plants which grow beneath their shade, in the degree
of their development; they are constructed on precisely the
same plan, and in all the varieties of their growth, for the most
part, are reducible to either one or the other of the two follow-
ing forms of vegetable organization.

The exogenous (?«, outward, and yevrdew, to produce,) or
outside-growing stem, so called, because this kind of stem
increases in diameter by successive annual additions of bun-
dles of vascular and fibrous tissue to its.outside. Such a stem
exhibits on the cross-section a number of concentric rings of
wood, which mark the successive annual growths of the tree,
surrounding a central column of pith, the whole enclosed
by a hollow cylinder of bark. The forest trees of the northern
United States, and the major part of our herbaceous plants,
are all constructed on this plan; and the cross-sections of an
oak branch, or of any other tree, will show the rings, and the
nature of an exogenous stem.

The endogenous (dor, within,) or inside-growing stem, so
called, because it increases in diameter by successive additions
of fibro-vascular and cellular matter to its inside. The growth
of these plants is carried on by means of the thick cluster of

ORGANIZATION OF THE STEM. 47

leaves with which they are terminated superiorly, and from
them the new vegetable matter descends along the centre of
the stem, and pushes outward the parts first formed. -

The oldest and hardest part of the stem of an endogen is
that nearest the circumference ; for the more the external parts
are pressed by the descent internally of new vegetable matter,
the denser must they necessarily become. It is owing to
this external hardness that many endogenous plants have no
lateral buds or branches, because they are unable to penetrate
the hard casing of the stem.

On the cross-section, the stem of an endogen is not distin-
guishable into bark, wood, and pith, neither does it present
any appearance of concentric rings; for the stem of an endo- *
gen is composed of separate bundles of vascular or woody
tissue irregularly imbedded in a mass of cellular tissue, which
bundles are distinctly traceable down into the stem from the
base of the leaves at its summit, and then curving outwards
they generally terminate in the bark. Hence on the cross-
section the cut ends of these bundles are visible in the form
of dots, interspersed through the uniform cellular tissue, with-
out any apparent order, although more commonly crowded
towards the circumference. Hence, also, we see the reason
why the bark of an endogen is inseparable from the rest of the
stem without a laceration of its fibres.

The plants whose structure is endogenous in the northern
United States are few, and, with the exception of the green
brier, entirely herbaceous. The grasses, the iris, the Indian
corn, are humble representatives of endogenous plants, which
attain their full development and display their noble arborescent
forms only under the influence of a tropical sun. The palms,
screw pines, plantains, and bananas of the tropics, are all endo-

48 COMPOUND ORGANS OF PLANTS.

genous, and present a striking contrast to the exogens of tem-
perate latitudes. A tall, cylindrical and unbranched stem rises
to the height of from 100 to 150 feet, crowned at the summit
with a magnificent cluster of leaves, many feet in length, bend-
ing elegantly downwards, and presenting altogether one of the
most graceful and beautiful objects that can adorn a landscape.

The Exogenous Stem.—Since the exogenous class of plants
is by far the largest in every part of the world, and embraces
all the trees and shrubs with which we are familiar in northern
climates, the structure of this kind of stem demands a more
detailed and particular investigation. Every exogenous stem
presents, on the cross-section, an arrangement of matter into
three parts, called, respectively, the bark, the wood, and the
pith. To obtain, however, a clear idea of the origin of
the exogenous stems, it is necessary to follow the course of
the development of the stem from the embryo state.

The first year’s growth.—If we place a seed in the ground
at the temperature of 32° Fahr., it will remain inactive until
it finally decays; but, if the earth be moist and above the
temperature of 82°, and the seed be effectually screened from
the action of the light, its integuments will gradually imbibe
moisture, soften, and swell, oxygen will be absorbed, carbonic
acid expelled, and the vital action of the embryo will com-
mence. It now elongates downwards into the earth by its
radicle, and upwards into the air by its plumule, or young
stem, lifting the cotyledons above the earth’s surface. The
cotyledons thus elevated acquire a green color, by the deposi-
tion of carbon absorbed from the atmosphere under the influ-
ence of solar light, and ultimately assume the form of two
opposite leaves. The process of germination is now completed,
and the root, stem and leaves being formed, we have a simple

ORGANIZATION OF THE STEM. 49

plant perfect in all its parts and dependent for its future
growth and sustenance on its leaves and roots.

Let us now examine the successive modifications of the inter-
nal structure, from the commencement of germination to the
growth of the first pair of leaves and the completion of this
the first stage of vegetation. At first the embryo consists
wholly of cellular tissue; as soon, however, as it begins to
grow, even while the cotyledons only are developing, some of
the cells begin to elongate into tubes longitudinally, assuming
the form of vascular and woody fibre. These nascent wood-cells
extend upwards into the cotyledons, and downwards into the
radicle, and are finally seen in a cylindrical form in the centre
of the stem. This longitudinal elongation of the cells does not
take place at random, but certain determinate cells only
thus change their character, whilst the others wholly retain
or depart but slightly from their primitive form. <A horizontal
section of the plumule at this stage of development will show
this.

The sap elaborated in the first pair of leaves contributes to
the upward growth of the plumule or young stem, and to the
development of the second paif of leaves; the new wood-cells
extend through them to form their frame-work, making the
woody stratum in the second internode as it lengthens, and con-
iributing at the same time to the increase of the stem beneath
them; and the same process is repeated throughout the whole
growth of the season with every fresh development of leaves.

The woody fibre having rapidly increased, ascending and
descending the stem with the growth of every new set of
leaves, the medullary rays ultimately become so much com-
pressed, that they assume the form of fine lines radiating from
the centre to the circumference of the stem.

5x :

50 COMPOUND ORGANS OF PLANTS. ,

Tn exogenous stems of a single year’s growth we therefore
observe, a central cellular pith, a zone of woody fibre and vas-
cular tissue, an exterior coating of bark, and medullary rays
passing from the pith to the bark. This is the complete struc-
ture of exogenous herbaceous stems which die down to the
ground annually.

In exogenous stems which are not annual, at the close of
the growing season the stem ceases to elongate, the old leaves
gradually fall off, the new leaves, instead of expanding after
their formation, retain their rudimentary condition, harden and
fold over one another, and a bud is produced, the winter's
residence of the shoot.

The second year’s growth.—The next year, with the return of
light and heat to the earth in Spring, vegetation re-commences.
The resinous exudation on the buds is melted by the heat of
the sun; the scales fall off, the leaves expand, and are sepa-
rated by the growth of the internodes, the buds terminal and
lateral are in this manner elongated into shoots, and are now
to the parent shoot what the young leaves were to it the first
year; that is, they perform precisely the same functions, and
contribute by their downward growth, and their deposit of
woody and fibrous matter, to the increased diameter of the
parent shoot.

With the development of the buds into shoots and leaves,
the sap is set into circulation through the system of the plant,
and the bark and wood which, at the close of the growing
season, or in autumn, firmly adhered together, are now easily
separable from each other, by the formation between them of
a stratum of mucilaginous, organizable matter, termed cam-
bium. This cambium is nothing more than the ordinary sap,
the water of which having been evaporated in the leaves, is

ORGANIZATION OF THE STEM. 51

necessarily thickened and well charged with assimilated mat-
ters, and is interposed between the wood and bark where
growth is going on. “It is quite wrong,” says Dr. Gray, «to
suppose that there is any real interruption between the wood
and the bark at this or any other period of time, leaving a
space filled with extravasated sap. A series of delicate slices
will at any time show that the bark and wood are always
organically connected with each other, by a very delicate tissue
of vitally active partly grown cells.” The cambium thus
deposited between the wood and bark becomes organized into
cells, and forms a new addition of matter to each. Hence, the
forming stratum is termed the cambium layer, the inner por-
tion of which forms wood, and the outer, bark. It is when
this process of growth is most rapidly going on, in spring or
early summer, and the whole cambium layer is gorged with
the flow of sap, that the bark and wood are so easily separated.
But the separation is effected by the rending of a delicate new
tissue.

At the end of the second year, the cambium layer of new
wood and bark hardens, the second annual layer or ring of
wood and bark is formed, and the bark and wood again adhere
firmly together. The new shoots are prepared for winter in
precisely the same manner as the first year’s shoot was pre-
pared, and are elongated cones as was the first.

In like manner will the plant continue to grow throughout
the third, fourth, and succeeding years, each annual growth
being only a repetition of the same phenomena.

After a certain number of years the tree arrives at the full
perfection of its growth, the outer layers of bark now become
fissured and rent, and are exfoliated or thrown off from the
stem, and the alburnum or sapwood becomes changed into

52 COMPOUND ORGANS OF PLANTS.

duramen or heartwood, which ultimately decays and falls away
leaving the interior of the stem rotten and hollow. These
changes in the external and internal appearance of the stem
are the necessary results of the following peculiarities of its
growth.

We have seen that one layer of bark and one layer of wood
is annually deposited from the viscid mucilaginous matter
called cambium, which makes its appearance between the bark
and the wood in spring. It follows, that the number of annual
layers or rings of bark ought to correspond to the number of
annual layers or rings of wood. Sometimes in the bark of
young shoots of two or three years growth these annual
deposits may be traced, but in general the successive layers of
bark are so amalgamated by the internal growth and conse-
quent pressure of new strata of bark, that it is impossible to
distinguish them.’ To the same cause is to be attributed the
fissuring and exfoliation of the outer layers of bark. The
diameter of the wood is a constantly increasing quantity,
because the growth of the wood is exogenous, each new layer
of wood being deposited on the outside of the last annual layer,
and therefore each ring of the wood remains unaltered in its
dimensions and position until it finally decays; on the other
hand, an increase in the diameter of the bark is constantly
prevented by the endogenous growth of the bark, each new
layer of bark being deposited on the inside of the last annual
layer ; and as new layers of bark are deposited internally, the
previous annual layers are subjected to gradual but incessant
distention, and finally unable to bear the stretch, are fissured
and torn into clefts and rents, causing that cracked and rugged
appearance of the bark of trees with which all are familiar.
Hence it is that on the cross-section the bark bears but a

ORGANIZATION OF THE STEM. 53

small proportion in thickness to the wood; the amount of bark
which remains deposited about the wood is exactly propor-
tionate to the stretch or tension to which it will submit, vary-
ing greatly in different species. In the old trunks of some
pines and firs, it sometimes attains the thickness of from eight
to twelve inches, whilst in the Platanus occidentalis, or com-
mon plane-tree, after the eighth or tenth year, all the epi-
phlceum or old and outer layers of bark fall away entirely in
the form of brittle plates.

The duramen or heartwood.—The sap chiefly circulates in
the inner bark and alburnum where growth is going on, the
new and fresh tissues being most active in its transmission.
The walls of the cells soon begin to thicken by the internal
deposition of mineral matter or sclerogen imbibed through
the pores of the roots with the sap, and what was once sap-
wood is every year, by the development of new rings of wood
removed farther and farther from the region of growth; after
a few years, therefore, it ceases to take part in the vital opera-
tions of the plant, its color changes, and it becomes what is
called duramen or heartwood

As the duramen or heartwood does not assist in maintaining
the functions of the tree, it may decay without injury to the
vitality of the plant. Hence it is that we sometimes see old
trees covered with the most luxuriant foliage, although their
inside is totally gone.

Having taken a cursory view of the development of an
exogenous stem, from the period when it first emerges from its
cotyledons or seed leaves, to that term of its existence when it
begins to show signs of decay in its interior, we shall now
attempt a more careful analysis of the different layers of bark,

54 COMPOUND ORGANS OF PLANTS.

wood, and pith which the stem exhibits on its transverse
section.

Fig. 11.

ABOU XH)

Fig. 11 shows a transverse section A, and vertical section B, of an exogenous or
dicotyledonous stem of three years’ growth. “In both sections, a represents the
cellular tissue of the pith, bb, the dotted ducts, and c ¢, the woody fibre of the
successive annual layers; d@ d, the spiral vessels of the medullary sheath; e é,
cambium layer; ff, liber; gg, cellular envelope; h, corky layer; ¢ 7, medullary
rays. In the vertical section the medullary ray is shown in only part of its
length ; since the continuity of the medullary rays from the pith to the bark, owing
to the slight flexure which always ocours in them, is rarely or never shown by such
a section.”*

The Bark.—¥From this section, it is evident that the bark,
anatomically considered, may be subdivided into four parts.

1. Lhe epidermis or general outer integument. The nature
of this investment has been already examined pp. (18-24). It
only remains at present to add, that in forest trees and larger

* See Carpenter’s “Principles of Physiology, General and Comparative,’’
3d edition, 1851.

ORGANIZATION OF THE STEM. 55 .

shrubs, the bodies of which are of a firm and vigorous texture,
it isa part of little importance, excepting in the young and
tender state of the plant; but in reeds, grasses, and other
plants with hollow stems, it is of great use and is exceedingly
strong, being chiefly composed of silica, or flint. The epidermis
is not represented in this section.

2. The epiphiceum (xi upon, provos bark), or corky envelope,
shown in the section at hh. This is the outer covering of the

_ bark, and consists of cubical or flattened tubular cells, without
chlorophyl, placed close together and elongated in a horizontal
direction. It is this part of the bark which gives to the trunks
of trees their peculiar color and rugged appearance ; generally
some shade of ash-color or brown.

In Quercus suber, the cork oak, the epiphloeum consists of
numerous strata of cells, forming the substance called cork;
hence the name corky envelope, which is given to it. So also
the branches and branchlets of “Liquidambar styraciflua, the
sweet gum-tree, and of Ulmus racemosa, one of the elms of the
northern United States, are winged with corky ridges, the
result of an unusual development of the epiphlceum of the bark.
In the currant and birch, the epiphleum is composed of only
a few layers of cells, and may be seen peeling off in thin cir-
cular pieces from the trunks of these trees. When the
epiphloeum is very thick, it is simply fissured or rent, in which
state it remains attached to the outside of the stem, forming an
excellent protective envelope to the inner and vitally active
layers of bark.

3. The mesophiccum or cellular envelope, represented at g. g.
This lies immediately on the outside of the liber. Its cells
contain chlorophyl, and are developed vertically. It is, there-
fore, that part of the bark which is colored green, and which

56 COMPOUND ORGANS OF PLANTS.

gives to young ‘shoots their green hues, as it shines through the
transparent membrane, the cuticle. In the shoots of young
trees, in the spring, it may be readily perceived. The bark of
a young stem the first year always assumes this color, from
the production of chlorophyl in its superficial cells, owing to
their direct exposure to the solar light.

The mesophlosum or green layer of bark, does not grow at
all after the first or second year. It is excluded from the light
by the gradually exterior deposition of layers of epiphleum,
and finally perishes never to be renewed again.

4. The endophleum or inner bark, called also the liber, ff.
This constitutes the fibrous portion of the bark, the corky and
cellular envelopes being composed exclusively of cellular tissue.
It is in the fibrous portion of the bark that the sap vessels are
contained, which convey the sap from the roots to the highest
extremities of the plant; hence the endophleum continues to
grow throughout the life of the plant, being formed in conjunc-
tion with the alburnum or sapwood directly from the cambium
layer.

The endophleum or inner bark possesses considerable
strength and many useful properties. The inner layers of
Tilia Europoea, or the lime tree, when separated by maceration
in water, form the common bass or matting used by gardeners,
and the woody fibre which is used for the manufacture of
cordage in all exogenous plants, as in hemp, flax, &e., belongs
to the endophleeum or inner bark, and not to the wood.

The cambiwm layer, e e. This layer has been already
described, pp. (50-1). In herbaceous plants it is not able to
organize itself, because the stem dies down to the ground the
first year. In every other respect the herbaceous stem offers
the same structure as the ‘ligneous, being composed equally of

ORGANIZATION OF THE STEM. 57

bark, of wood, and of pith, but the cambium layer is not there,
and therefore it wants the elements necessary to the forma-
tion of a new layer of bark and a new layer of wood.

In the second year, the gelatinous tissue of the cambium is
subjected to the following changes. We have seen that it occu-
pies an intermediate position between the bark and the wood.
This zone of cambium cells produces at every point beds of the
same nature as those with which it is in immediate contact,
and is developed into. ligneous and cortical fibre, preserving its
cellular organization only in those portions which correspond
to the medullary rays. The inner portion of the cambium
layer forms the wood, the outer portion the bark, and the new
cells of both layers thus mould themselves entirely on the
older cells throughout all their points of contact.

That the bark increases in diameter by the deposition of new
layers of bark internally, was first proved by Duhamel, a cele-
brated French physiologist, by the following simple experiment.
He passed a metallic thread between the liber and wood of a
young tree, and cutting down the tree several years after, he
found, on examination of the cross-section, that rings of bark
coinciding in number with the years elapsed since he placed
the wire next the wood, had grown between the wire and the
wood, so that the wire was separated from the wood by a con-
siderable thickness of bark. No experiment can be imagined
more decisive than this, of the growth of the bark in diameter
by internal deposition of matter.

The wood.—This consists of two parts, the alburnum or sap
wood, and the duramen or heart wood. The alburnum or sap
wood, so called because the sap circulates through it, and also
in allusion to its white or pale color. The alburnum is the
zone or ring of wood last formed. It consists of elongated

6

58 COMPOUND ORGANS OF PLANTS.

tubes of woody fibre, ¢ c, intermixed with bothrenchyma or
porous vessels, 6b. On holding up a thin traverse section of
an oak or ash stem to the light, the porous vessels will be seen
in the form of large round openings in the tissue, which,
situated near the margin of each woody circle or zone, renders
apparent the annual growths of the stem. In the maple,
plane, and lime tree, these openings are smaller and more dif-
fused, and hence there is an indistinctness in the line of demar-
cation between the successive zones.

This new layer of wood gradually loses its softness as the
season advances, and towards the middle of winter is condensed
into a solid ring of wood. In this country, and in Europe
generally, there is a periodical check to vegetation during the
colder part of the year, which occasions the annual layers
found in the stem of exogenous trees. These annual rings,
which are distinctly seen in most trees of temperate climates
when a section of their stem is examined, serve as natural
marks by which to distinguish their age. Thus, suppose an
elm, or any other tree to be felled, and the section near the
ground to have thirty-five circles, or rings of wood, it may be
inferred that the tree is thirty-five years of age.

This computation, however, can only be made in trees which
have these rings distinctly marked, and even then there are
sources of deception of which it is proper that the student
should be informed. For example, a warm spring followed by
weather cold enough to check vegetation, will leave a ring in
the stem, and the subsequent growth of the stem, with the
return of warm weather will give on the cross-section the
appearance of two rings of wood, or of two years growth, to the
growth of one year; on the other hand a warm winter, by
keeping the tree constantly growing without check, will give

ORGANIZATION OF THE STEM. 59

the appearance of one layer to the growth of two years. Not-
withstanding this, practical men find counting the concentric
circles of exogenous stems, to be the best mode which has yet
been discovered for ascertaining their age, as in ordinary cases
only one growth is made in the course of a year.

In tropical countries, where the temperature is compara-
tively speaking pretty much the same throughout the year,
these rings are very indistinctly marked. In tropical coun-
tries, vegetation is not liable to that periodical check which it
receives in colder regions, and therefore the cross-section of
the stems of exogenous trees in many instances do not disclose
these rings, or any separation of the wood into concentric
layers.

As the same development of woody and cortical matter takes
place in the branches as well as in the stems of exogenous
trees, therefore, the time when a branch was first given off
from the stem may be computed by counting the circles on the
stem and branch respectively. If there are, for instance,
thirty rings in the stem, twenty in one of its branches, and
five visible on the cross-section of another, then the tree must
have been ten years old when the first branch was developed,
and twenty-five years of age when it formed the second.

If we carefully examine the rings on the cross-section of an
exogenous stem, we shall soon perceive that they have not the
same geometrical centre, that their breadth varies, and that
they are occasionally thicker on one side of the tree than on
the other. A variety of causes contribute to produce these
effects. The variable breadth of the rings depends on the
variability of the seasons; for more wood will necessarily be
deposited when the season is favorable for vegetable growth,
than when the contrary is the case. Moreover the circles will

60 COMPOUND ORGANS OF PLANTS.

be broadest on the side of the tree where there is the most
wood deposited, and this is invariably on the side which
contains the greatest number of branches and leaves. In a
solitary tree, other conditions being favorable, the rings are
generally the broadest on the south side of the stem.

The Duramen or Heart-woodi—We have already stated
that the walls of the fully developed fibre-cells through which
the sap circulates, become thickened by the deposition of
matter in layers on their interior surface, until at length the
cavities of the cells are almost entirely closed; when this
happens, the sap can no longer permeate the wails of the cells,
and their vital functions cease.

This solidification of the wood-cells is usually connected
with a change in the color of the wood, more or less marked.
Sometimes this change is made suddenly and without any.
‘intermediate shades, as in the wood of the ebony and logwood,
of which the heart-wood is black or deep red and the albur-
num almost white; but it frequently happens, that there exists
no sensible difference of color between the alburnum and heart-
wood, as for example, in trees with white wood, the color of
the heart-wood never changing except from incipient decay.

This older, more solidified, and harder wood, which occupies
the centre of the trunk, is the part principally valued by work-
men as most suitable for economic purposes. The various fancy
colored woods employed by the turner and cabinetmaker con-
sist of the heart-wood only, which assumes different colors in
different ‘species, being black in the ebony, bright yellow in the
barberry, purplish-red in the cedar wood, and dark-brown in the
black walnut. The alburnum in all these trees, even in the
ebony itself, is always white, and is regarded by workmen as
a part of the tree of little or no value.

ORGANIZATION OF THE STEM. 61

The ligneous zones, considered collectively, are more indu-
rated towards the interior of the stem, because, in fact, these
are the most ancient deposits; but, examined separately, they
are more solid in their exterior than in their interior parts;
because the latter was formed in early spring, at a period when
the nutritive sap was more aqueous and less condensed.

The Medullary Sheath.—This is a very thin zone of vascu-
lar tissue and spiral fibre, immediately surrounding the pith,
shown at dd. It may be readily seen in the traverse section
of a young exogenous shoot by its green color, which appears
deeper as contrasted with the white of the pith which it sur-
rounds. Jf we scoop out the pith of the shoot from the
ligneous cylinder which surrounds it, we shall obtain a longi-
tudinal view of the medullary sheath, which will appear like
a green layer on the interior surface of the cylinder. The
medullary sheath is the earliest formed portion of the vascular
system, and is developed with the upward elongation of the
stem, sending its woody fibre and spirals into each young shoot
and leaf to form its veins. The medullary sheath is the only
part of an exogenous stem in which spiral vessels occur.

The Pith, represented at a, consists of soft cellular tissue, .
and is formed as the stem elongates. At first it abounds with
nutritive matter, which serves to nourish the growing bud
resting on its summit; this office fulfilled, is becomes dry and
dies, assuming the appearance and structure of wood, insomuch
that in old stems, there is scarcely such a thing as pith to be
seen. Herbs and young shrubs, in proportion to their bulk,
have more pith than trees. Many herbaceous stems expand so
rapidly during their early growth that they become hollow,
the pith being torn away by the distension and its remains

6*

62 COMPOUND ORGANS OF PLANTS.

forming a mere lining to the cavity, as in grasses and other
herbs.

A species of Aischynomene, growing in China, has the whole
of its stem, which is about an inch thick, composed of a mass
of pith, covered by a very thin epidermis. Rice-paper is pro-
cured from the herbaceous stem of this plant by the following
process. The centre column of pith is cut spirally round the
axis with a sharp instrument into a thin lamina, which is then
unrolled, and may be made into sheets containing about a foot
square. The medullary sheath and the concentric zones of
wood are traversed by

The Medullary Rays.—These are numerous thin plates of
condensed cellular tissue, which pass from the pith to the cellu-
lar system of the bark, and maintain a communication between
them. These plates of cellular matter are to be seen on
the surface of the cross-section of most exogenous stems,
on which they appear as fine lines, radiating from the centre
to the circumference, but cannot be traced continuously to
any great extent in a vertical direction. The medullary
rays constitute the silver grain of the carpenters. They are
the remains of the cellular system of the stem, condensed into
lines by the adjacent pressure of the woody wedges. The cellu-
lar system of the stem is first formed in a horizontal direction,
and constitutes the matrix, or bed, into which ascends and
descends, in a longitudinal direction, the fibro-vascular or
woody system. The wood of the exogen is, in fact, made up
of a number of wedges of longitudinal fibro-vascular tissue,
embedded in the horizontal cellular tissue of the stem. The
base of each wedge is in contact with the inner surface of the
bark; the apex is next the pith and its sides are bounded by
the medullary rays, which are, as before stated, the remains of

ORGANIZATION OF THE STEM. 63

the horizontal cellular system, condensed by pressure of the
longitudinal wedges into fine lines of cellular matter.

On the whole, the organization of an exogenous stem, con-
sidered collectively, presents three distinct systems, the cortical,
ligneous and medullary system, or the bark, the wood and the
pith; and from the mode of its development, it is evident that
the wood has a constant tendency to solidify itself, and the
bark to destroy itself; hence vitality soon ceases in the former,
and the exfoliation and fall of the different parts of the latter,
first the’epidermis, then the suberous cellules, the cortical pith
and even the liber.

The stem of an old exogenous tree, therefore, consists of a
curious conjunction of dead and living matter, and the rings of
wood not only mark the growths of successive years, but the
number of generations of spontaneously grafted individuals
which the stem has sustained. No part of such a tree is alive
now that was living a few years ago. The leaves have fallen
which the tree then bore, and the nodes from which they
sprung are deeply buried in the interior of the stem, beneath
the wood formed by the generations of buds and leaves that
succeeded them; whilst the living bark that then covered the
stem in immediate contact with the wood has been separated
from it by the internal growth and deposition of other strata
of bark, and is now visible on the outside of the stem in the
form of dead and fissured layers, or else has been thrown off
from its surface altogether. Thus in the coral tree, far beneath
the ocean wave, where mineral matter assumes a vegetable
form, the recent shoots and surface alone are alive, all is dead

along the central axis.

64 COMPOUND ORGANS OF PLANTS.

CHAPTER V.
ON THE DEVELOPMENT OF THE BUDS AND BRANCHES.

Tue stem or aerial portion of the axophyte possesses exelu-
sively a force of lateral expansion, by means of which it
projects into the atmosphere numerous dilated appendages, in
the form of membranaceous expansions of its cells and fibres,
more or less flattened, and of a green color, which are termed
leaves. Certain definite cells of the axophyte appear to have
a natural tendency to this lateral growth, and, therefore, these
leaves are produced symmetrically at certain definite points on
the stem called nodes (nodus, a knot;) so called because these
parts of the stem are internally more solid and compact than
the other parts, in consequence of the vertical fibres of the
stem being interwoven with those which are sent off horizon-
tally into the leaf. These nodes are very conspicuous in the
bamboo, Indian corn, and all plants with hollow stems, which
stems, on examination, will be found to be solid at these
points. The naked intervals of stem between the nodes are
termed internodes.

Before their expansion these leaves, together with the
branches on which they are borne, are enclosed in a particular
organ termed a bud. All branches begin and terminate in a
bud. <A bud is, therefore, clearly an undeveloped branch.

Now the bud, or undeveloped branch or stem, is made up of
a succession of these leaf-bearing points or nodes, the inter-
nodes between which have not been developed, so that these
nodes or leaf-bearing points are brought into close proximity,

BUDS AND BRANCHES.. 65

Fig. 12.

A year’s growth of the horse-chestnut branch, crowned with a terminal bud; a,
scars left by the bud-scales of the previous year; b, leaf-scars, with round dots, show-
ing the points of issue of the fasciculi, or bundles of woody fibre which form the
petiole; ¢, axillary buds, developed at the base of the petiole of the fallen leaves.

and the leaves themselves developed in a rudimentary state,
assume a scale-like appearance, and overlap each other symme-
trically in accordance with their natural arrangement on the
stem. The formation of buds is the natural result of the
cessation of the growth of the internodes, and the partial
development of the leaves at the nodes.

That the scales of buds are leaves in an imperfectly formed
or rudimentary state, is evident from the fact that they are the

66 COMPOUND ORGANS OF PLANTS.

last leaves of the season, developed at a period when the sap
is ceasing to flow, and when the vital powers of plants have
become almost torpid. The transition of scales into the
ordinary leaves of the stem is well seen in the spring, in the
expanding buds of the hickory or horse-chestnut, where the
“gradual passage of one into the other may be distinctly
traced.

Buds originate in the horizontal or cellular system, and may
be distinctly traced in young branches to the pith or medullary
rays, at the extremities of which they are invariably found
when they take a lateral development. This may be easily
verified by making a section through the centre of one of the
lateral buds, at right angles to the surface of the stem, when
the medullary ray will be seen on the surface of the section in
the form of a white line, which, proceeding from the centre of
the bud, traverses the several rings or annual deposits of wood,
and terminates in the pith at the centre of the stem. The
central cellular portion of every bud is therefore in direct
communication with the interior pith of the young shoot by
means of the medullary rays, at the extremities of which they
are formed.

Buds are formed—some in the early part of the summer,
others late in autumn, before the leaves fall from the trees—in
the axilla of the leaves, that is, in the angle formed by the leaf-
stalk and the stem. Examine the branch of any tree before it
has cast its leaves, and you will find at the base of the petiole
or leaf-stalk, the buds for the ensuing year. Hence in
autumn, after the leaves have fallen, these buds remain
attached to the branches.

Linnzeus called buds the hybernaculum or winter residence
of the branch; and the term is very appropriate, because it

BUDS AND BRANCHES, 67

expresses admirably the purposes for which the buds are
formed.

The scales which envelope the bud are clearly designed to
protect the embryo branch and leaves of the next season, which
they surround, against the humidity and cold of winter. They
vary in their texture, external covering, and thickness, in dif-
ferent plants. In the beech and lime tree, the bud scales are
thin and dry; in the willow and magnolia, thick and downy,
and in the horse-chestnut and balsam poplar, they are covered
externally with a plentiful exudation of gummy resin, and
thickly clothed internally with a woolly substance. By this
beautiful provision both wet and cold are effectually excluded.
Plants are most unquestionably a peculiar form of life, and
when we see them thus modifying their organs to escape what
is hurtful to their existence in the air, and constantly availing
themselves in the development of their roots of what is con-
ducive to their growth in the earth, we must admit them to be
somewhat elevated in the scale of nature and very far removed
from the conditions of inorganic matter.

In the Smilax rotundifolia, or common green brier, the
buds are protected through the winter by the dilated and per-
sistent base of the petiole or stalk of the old leaves, which
remains on this shrub throughout the winter and falls away in
the spring. In the Platanus occidentalis, or Plane tree, we
seek in vain for the buds in their ordinary situation, the axils
of the leaves, for they are protected during their growth, and
are concealed within the swollen base of the petiole. This is
well seen in autumn, when, on removing the leaf from the
stem, the base of the stalk is found to form a cap or covering
to the leaf buds.

Buds contain in their interior, in an embryo state, the whole

68 COMPOUND ORGANS OF PLANTS.

plan of the next year’s growth, the nodes and even the leaves
of the future stem. On the approach of winter the vegetable
machinery stops, but there is no disarrangement of its parts,
on the contrary, all is ready in the bud, and awaiting the
stimulus of the returning light and heat.

The young leaves are beautifully folded together in the bud
in such a manner as to occupy the least possible space, the
peculiar mode varying in different plants. The arrangement of
the leaves in the bud is termed their vernation or preefoliation.
Any one can examine it in the spring with the certainty of
being very much interested, by cutting across the leaf-buds
with a sharp knife, when they are swelling and before they
have begun to expand.

On the approach of spring, the leaf buds throw off their
scales, and the leaves which were at first all crowded and
closely packed together in the bud, become separated from
each other by the elongation of their axis of growth, or the
formation of internodes or naked intervals of stem between
them, much after the mode in which the joints of a pocket
telescope are drawn out one after the other; whilst, at the
base, or in the axilla of every leaf-stalk, is seen to form, as the
season advances, buds capable in their turn of being developed
into branches, or a provision for the growth of the ensuing
season.

Now it is the growth of the terminal bud which produces
the elongation of the stem, whilst the development of the
axillary buds produces the branches; and as the arrangement
of axillary buds depends on that of the leaves, in the axils of
which they grow, and as the bud is the germ of the future
branch, it is evident that the development of the branches,
together with all their subsequent ramifications, must follow

BUDS AND BRANCIIES. 69

the same law as that which governs the arrangement and
position of the leaves. If the leaves be opposite, the
branches will be opposite; if the leaves be alternate, the
branches will be alternate; and go on. This symmetrical
arrangement of the branches is interfered with and obscured by
the operation of the following causes :—

The non-development of some of the axillary Luds.—As the
primary plant is only called forth from seed by certain condi-
tions of heat, light and moisture favorable to its development,
without which it remains latent in the seed, so the branches
only protrude from axillary buds when circumstances are
favorable, otherwise the buds remain latent on the stem, and
no branches proceed from them. Now many of the buds in
the axils of the leaves do not grow; because their growth is
checked by the rapid growth of some few leading buds, which
monopolize all the nutriment, leaving them only just sufficient
to carry them forward with the increasing thickness of the stem,
and to maintain their position on its surface, where they remain
ready for action in case the growth of the other buds is checked
by untimely frost, or other causes. In this manner, trees,
whose young and tender foliage and branches has sustained
injury by the cold in early spring, soon become re-clothed with
verdure. On this principle, also, trees are pruned and trained
against walls, or other supports. Certain leading shoots and
buds are cut, in order that the supply of sap they were mono-
polizing may flow to certain lateral and latent buds, and cause
their growth in the proper direction. In general the sap has
tendency to rise in greater force and abundance towards the
extremity of the branches,.the result of this is that the inferior
leaves are the first to become detached from the branches, and
their buds not receiving enough nourishment to bring them to

i

(

70 COMPOUND ORGANS OF PLANTS.

a perfect state, become abortive or incompletely developed. It
is in fact almost always the inferior buds which are thus reduced
to a rudimentary condition. The light does not get access to
them so freely as to the buds towards the summit of the
branches, and hence the lower part of the branches is generally
naked and deprived of branchlets. The symmetrical arrange-
ment of the branches with the leaves is also prevented.

By the growth of adventitious or irregular buds, that is to say,
of buds which come in parts of the stem, between the leaves, and
not in their place in the axils of the leaves. Sometimes, owing
to the growth of the leading buds, the growth of the latent
axillary buds is checked altogether, in which case they sink
beneath the surface of the stem, and are buried beneath the
succeeding layers of wood; but their vitality is not destroyed
so long as they remain at a certain depth in the stem, that is
to say, in the alburnum or sap-wood. The trunks and branches
of trees, therefore, always contain an immense number of these
buried buds, and should some of the leading branches be broken
off by high winds, or sustain injuries from other causes of this
character, then the flow of sap to them becomes so powerful
that they will force their way through the wood to the surface,
although that wood be the successive growths of years, and
break forth into branches. All must be familiar with the
sight of willows and other trees, whose main branches have
been thus broken, and whose trunks have, nevertheless, been
covered with young branches and shoots, the growth of buds
which have been buried in their wood, and for years dormant
beneath their surface.

From these facts it is plain that those forms of life which we
call plants, although rooted to the soil, and more exposed by
this circumstance than any other living being, are nevertheless

BUDS AND BRANCIIES. val

far from being destitute of a power to escape. It is true that
they are exposed to the inclemency of the season, and are
threatened with destruction on every side, but so powerful and
varied are the defences with which nature has furnished them,
that they seem to be all but indestructible. How innumerable
are the buds with which a tree is covered! How complete
their protective apparatus against the winter’s cold!, We have
seen that each bud, although it remains in union with the
parent tree, is nevertheless capable of forming the germ of an
independent life. If not developed, it only awaits the destruc-
tion of its associates to enter the breach, repair the injury, and
continue by its growth the battle of the living principle in the
plant against the hostile forces of nature. Endowed with such
powers of defence, a tree will grow and lift its majestic and
massive stem for centuries to the air and light of heaven, and
if after thus long and bravely conflicting with nature, it should
be finally prostrated by the power of the tempest, if its con-
nection with the soil still continues, the reserved and buried
buds of other years shal] issue forth a new phalanx of defence,

and renew successfully the struggle of the plant for life.
e

72 COMPOUND ORGANS OF PLANTS.

CHAPTER VI.
THE LEAVES. .

LEAVES are contrivances by which the green absorbent sur-
face of the plant is increased, so that the greatest practicable
amount of food is taken from the air. The entire structure of
the leaf proves it to be put forth for this purpose.

The leaf is simply an expansion of the green cellular bark
of the young shoot, and is formed by the spread of the woody
fibre which issues from its side, carrying with it at the same
time the bark, which thus becomes expanded horizontally, to
the air and light of heaven.

When the leaf is fully developed it consists of two parts,
viz. : the expanded portion called the lamina or blade, and its
little stalk or support, which is termed the petiole. Some-
times, however, the petiole is wholly absent from the leaf, the
spread of the woody fibre, together with the expansion of the
green young bark of the young shoot, taking place at its sur-
face. In this case the leaf is said to be sessile. So also this
expansion of bark does not always take place at a single point
of the stem, but is extended down the stem a little and then
spreads out horizontally, producing a decurrent leaf. The
leaves of the Verbascum thapsus, or common mullein, are of
this description. Occasionally, as in the orange, the bark of
the petiole itself shows this tendency to expansion, when the
petiole is said to be winged. Most frequently, however, dis-
tinct fasciculi or bundles of woody fibre and spiral vessels
emerge from the side of the shoot, unite and form a petiole,

THE LEAVES. 73

and then diverge at some distance from the stem, forming the
expanded lamina of the leaf. The points of the stem from
which these fasciculi have issued are apparent on the scars left
by the fallen leaf stalks in the form of round dots, of a uniform
number and arrangement in each species of plant. Thus in
the apple, the pear, and the peach, the leaf is attached to the
stem by three fasciculi or bundles of woody fibre, and three
round dots may be distinctly seen on the leaf scar; and in the
horse-chestnut, from five to seven dots are visible on the leaf
scar, the number of fasciculi passing out of the stem and
uniting in the petiole being the same as the number of the
leaflets.

~The vascular or woody system which passes out of the
stem into the leaf is clearly designed to give to it the needful
strength and support, as well as to convey the sap to be aerated
in the leaf. This part of the leaf evidently constitutes its
framework or skeleton. The vascular and woody system in
exogenous leaves, as for example, that of the Cornus florida, or
Flowering dogwood, consists of a distinct midrib or keel, and
less elevated ribs, (costz,) which proceed from the sides of the
midrib and take a curvilinear direction to the margin and
apex of the leaf. On closer investigation the coste are seen
to communicate with each other by means of small transverse
fibres, which again branch and subdivide in various ways, the
last ramification or branchlets running together or anastomosing
amongst themselves, and the whole forming a delicate and
beautiful network.

Seen through the microscope, this vascular framework is found
to consist of woody fibre enclosing spiral vessels. This is its
constitution, from the main fasciculus or bundle of woody fibre
called the midrib or keel of the leaf, through the several fasci-

Tk

74 COMPOUND ORGANS OF PLANTS.

culi or costes which proceed from the sides of the midrib, each
fasciculus consisting of woody fibre enclosing spiral vessels
throughout all its ramifications.

The cellular system of the leaf—This substance forms its
principal part, filling up the meshes in the network, formed by
the vascular system. To the naked eye it appears as a struc-
tureless pulpy mass of a green color, called parenchyma (zapo,
beside or between, and yevuo, anything effused or spread out.)
Under the microscope the parenchyma of the leaf no longer
appears as an unformed mass, but as a beautiful and regular
arrangement of cells, which are so disposed as most effectively
to subserve those purposes of nutrition for which the leaf is
formed.

In all leaves which present one surface to the sky and the
other to the ground, there is between the upper and under
cuticle two strata of parenchyma differently arranged. In the
upper stratum of parenchyma, the cells are arranged in one or
more compact layers, vertically, or at right angles to the upper
surface of the leaf, so that they present the least possible

Fig. 13.

Fig. 13. Magnified view of the edge of a leaf. The parenchymais alone represented,
the woody tirsue being left out. @ and b, shuw the epidermis and denser parenchyma
of the upper surfuce of tho leaf; c,d, the looser parenchyma and epidermis cf its
Jower surface.

TIE LEAVES. 75

amount of surface to the sun; whilst, in the lower stratum of
parenchyma, the cells are arranged horizontally, having amongst
them numerous intercellular passages, or cavities filled with air,
which communicate freely with each other through the sub-
stance of the leaf, and with the external air by means of the
stomata or pores in the epidermis.

The dense parenchyma of the upper surface of the leaf
accounts for its deeper tint, and is well adapted to restrain
the excessive evaporation to which the fluids in the upper
stratum of cells are liable, by their direct exposure to the sun;
whilst the loose parenchyma of the lower surface is the cause
of the lighter tint of the underside of the leaf, which, together
with the pores of the cuticle, is well calculated to give the air
free access to all parts of the leaf, from which source plants
derive the greater part of their nutriment. Leaves growing
erect,possess uniformity of structure in both strata of paren-
chyma.

The vegetable membrane which forms the walls of the
cells of the parenchyma, is perfectly white and colorless.
The green color of the leaf is found to be caused by the forma-
tion of granules of green matter in the cells, which either float
free in the sap contained in their cavities, or else collect into
grains and adhere to the walls or sides of the cells. This sub-
stance is called chlorophyl (xkopos, green, and pvaaror, a leaf), in
contradistinction to chromule (zp2ya, a color), which is the
term employed by botanists to designate the colored substances
with which the cells of flowers are filled.

Chlorophyl appears to belong ‘to the class of waxy bodies.
It is soluble in alcohol or ether, but not in water, and is
developed only in those cells which are exposed to the action
of light. It is therefore only formed in the superficial strata

76 COMPOUND ORGANS OF PLANTS.

of cells. Chlorophyl is not developed in the external investing
layer of cells called the epidermis, nor in the woody fibre
through which the crude sap circulates, both the epidermis
and woody fibre being from this cause white and transparent ;
but it is formed in the superficial strata of cells immediately
beneath the epidermis, and gives to the leaves and young
shoots their green hues.

That the chlorophyl, or green matter in plants, is produced
by the effect of light, is evident from the fact that it is decom-
posed and disappears when plants are made to grow in the
dark. The celery served at table is blanched or rendered
white by covering the stems with earth, so that the light
cannot gain access to them; and for the same reason, plants
exposed to the full sunshine have a deeper tint than those
which grow in the shade.

The epidermal system of the leaves, together with the val-
vular action of their pores has been already described, (page
22,) and we have seen how beautifully their stomata control.
the evaporation from their surface. But these organs have
other uses. They are the instruments by which the leaves
communicate directly with atmosphere, and by which vegetable
breathing or respiration is carried on. Vegetables respire as
well as animals, and the sap of plants which is analogous to
the blood of animals, must be brought into contact with the
atmosphere, like the blood, and be thoroughly aerated in the
leaves, before it can be converted into nutritive fluid.

Bonnet was the first who observed that leaves, when plunged
into water and exposed to the action of sunlight, disengaged
gas. He also found by experiment, that the same amount of
gas was evolved when the leaves were immersed in water
which had been previously boiled, and therefore completely

-

THE LEAVES. we

deprived of its air. The gas was therefore clearly evolved by
the leaves. Priestly recognized this gas to be oxygen, and
Ingenhouse showed that light was indispensable to its manifes-
tation, singe it ceased to evolve itself from the leaves in dark-
ness. Such was the state of the question when Sennebier fully
demonstrated by experiment, that the oxygen evolved from the
leaves was the result of their decomposition of the carbonic
acid which was contained in them.

This carbonic acid is chiefly abstracted from the atmosphere
by means of the stomata or pores of the leaves. very onc
must be aware that neither plants nor animals could live with-
out air, and if they both lived on the same air, the atmosphere
would soon become unfit for respiration. But the air taken
into the lungs of animals in the act of inspiration, imparts its
oxygen to the dark venous blood in the lungs, which combining
with the carbon of the blood forms carbonic acid; this gas is
expelled from the lungs in the act of expiration (ex, out, spiro,
to breathe.) The blood thus oxygenated by breathing, loses
its dark color and is changed into that bright red arterial
stream which again circulates through the system for its nutri-
tion. Now the atmosphere would soon be thoroughly poisoned
by animals, but for the purifying influence exerted on it by
the vegetable creation. Carbonic acid, CO2, which is com-
posed of one equivalent of carbon and two equivalents of
oxygen, is taken into the plant through the pores of its leaves,
and under the influence of solar light decomposed, the plant
fixing the carbon, which when thus assimilated, forms the chlo-
rophyl or green matter in them; whilst the oxygen is set free
and escapes into the atmosphere, which gas is the food of
animals.

This inspiration of the carbonic acid of the atmosphere,

78 COMPOUND ORGANS OF PLANTS.

together with the assimilation of the carbon and the expira-
tion of the oxygen, constitutes what may be truly denominated
vegetable breathing or respiration.

These results take place only when the plant is exposed to
the direct rays of the sun. Recent experiments have shown
that the process ceases when the sun is behind the clouds, and
that not only during the night, but even under the influence
of diffused daylight, the exhalation of oxygen is stopped.

The exhalation of oxygen gas from the leaves of plants is
the only provision that we know of for keeping up its supply
in the atmosphere. The prevailing chemical tendencies are to
take oxygen from the air. Were it not for the copious sup-
plies of this gas poured into the atmosphere from the pores of
plants, animal life could not exist. Hence the perfect adap-
tation of the two kingdoms of nature, each removing from the
atmosphere what would be noxious to the other, each yielding
to the atmosphere what is essential to the life of the other.

To show that plants give out oxygen in sunshine. Fill a
jar with water, and invert it in a vessel containing the same
fluid. Introduce beneath the jar a sprig of mint, or any other
living plant. After a while bubbles of gas will collect on the
leaves and ascend to the top of the jar, displacing the water.
If the air thus collected be tested, it will be found to be pure
oxygen gas. If the vessel be placed in the shade, the bubbles
of gas will disappear from the leaves.

The leaves are not the only organs of vegetable respiration.
The young branches, the scales, in a word, all the herbaceous
and green parts of plants act on the atmosphere in a similar
manner to the leaves. They take in carbonic acid from the
atmosphere, assimilate the carbon, and give out the oxygen.

Form of Leaves.—It has been stated that the leaf of a plant

THE LEAVES. 79

is simply an expansion of the wood and bark of its stem, the
wood issuing from the side of the shoot whilst in its green
young state in fibrous bundles, which carry with them at the
same time the green cellular bark of the shoot, and then by
their expansion spread it out to the air and light of heaven.
There must, therefore, be a natural adaptation and corres-
pondence between the spread of the woody fibre which
constitutes the framework of the leaf, and the peculiarities
of its form. This idea was first suggested by Decandolle.
According to him, the shape of leaves depends on the mode
in which the fibres diverge when they leave the side of the
shoot, and upon the quantity of parenchyma or bark which
they carry with them; and by him this arbitrary nomenclature
of form was rendered intelligible and reduced to something
like system based on scientific principles. Decandolle distin-
guishes three principal modes in the venation of leaves, viz. :
the net-veined, the parallel-veined, and the fork-veined.

1. The reticulated or net-veined leaves are characteristic of
exogens, which are justly regarded as the most highly organ-
ized plants in the vegetable world. Two modifications of net-
veined structure have been observed, the feather-veined and
radiate-veined; the leaves of the chestnut are good examples
of the former, and those of the garden nasturtium of the latter.
The margins of net-veined or exogenous leaves are very seldom
entire, but most frequently notched in various ways, described
in books as dentate or toothed, crenate or scolloped, serrate or
having teeth like a saw, of which last we have a good example
in the leaf of the rose. The cause of these incisions has not
been clearly ascertained.

2. The parallel-veined leaves are the distinguishing feature
of endogens, which are considered humbler in their organic

4

80 CQMPOUND ORGANS OF PLANTS.

structure. That nature has been less elaborate in their forma-
tion, will be evident to any one who will only take the trouble
to compare a lily leaf with that of a rose. If held up to the
light, the intricate and highly complex ramifications of the
fibrous structure of the exogenous leaf of the rose will be seen
in striking contrast with the extreme simplicity of the endo-
genous leaf of the lily. Two different modes of venation have
algo. been noted in endogens, the curve-veined and the straight-
veined. In the first instance, the veins run in parallel curves
from the base to the apex of the leaf, and in the other case
proceed in right lines. The plantain and Hemerocallis or
day-lily, are good examples of the first, and grasses of the last
method of venation. The margin of endogenous leaves is inva-
riably entire, and never marked with indentations of any kind.

8. The fork-veined leaves, which are peculiar to ferns, plauts
still lower in the scale of organization. It may be proper to
qualify these divisions and sub-divisions, by remarking that
they are not intended accurately to define the boundarics
between the different modes of venation. There is an
approach to the forked method of venation in some exo-
genous plants, as in clover, and doubtless there are many
intermediate forms. All classification is but an approxima-
tion to that order which obtains in nature. All that Decan-
dolle intended, was to point out some of the principal modes
in which the woody matter of leaves was distributed through
their parenchyma, and to call attention to the fact that the
variety of their form is the result of one or the other of
these modes of distribution. The student will now under-
stand that leaves assume the linear, lanceolate, ovate or
orbicular form, according to the greater or less degree of
divergence of the woody fibre constituting thcir framework.

THE LEAVES. 81

Simplicity in causes and variety in effects mark all the opera-
tions of nature !

The distribution of leaves about the stem.—All who notice
plants much have frequently observed the regularity and sym-
metry with which leaves are arranged around the stem. Some-
times they spring from its sides in pairs, crossing each other at
right angles, as in the mint family, or in beautiful whorls, as
in the Galium or bedstraw tribe; and again, they are scattered
along the stem on either side, but still with an apparent regu-
larity, and certainly not at random. ‘These peculiarities of
their distribution are produced by a combination of the two
following causes.

1. The manner in which the stem grows. If the elongation
of the stem and the growth of the leaves be simultaneous, the
leaves will be scattered on all sides of the stem, and will be
few or numerous, according to the greater or less degree of
rapidity with which they are developed; but if the elongation
of the stem is periodically checked, and the growth of the
leaves at the same time continues, they will neccssarily start
out from the same point of the stem in pairs or in whorls,
according to the length of time taken up before the stem
again elongates. This is well seen in Lysimachia quadrifolia,
which in ordinary circumstances bears whorls of four and six
leaves ; these, when the growth of the stem is rapid, become
alternate. We have also instances of the operation of this law
in the Conifere, or pine family. The Larch has leaves de-
veloped in fascicles or bundles. These leaves are without any
lamina or blade, rigid and needle-shaped or linear. They are
brought together in consequence of their rapid development
and the non-elongation of their axis of growth. That this is
really the cause of their fascicled character, is evident on close

8

82 COMPOUND ORGANS OF PLANTS.

inspection of the young shoots of the larch, which by their
rapid growth do not admit of any fascicular development of
their leaves. On these shoots the young leaves of the larch
will be found to be scattered, not fascicled, clearly showing
their natural arrangement, and proving that the fascicles are
the result of the development of the leaves and the non-de-
velopment of the nodes of the stem.

The spiral growth of the leaves. This is most readily
perceived in such plants as have their leaves distributed
alternately on either side of the stem. If a thread be wound
about the stem so as to touch the basis of the first, second,
third, fourth, and succeeding leaves, it will be found to de-
scribe an ascending spiral around the stem, and with such
accuracy that the law may be expressed numerically. The
observations of Dr. Gray, on leaf arrangement, are too
interesting to be omitted in this place. If we write down
in order the series of fractions which represent the simpler
forms of leaf ae as determined by observation, viz. :
3, 4) 3) & Per vy 38) We can hardly fail to perceive the
relation that they bear to each other. For the numerator of
each is composed of the sum of the numerators of the two
preceding fractions, and the denominator of the sum of the two
preceding denominators. Also, the numerator of each fraction
is the denominator of the next but one preceding. We may
carry out the series by spplying this poi law, when we
obtain the farther terms, #3, 9%, 13, 21, &. Now these
numbers are those which are actually verified from observation,
and, with some abnormal exceptions, this series comprises all
the cases that occur.”

That the interest which attaches to the above extract may
be fully appreciated, I remark, that the fractions severally

THE LEAVES. 83

represent different kinds of spirals, the numerator denoting the
number of times that the thread winds round the stem before
it touches the base of a leaf directly over the one it began
with, whilst the denominator expresses the number of leaves
it touches in its course, before it arrives at that leaf thus
situated. Thus the fraction ? denotes that the thread winds
twice round the stem, powering the bases of five leaves in its
course ; consequently, the oe leaf stands directly over the
first.

But the most curious and wonderful thing is that the higher
fractions 5,, 8, &c., as developed by the application of this
numerical law, are positively realized in nature. For the
same principle of arrangement extends to all those parts of
plants which are modifications of leaves, and these numbers
are actually verified when we come to examine the rosettes of
the houseleek, and the scales of pine cones. It is the com-
bination of both these causes, the tendency to spiral develop-
ment, combined with the peculiarities of stem growth, which
disposes the leaves of plants with so much regularity and
symmetrical beauty around their stems.

Leaves sometimes assume very curious forms.—Sometimes
the lamina or thin expanded portion of the leaf becomes nearly
or altogether abortive, and the petiole itself assumes a leaf-like
appearance. This modification of structure is termed a phyllo-
dium. The leaves of the New Holland acacias are all more
or less formed into phyllodia. These plants have compound
pinnate leaves, and just in proportion as the pinnze of the limb
are suppressed, is their petiole expanded and leaf-like. In
young acacias, and occasionally in old ones which have been
freely pruned, all the intermediate states between a compound
pinnate leaf and a simply expanded petiole may be observed.

84 COMPOUND ORGANS OF PLANTS.

Decandolle considers that the sheathing leaves of endogenous
plants which are not furnished with a distinct limb, are only
expanded petioles. The leaves of the Hyacinth, and Iris
versicolor, or common blue flag of the pools, are of this nature.
Such leaves are sometimes met with even in the higher order
of exogenous plants, as, for instance, Ranunculus flammula or
Spearwort crowfoot, a common aquatic plant.

Sometimes the edges of the lamina or blade cohere together,
producing still stranger modifications of leaf structure. It is
well known that the parts of plants which grow closely
together, are apt to become coherent. Accidental unions of
this kind amongst the leaves of plants are of common occur-
rence. In some species these unions occur after the plant is
considerably grown, as in the garden honeysuckle, the upper
leaves of which usually cohere together by their bases, owing
to their sessile character, and form what botanists call a connate
leaf. So also, the numerously crowded and closely compact
leaves, constituting the calyx or cup of the Marygold or Holly-
hock flowers, will be found to be more or less united with each
other. Jn other instances, where the cohesion of the leaves
with each other or with the stem is of constant occurrence in
every stage of vegetable development, this union appears to
take place at a much earlier period. In this case, whilst the
plant lies folded up within the seed and its texture is yet deli-
cate, the numerous vessels of its organs which are thus brought
into close contact anastomose ; that is, run together or unite with
one another by means of the elaborated juices which nourish
them, thus producing those cohesions of the parts of plants which
are visible in their after developments.

If these views be correct, they will serve to explain the
nature of the hollow leaves of Sarracenia purpurea, or the side

THE LEAVES. 85

saddle flower, with its leafy cups half filled with water and
dead insects, which abounds in the bogs of the Northern and
Middle States. This pitcher may be conceived to be formed
by the cohesion of the edges of a partly formed phyllodium.
If we imagine a dilated petiole with its partially formed lamina
to curve over and unite at its edges, a leaf like that of the
Sarracenia will evidently be formed, in which the pitcher will
be simply a hollow petiole, whilst the hood at its summit is
produced by its abortive lamina or blade.

In Utricularia, or bladderwort, the leaves form sacs galled
ampulla, which are filled with air, and float the plant in the
water at the time of flowering.

But the most remarkable case of leaf cohesion, is seen in the
Nepenthes distillatoria, or pitcher plant of the Hast Indies. In
this instance, the petiole when it first leaves the side of the stem,
isround, or of its usual shape, then it expands into a leaf-like organ
or phyllodium, and next it is contracted into a tendril, finally it
formsinto a phyllodium, the sides of which cohere together so as to
form a pitcher, which is surmounted at the summit by the
abortive lamina or blade, in the shape of a lid. This pitcher
is constantly filled with about half a pint of pure water, which
is not collected from without, as in the Sarracenia, but is
secreted by the plant: for the lid surmounting their summit
constantly and accurately closes the orifice of these pitchers,
and their internal surface is of a glandular structure. In
Ceylon, where this plant is common, it is called by the natives
by a word the signification of which is monkey cup; because
these cunning animals when thirsty, and there is no stream at
hand, open thelid and drink the contents. Men also travelling
or hunting in the woods, often find the water contained in these
vegetable pitchers a means of assuaging their thirst.

gx

86 COMPOUND ORGANS OF PLANTS.

The fall of the leaf.—There is no subject on which botanists
have entertained a greater variety of opinion than on the fall
of leaves. The causes which produce their excision from the
stems and branches of plants are so exceedingly complicated,
that a much more advanced condition of botanical science seems
to be necessary before they will be clearly and accurately
understood. It is obvious that leaves are thrown off by plants
because they are no longer of any service to them, and the
means by which nature effects their separation are truly won-
derful, and at the same time instructive.

The causes which produce the decay and fall of leaves are
partly chemical and mechanical. The water which enters the
roots of plants as it percolates the soil, dissolves a small portion
of earthy matter. This is partly deposited in the woody and
fibrous tissues of the stem, but principally in the cellular tissue
of the leaves, by the evaporation which is continually taking
place at their surface. In this manner the interior walls of the
leaf cells become encrusted or thickened by deposits of mineral
matter, just as earthy matter accumulates at the bottom of a
pot used for culinary purposes, and the leaf is thus rendered
finally unfit for the performance of its functions. The mineral
matter deposited in the cells is sometimes beautifully crystal-
lized, the earths or bases taken up by the roots uniting with the
acids formed in the vegetable organs. The most common kinds
of crystals are those of the carbonate and oxalate of lime which
are of different sizes and forms, rhomboidal, cubical and pris-
matic; but the most prevalent form is the acicular or needle-
shaped. It is to this form that the term raphides (raphis a
needle) was originally applied by Decandolle, although it is
now used indiscriminately in reference to all cellular crystals.

In the autumnal months, the light becomes less powerful, the

THE LEAVES. 87

leaves lose their green color, and their cells becoming gradually
and entirely choked up with mineral matter, the sap no longer
circulates through them. They absorb oxygen from the air,
and the result of their different degrees of oxidation is seen in
all that variety of autumnal tint, which casts such a charm over
the dying landscape.

Whilst these chemical changes are taking place, nature is at
the same time preparing to effect the mechanical excision of the
leaf from the plant.

Now, at first, all leaves are contiguous with the stem. As
they grow, an interruption of their tissue takes place at the
base of their footstalk, by means of which a more or less com-
plete articulation is formed. This articulation is produced by
the continuation of the growth of the stem after the leaf has
attained its full growth, which it generally does in a few weeks.
The growth of the leaf being completed, all its functions lan-
guish in consequence of the increased deposition of mineral
matter within its cells, and the base of the petiole or footstalk
being no longer able to adapt itself to the increasing diameter
of the stem, a fracture between the base and stem necessarily
ensues; the excision advances from without inwards, until it
finally reaches the bundles of woody fibre, which are the main
support of the leaf.

Whilst, however, nature is forming a wound, she is at the
same time making provision to heal the same; for the cuticle
or epidermis of the stem is seen to grow over the surface of
the scar, so that when the leaf is detached the tree does not
suffer from the effects of an open wound. The provision for
separation being thus completed the leaf is detached by the
growth of the bud at its base, by the force of the winds, or
even by its own weight. Such is the philosophy of the fall of

88 COMPOUND ORGANS OF PLANTS.

leaves, and we cannot help admiring the interesting and won-
derful provision by which nature heals the wounds even before
they are absolutely made, and affords a safe covering from
atmospheric changes before the parts can be subject to them.

The decay and fall of leaves is, therefore, not the result of
frost, as is commonly supposed, for leaves begin to languish
and change color (as happens with the red maple, especially,)
and even fall, often before the autumnal frosts make their
appearance, and when vegetation is destroyed by frost the
leaves blacken and wither but remain attached to the stem;
but the death and fall of the leaf is produced by a regular
vital process, which commences with the first formation of this
organ, and is completed only when it is no longer uscful.
There is no denying, however, that the frosts of autumn, by
suddenly contracting the tissues at the base of the petiole,
accelerate the fall of leaves. All must have noticed,-on a
frosty morning in autumn, that the slightest breath of air
moving amongst the decayed and dying leaves, will bring them
in complete showers from the trees to the ground.

In general, we may say, that the duration of life in leaves is
inversely as the force of the evaporation which takes place
from their surface. For we find that the leaves of herbaceous
plants, or of trees which evaporate a great deal, fall before the
end of the year, whilst the leaves of succulent plants, or of
evergreens, which latter are of a hard and leathery texture, and
evaporate but little, often last for several years. In pines,
firs, and evergreen trees and shrubs, there is an annual fall of
leaves in the spring of the year whilst the growth of the
season is taking place; but as this leaf-fall is only partial, con-
sisting of one-half or one-third at a time, there is always a
sufficient number left on such trees to keep them clothed with

NATURE AND SOURCES OF FOOD, 89

perpetual verdure. Hence it is, that the entire foliage of such
trees consists of leaves which have been attached to the stem
from one to three or five successive years.

In the beech and hornbeam, the leaves wither in autumn,
and hang on the branches in a dead state through the winter.
Such leaves, when examined, will be found to be contiguous
with the stem at the base of their petiole, and therefore with-
out that articulation or joint which so materially aids in the
disruption of the leaf from the stem. These dead leaves fall
off when the new leaves expand in the spring.

Most of the trees of this country have deciduous leaves, and
in winter our woods are bare and no longer cast their shadows
on the earth ; but the forests of tropical climates are evergreen,
and usually retain the same appearance throughout the year.
A perpetual shade is thus afforded by nature, which in some
measure gives relief against the continuous heat of these
regions.

¢

CHAPTER VII.

ON THE NATURE AND SOURCES OF THE FOOD ASSIMILATED
BY PLANTS.

THE investigation of the nature and sources of those sub-
stances assimilated by the nutritive organs of plants, is neces-
sary to a clear understanding of their physiological action, and
will very properly close this part of the subject.

These substances can only be determined by chemical analy-
sis. Plants have been examined chemically by Liebig, Mulder,

90 COMPOUND ORGANS OF PLANTS.

and Johnson, and we are about to lay before the student the
results of the labors of these philosophers.

The solid part of plants, chemically considered, is found to
consist of organic and inorganic matter; the first may be burnt
away and is derived from the atmosphere, the second is incom-
bustible and is derived from the soil in which the plant grows.

* To show the organic and inorganic matter in plants. Burn
a piece of wood or straw.in the flame of a lamp. The part
which burns is organic matter and passes again into the atmo-
sphere from whence it was taken; the incombustible ash that
remains is the inorganic matter in the plant, which was derived
from the soil.

The organic part of plants is composed of four substances,
carbon, or charcoal, more than one-half, oxygen one-third,
hydrogen one-twentieth, and nitrogen one-fiftieth.

The inorganic part of plants, or the ash remaining after the
combustion of the organic matter in them, consists of no less
than eleven different substances, viz: potash, soda, lime, mag-
nesia, silica, oxide of iron, oxide of manganese, sulphur, sul-
phuric acid, phosphoric acid, and chlorine. .

The carbon or charcoal in plants composes more than one-
half of their entire bulk. If a green leaf or a piece of wood
be charred (which may be done by heating it in a close vessel
out of contact with the air,) all the hydrogen and oxygen in
the plant will be driven off, and what remains will be the
amount of carbon in the plant, together with a small per-
centage of inorganic matter. The leaf or specimen of wood
which has been thus carbonized will be found to preserve its
form and bulk uninjured, even to that of the most delicate
cells and vessels, but will be considerably lighter. A piece oi
common stove charcoal is a beautiful instance of wood which

NATURE AND SOURCES OF FOOD. 91

has been thus treated, and evinces that charcoal is the principal
constituent in the material out of which a plant is constructed.

The carbon found in plants is derived from the atmosphere
and from the decomposing vegetable matter in the soil. It has
been shown how plants take in carbon from the atmosphere,
through the pores of their leaves, in the form of carbonic acid
gas. But the atmosphere is not the only source ; the soil also
contains an immense quantity, for carbonic acid is given off not
only by the lungs of animals, but by burning bodies, and by
the decaying animal and vegetable matter in the soil.

When we burn a plant and thus effect a separation between
its organic and inorganic constituents, restoring the former
to the atmosphere and isolating the latter under the form of
ashes, the process of combustion is only the result of the rapid
union of the oxygen of the air with the carbon in the leaf, and
the consequent formation of carbonic acid gas. Now precisely
the same process occurs in nature when plants decay and disap-
pear from the earth’s surface.

The decay of vegetable bodies in the soil, as Liebig has
shown, is only a slower process of combustion, being produced
by precisely the same cause, viz., the union of the oxygen of
the air with the carbon in the plant, with the consequent pro-
duction of carbonic acid gas.

Hence we see the reason why wood when it gradually
decays becomes brown and ultimately black, presenting pre-
cisely the same appearance as if it had been burnt with fire.

In the process of decay, or as it is termed chemically,
eremacausis, that is slow burning, the oxidation of the vege-
table is so slow that neither heat nor light is evolved ;. hence
the products of ‘the vegetable decomposition are aqueous as
well as gaseous, or the body, popularly speaking, putrefies.

92 CUMPOUND ORGANS OF PLANTS.

To this decaying animal and vegetable matter the term humus
is applied. It constitutes the brown or black portion of every
soil. Wherever it exists, there plants spring up the most readily,
whilst in places devoid of it, they are stunted and dwarfed in
their growth and decidedly inferior both in organization and
beauty. Thus, though carbonic acid is principally absorbed
from the air by the leaves, the roots of plants also find it in
every soil which contains humus; for humus consists in
decaying organic matter, that is, organic matter resolving itself
by a sort of slow combustion into carbonic acid and water.

Carbonic acid makes up, on the average, only one two-
thousandth part of the bulk of the atmosphere. It is,
however, very soluble in water, and its accumulation in the
air like that of ammonia is mainly prevented by the rains
which greedily absorb and wash it down to the earth, from
whence it is imbibed by the root. In this manner carbonic
acid enters the system of the plant by the roots as well as by
the leaves. ;

Hydrogen and the greater part of the oxygen enter the
plant by the roots in the form of water (H O), which consists
of these two gases in chemical union. These two gases indis-
solubly bound together in the form of water, which circulating
through nature on entering the system of plants, is neverthe-
less readily decomposed by the powers of vitality.

Nitrogen enters by the roots chiefly in the form of nitric
acid and ammonia. The former is produced during the passage
of electricity through the air; the latter is copiously evolved
from compost heaps and from decaying vegetable and animal
matter.

To test the presence of ammonia in the compost heap. Dip
a glass tube in hydrochloric acid (spirit of salts) and hold it

NATURE AND SOURCES OF FOOD. 93

over the heap. If-ammonia be present, copious white fumes
will be perceived, which result from the chemical union of the
hydrochloric acid gas with the ammoniacal gas, and the forma-
tion of a salt, the hydrochlorate of ammonia, or the sal
ammoniac of the stores.

Although ammonia is constantly rising in vast quantities
into the atmosphere from decaying animal and vegetable
matter, it is nevertheless easily soluble in water, and is there-
fore prevented from accumulating there by the aqueous vapor
of the atmosphere, which, when it is precipitated thence in the
form of rain, conveys the ammonia in solution to the roots of
plants. That this is the fact is evident because ammonia can
be detected in rain water and in the sap of plants, and also
because all manures such as guano, which contain a great
amount of ammonia, are found to be fertilizing to soils.

The combustible or organic part of the plant forms by far
the greater part of its structure. This is evident’ from the
small amount of ash or inorganic matter left after its incinera-
tion. It follows that plants derive the materials of their
growth mainly from the atmosphere.

That certain plants derive the greater part of their food
from the atmosphere, affords an explanation of the process by
which nature changes the barren rock into the fertile soil.
The first plants which clothe the surface of the newly formed
coral reef, or of our common rocks, are lichens and mosses;
plants which derive the greater part, if not the whole of their
nutriment, entirely from the atmosphere. Now plants can only
grow in proportion to the quantities of food afforded them.
Lichens and mosses are plants of very humble growth and
exceedingly simple structure, consisting of, comparatively
speaking, only a few cells. Successive generations of these

9

94 COMPOUND ORGANS OF PLANTS.

atmospheric cryptogamia flourish and die, forming a humus
for the growth of grasses, ferns, and more highly organized
plants; until at length there is formed on the surface of that
once barren rock a sufficiency of humus for the nutrition of all
the varieties of vegetable organization found in the fertile
meadow, the tangled thicket, and the widely extended forest.
Finally man comes to take possession of the new domain
which nature has thus been carefully preparing for him, and
life reaches its highest stage of development.

The inorganic matter constituting the ash which remains after
the combustion of the plant, is wholly absorbed from the soil,
and enters the plant in a state of solution by the pores of the
roots. Some persons have supposed that these mineral matters
were produced by the plants themselves, and not derived from
without. It is true that the earths, such as silica or sand,
alumina or clay, are insoluble by themselves in water, and that
the subdivision of the matter of which they are composed must
be carried to an almost infinite degree of minuteness, before
they can pass into the system of the plant through the minute
pores of the roots; but all the earths are soluble with the
alkalies, such as potash, which enters largely into the composi-
tion of all rocks, and as these earths are furnished to the soil
by the slow decomposition or disintegration of rocks, there can
be no doubt that the water, as it percolates the soil impreg-
nated with potash and carbonic acid, effects their solution to
such an extent that they pass unimpeded into the system of
the plant along with the water which is imbibed by the root.

Each species of plant, according to its peculiar constitution,
retains a greater or less amount of one or more of these earthy
ingredients. Thus, nearly all plants retain a quantity of
potash; wheat, a certain amount of silex; some aquatic plants

&

NATURE AND SOURCES OF FOOD. 95

accumulate iron so that on décaying they leave a sediment of
iron rust in the water; chlorite is found in all marine plants;
phosphorus in the onion; and sulphur in mustard seed, in
celery, and in ginger. The immense quantities of water vari-
ously impregnated with these foreign bodies, which pass
through a plant, being condensed by evaporation in the leaves,
is sufficient to account for their presence, in appreciable
quantities in the plant, however minute may be their propor-
tion in the water which the roots imbibe. Hence it is found
that plants will not grow in distilled water, or water freed from
all foreign ingredients; and also that the water exhaled by”
plants is so pure that not a trace of foreign matter is discover-
able in it; the stomata or pores of the leaves are in fact the
most perfect stills in the great laboratory of nature. About
two-thirds of the fluid taken up by the spongioles of the roots,
is evaporated from the leaves of plants in the form of water,
and consequently about one-third remains in the plant in a
highly concentrated state, and contains the carbonic acid and
other earthy ingredients which happen to be dissolved in the
fluid when first presented to the roots.

Although the ash or inorganic matter in plants constitutes a
very small proportion of their substance, yet its importance is
not on this account to be underrated. The small per centage
of inorganic matter contained in them appears to be absolutely
necessary to their healthy growth. It is for this reason that
the soil exercises such’a marked influence on the distribution
of species. It is impossible to examine the plants which spring
up spontaneously in any district, without arriving at the conclu-
sion that they are influenced in the development of the pecu-
liarities of their organization, by certain inorganic matters which
abound in the soils in which they grow. The barren mountain

96 COMPOUND ORGANS OF PLANTS.

and the fertile valley, the sandy soil and the marshy swamp,
the margin of rivers and shores of the ocean, have all their
peculiar species of plants. The chemical composition of the
ash of a plant being known, scientific conclusions can be drawn
as to the soil most suitable for its growth.

A good soil must contain all the substances found in the ash
of the plant. This is a matter of great importance to the agri-
culturist. If we give abundant and vigorous food to an animal
it becomes strong and fat; if its food be small in quantity and
poor in quality, it becomes poor and lean. Just the same
happens to a plant. Plants will grow vigorously and fruit
plentifully when there is an abundance of that kind of food in
the soil which is the most suited to their organization; and
their growth will be checked and their fruit injured by any
deficiency in their proper food. Nature is a wise and perfect
cultivator. Some plants are found in a moist soil, others in a
dry one. Some seek the cool shade, others the warm sunshine ;
some are natives of lofty and barren mountains, others of lowly
and fertile valleys; some fixed to rocks delight in the noisy
waves of the sea; others attached to stones in brooks and
rivers grow beautifully in their quiet waters. All plants, how-
ever, are placed by nature in soils which are chemically and
physically adapted to promote their growth, so that they may
answer her grand and secret purposes in the development of
their organization.

The motion of the sap in plants.—The function of nutrition,
which in the higher animals comprises a variety of distinct
processes, is reduced in plants to the utmost degree of simpli-
city. When water charged with nutritive substances from the
soil enters the cellular extremities of the roots, it immediately
fills the cells and vessels of the plant, and becoming subjected

NATURE AND SOURCES OF FOOD. 97

to their vital action, undergoes a change of properties. The
water thus altered is called the crude or ascending sap. This
fluid, in the active periods of vegetation, is incessantly in
motion, and is unquestionably analogous to the blood of ani-
mals. Butthe motion of the sap in plantsis a great deal more
complicated and altogether different from the circulation of the
blood in animals. The sap is not, like the blood, confined to a
separate system of vessels, for owing to the manner in which
the vascular and cellular tissues are interwoven with each
other, and the general permeability of all the organs, a general
transfusion of the sap from cell to cell takes place endosmoti-
cally in every direction. This is particularly the case at the
commencement of growth, as in germinating plantlets, or
developing leaf-buds, but as soon as woody fibre and vascular
tissue or ducts are formed, they take the most active part in
the upward conveyance of the sap for which they are well
adapted by their tubular and capillary character.

The current of ascending sap flows through the vitally active
and forming cells of the alburnum or sap-wood, situated nearest
the bark, and not at all through the dead wood cells of the
duramen or heart-wood, situated in the interior of the stem.
It is this interposed stratum of sap which renders the bark and
wood so easily separable in the spring of the year.

The sap in plants appears to be set in motion by the expan-
sion of the buds. The extremities of the branches are always
more herbaceous than the part of the branches immediately
below them, and therefore are the first to be affected by an
increase of temperature in early spring. So soon as the extre-
mities of the branches together with the buds begin to swell,
the cells of which they are composed attract the sap from the
tissues in their immediate neighborhood, which tissues are

9x

98 COMPOUND ORGANS OF PLANTS.

Fig. 14.

Fig. 14. Exyeriment of Hales to show the force with which the cap ascends. ¢. Stock
of vine cut. ¢ Tube with double curvature fastened to the top of the stock by a
copper cap v, which is secured by a lute aud piece of bladder m. 2m. Leve' of the

mercuriz] column in the two branches of the tube at the commencement of the experi-

ean oF al Daaal at ite elaca

NATURE AND SOURCES OF FOOD. 99

again refilled by the flow of the sap from the subjaccnt tissucs,
and in this manner the sap is gradually set in motion from the
extremities of the branches to the roots through the entire sys-
tem of the plant. When at length the young branches have
developed themselves from the buds, and the leaves are spread
abroad in the atmosphere, the ascent of the sap becomes
powerfully accelerated by the evaporation which takes place
from their surface.

The height to which the sap rises in forest trees is very
great, and the force with which it ascends is very considerable.
The force with which the sap ascends in the stem of the vine
was measured by Hales, a celebrated English physician. In
the early part of the month of April, he fitted a bent tube to
one extremity of the stem of a grape-vine, which he had cut
down to about two and a half feet above the ground. This
tube was graduated and its curve filled with mercury. Ina
few days he found that the ascending force of the sap had raised
the mercury upwards of 88 inches. Now, since the pressure
of the atmosphere supports a column of mercury varying from
28 to 30 inches in height, it follows that the ascending force
of the sap is greater than the pressure of the atmosphere. In
some of his experiments, Hales calculated that the ascending
force of the sap in the stem of the vine was five times greater
than that which impels the blood through the principal artery
of the horse. A piece of bladder tied over the stump of another
vine, from which a piece had been cut off early in May, was
torn into shreds by the rising of the sap.

As the sap rising in the stem attains a greater distance from
the root, it becomes less watery and more thick and mucilagi-
nous. It finds, in effect, amassed in the tissues which it tra-
verses, portions of gum, sugar, starch, &c., left in them by the

100 COMPOUND ORGANS OF PLANTS.

growth of the previous year, which it re-dissolves and carries
along with it; so that the sap which circulates in the superior
parts of a plant offers a composition more rich in organic prin-
ciples.

It.is, however, principally in the leaves that the sap under-
goes those changes which render it subservient to the growth
and nutrition of the plant. In the leaves, the sap is exposed
to the influences of the light and air, and is thickened and
condensed by the evaporation of the useless water. Under the
influence of light, the oxygen of the carbonic acid is given off
from the leaves into the atmosphere, and the carbon is fixed,
chlorophyl being formed in the cells. The sap is distributed
to all parts of the leaf by means of the veins in the leaf,
which are immediately connected with the alburnum or sap-
wood of the stem. The mechanism of the leaf and the action
of the pores has been already explained. Not only the leaves,
but the young branches, scales, and all the herbaceous or green
parts of the plant, act on the atmosphere in a similar manner
to the leaves.

After having been elaborated in the leaves, the sap, which is
now called the proper juice, re-descends from the leaves towards
the root.

The vascular and cellular system of the leaf not only offers
the same composition as the stem, but it preserves the same
relative situation in the leaf as in the stem; those vessels
which occupied the interior of the stem next to the pith,
becoming superior in the leaf whilst the more external vessels
become inferior, and all retaining the same relative parallelism
in the petiole and lamina.

Now the fibro-vascular tissue which thus issues from the
stem into the petiole, consists of two layers of vessels, an

NATURE AND SOURCES OF FOOD. 101

Fig. 15. Vertical section through a young branch and petiole, showing the manner
in which the vascular and cellular tissues of the leaf communicate with those of the
stem. m pith of the stem; fv fibro-vascular tissue next the pith passing into the
petiole which is articulated to the axis; pc, pc parenchyma of the stem; b, bud in the
axil of the leaf; c, cushion or swelling below the leaf; /, forming fracture.
ex-current layer situated on the upper surface of the petiole
and lamina, and which is immediately connected with the
alburnum of the stem, and a recurrent layer situated imme-
diately beneath the first layer, on the under surface of the
petiole and lamina, which is connected with the endophleum or
inner fibrous bark. The sap is brought from the albur-
num by the ex-current or upper-layer, into the leaf and distri-
buted to all parts of its upper surface; having undergone all
those chemical changes which render it suitable for vegetable
assimilation, or having been elaborated into proper juice,
then conveyed by the recurrent layer of fibres along the under
surface of the lamina and petiole into the bark, down which it
descends to the roots.

That the sap re-descends from the leaves to the roots by the

102 COMPOUND ORGANS OF PLANTS.

bark is evident from the following simple experiment. If a
ring of bark be removed from a tree in spring, the sap will
rise just the same as usual, but when the sap begins to descend,
a protuberance will be formed just above the ring, which is
occasioned by the accumulation of sap there, its farther descent
being stopped by the removal of the bark. The same effect
will be produced if we make a simple ligature or annular com-
pression of a young stem. At the end of a year or two a cir-
cular swelling will form itself immediately above the ligature.
This swelling is evidently produced by the sap, which descending
through the thickness of the bark from the summit of the
stem, and finding an obstacle which it cannot pass, accumulates
above that obstacle. All must have observed the distortions
which twining stems thus produce on the trunks of the trees
about which they entwine themselves.

So also we see the reason why the branch of a fruit tree,
when sterile, may be made to flower and fruit abundantly by
being girdled. This consists in removing a narrow ring of
bark from the branch, sufficient to arrest the downward course
of the elaborated sap, which is thus accumulated in the branch
sufficient in quantity to produce this desirable result.

The ascending and descending sap are very different both in
appearance and qualities. The ascending sap in all plants is
nearly the same, containing no noxious qualities even in the
most poisonous. We are told, by Berthellot, that the natives
of the Canary Islands tear off the bark from the poisonous
Euphorbia Canarensis, and find the ascending sap which they
obtain from the alburnum a refreshing drink, whilst the des-
cending sap is of so acrid a nature that it acts as a caustic,
burning the flesh off suchas happen to touch it. In the maple,
and some other plants, the ascending sap is so sweet that sugar

NATURE AND SOURCES OF FOOD. 103

may be obtained from it by evaporation, whilst the descending
sap of the same tree does not possess any sweetness.

Besides this general circulation of the sap through the entire
cellular and vascular system of the plant, an independent cir-
culation or movement of rotation has been observed in the cells
themselves, considered separately and individually. This is
well seen in those cells which form the hairs of plants which
are conveniently situated for observation. The string of bead-
like cells which compose the jointed hairs of the Tradescantia
Virginica, or the spiderwort, show this circulation distinctly
under a magnifying power of about 400 diameters. In the tubu-
lar cells of Chara, an aquatic plant growing in stagnant pools,
this circulation may be seen with an ordinary microscope. The
motion of the currents in the cells of these plants is rendered
visible by the minute grains of chlorophyl which they carry
along with them. The cause of these motions is at present
wholly unknown.

PART IV.

ON THE
ORGANS OF REPRODUCTION

IN PHANEROGAMOUS PLANTS.

PAR Vs

THE ORGANS OF REPRODUCTION IN PHANEROGAMOUS
PLANTS.

CHAPTER VIII.

GENERAL CONSIDERATIONS ON THE FLOWER.

ALL intelligent naturalists are agreed that a similar prin-
ciple and plan of structure pervades the whole chain of animal
organization, and that the same organs are developed under
different forms to meet the peculiar wants and self-preservative
instincts of the animal. Thus, the arm of man, the foreleg of
quadrupeds, the true wing of birds, and even the pectoral fin
of fishes, all represent one and the same organ developed under
widely different forms, in accordance with those purposes to
which they are subservient in the animal economy.

Now precisely the same is the principle and plan of struc-
ture in the vegetable world. The essential organs of plants
consist of the root, stem and leaves; no new organ is intro-
duced, but these common elements of vegetable structure are
developed in peculiar and appropriate forms to suit the several
wants of the plant.

When we:look at a plant in full bloom, we are apt to regard
it as an organized being of a very complex character, and to
look on the green leaves of its stem, and the several members

108 COMPOUND ORGANS OF PLANTS.

or component parts of its flower, as entirely distinct in their
derivation and character. A more extensive acquaintance with
floral structure soon, however, discloses the interesting and
important fact, that all the beautiful and highly organized parts
of the flower are only a series of progressively metamorphosed
leaves ; which have assumed these lovely colors and this pecu-
liar arrangement and form, in consequence of the peculiar
functions assigned them.

The green leaves on the stem and branches are concerned in
the functions of nutrition; they decompose carbonic acid gas,
and, under the influence of solar light, chlorophyl is formed in '
their cells, (zaweds green, and gvaroy a leaf,) so called because
it is the substance which gives to the leaves their green hues.
The leaves of the stem take their 'peculiar color and form in
consequence of their action on the atmosphere; they take in
food from the air, which, in connection with that absorbed by
the roots from the soil, contributes directly to the growth or
the extension of the parts of the plant.

The leaves which constitute the flower, on the other hand,
are concerned in the functions of reproduction, and are there-
fore modified in their structure, form, arrangement, and color,
so that they are beautifully adapted to the exercise of these
functions. The organs of reproduction which are collectively
designated as the flower, are therefore only a peculiar modifica-
tion of the organs of nutrition. A flower-bud only differs from
a leaf-bud in having no power of extension. Like the leaf-bud,
it is a shortened branch, the axis of which has not been elon-
gated, and however the parts of the flower may differ from the
ordinary leaves of the plant in appearance, we shall presently
show that they may all be referred to the leaf as a type, their
nature being precisely the same, and appearance dissimilar in
consequence of a difference in the functions assigned them.

GENERAL CONSIDERATIONS. 109

Hence when the student has acquired a knowledge of the
anatomy and functions of leaves, he is prepared to enter on
the consideration of the floral organs.

It has been shown that every plant which consists of more
than one cell, or of a series of cells united together, may be
divided into two distinct parts, to which separate functions are
assigned, a vegetative part and areproductive part. In the more
highly organized plants, the vegetative part of the plant con-
sists of the root, the stem, and the leaves, each having distinct
functions assigned in the vegetable economy. Now every
plant continues to grow so long as its vegetative cells continue
to develop; but when the plant acquires all its developments, or
arrives at an adult state, the reproductive cells show them-
selves, and growth stops in that direction: the whole force of
vegetation being expended in the production of the spore or
seed, the embryo or germ of the future plant.

In the more highly organized plants, the cells which are
connected with reproduction make their appearance in the
form of beautiful whorls of metamorphosed and colored leaves,
constituting that part of the plant which is popularly called
the flower; and we are about to trace those curious processes
which are carried on by them, or their physiological action in
the production of the embryo or seed, which contains within
itself the rudiments of future generations.

That flowering is an exhaustive process and’ therefore
necessarily causes the cessation of the growth or extension of
the parts of plants, is evident from the following facts. Plants
will continue to grow if the flower buds are removed as soon
as they are formed. This is often done by gardeners, who nip
off the young flower buds in order to encourage the growth of

the plant, which is thus enabled to accumulate a greater store
10*

110 COMPOUND ORGANS OF PLANTS.

of nutriment, and finally to produce finer flowers and fruit.
By removing their flower buds as soon as formed and thus
preventing the exhaustion consequent on flowering, annuals
may be changed into biennials, or even perennials, their life
being prolonged indefinitely; whilst the same plants left to
flower in the ordinary course of nature, perish as soon as they
flower and bear seed, whether during the first, second, or any
succeeding year. The actual consumption of nutriment in
flowering, is seen in the rapidity with which the farinaceous
and saccharine store accumulated in the roots of the beet and
carrot disappears as soon as these plants begin to flower,
leaving them light, dry and empty; so also the esculent roots
of radishes and turnips become fibrous and unfit for food, when
they are allowed to run to seed. When the branch of a fruit
tree is sterile, it may be made to flower and fruit abundantly
by being girdled. This consists in the removal of a narrow
ring of bark from the branch, sufficient to arrest the downward
course of the elaborated sap, which is thus accumulated in the
branch in a sufficient quantity to produce this desirable result.

The reproductive organs show themselves only at the epoch
when the plant acquires all its development, or arrives at an
adult state. The period when this event occurs depends on the
peculiar organization of the plant. At this time a change
takes place in the primary mode of development, the buds in
the axils of the leaves or at the extremities of the branches
cease to elongate, and the internodes or naked intervals of stem
between the leaves being non-developed, the leaves remain
crowded together in whorls, in a sort of rosette, and under-
going peculiar modifications in their form and color, a flower is
produced.

Every flower, when complete, consists of four whorls of pro-

GENERAL CONSIDERATIONS. 111

gressively metamorphosed leaves, called respectively the calyx,
the corolla, the stamens, and pistils. Of these four verticils,
the two outer whorls marked a and 3, in Fig. 16, are called

Fig. 16.

floral envelopes, and are considered to be merely accessary
organs, whose functions are to protect the two inner whorls,
the stamens and pistils marked ¢ and d, which are named
sexual organs, and which are by far the most important and
‘highly organized parts of the flower. A flower may be perfect
and reproduce itself without either calyx or corolla, but not
without stamens or pistils; for these last organs are imme-
diately connected with the formation of the seed, the germ of
the future plant, and without these secreting and all-important
bodies, it is impossible for fertilization to take place, or seed to
be produced.

The leaves of the flower, like those of the stem, are arranged
spirally about the axis of growth, and therefore the separate
pieces of each verticil alternate with each other. Thus the
petals or leaves of the corolla alternate with the sepals or leaves
of the calyx; that is to say, each petal is placed in the inter-
val between two sepals; the stamens alternate with the petals
and the pistils with the stamens.

The sepals of the calyx, or outermost of the floral whorls,

112 COMPOUND ORGANS OF PLANTS.

are usually colored green, and are the nearest allied to the
leaves of the stem, both in form and appearance; the petals of
the corolla, or innermost floral envelope, are usually of some
other color than green, as for instance, white, red, blue, yellow,
or some intermediate shade of these colors, and more delicate
and beautiful in their texture than the sepals. The stamens
markedc in fig. 16, are situated immediately within the corolla,
and surround the pistils marked d, or central organs of the
flower. The stamens are collectively termed the andrecium
(denp a male, and éexéov habitation), and are considered to be
the male organs of the plant. The pistils occupy the centre
of the flower, are surrounded by the stamens and floral
envelopes, and after flowering are changed into fruit, and con-
tain the seed. The pistils are collectively termed the gymne-
cium (yw a female, and éxiov a habitation), and are considered
to be the female organs of the plant.

All these organs of the flower are situated on the summit
of the peduncle or flower-stalk, and the part on which they
are situated has received the name of thalamus, torus, or
receptacle.

The different organs of the flower are verticillate leaves
brought into close proximity, in consequence of the non-devel-
opment of the floral internodes. This fact is beautifully con-
firmed by the appearance of an internode, or naked portion of
stem, in some species between one or more of the floral whorls,
by which they become separated from each other, just as the
whorls of leaves are separated on the stem. Thus the inter-
node, or naked interval of stem between the stamens and pistils
is developed in Euphorbia corollata, flowering spurge (Fig.
17), the pistil @ being elevated, after, it is fertilized, on a little
stalk, and thus lifted, as it were, from out of the midst of the

GENERAL CONSIDERATIONS. 113

stamens and floral envelopes; so also in the genus Gynandrop-
sis (Fig. 18), which belongs to the caper family, the stami-

Fig. 18.

nate leaves marked s are separated from those of the corolla c,
by the development of the internode or naked interval of stem
between them; and the pistil p is also separated from the sta-
mens by the development of another internode, and supported,
as it were, on a little stalk or pedicel. Usually, however, the
floral internodes remain undeveloped, and therefore such
appearances of the whorls may be justly regarded as an ab-
normal condition of things. Aberrant forms and monstrosi-
ties, whether in the vegetable or in the animal world, are
always exceedingly instructive, and furnish rich materials
towards cultivating and expanding our knowledge of the regu-
larly developing organism.
‘

114 COMPOUND ORGANS OF PLANTS.

CHAPTER IX.
THE INFLORESCENCE.

THE arrangement of flowers on the stem or floral axis is
called the inflorescence. Flower buds, like leaf buds, are either
terminal or lateral. Flowers are terminal when the bud
which terminates the axis of growth is a flower-bud. This of
course stops the farther growth of the plant in that direction.
Flowers are lateral when the bud which terminates the axis of
growth develops as a leaf-bud. In this case the floral axis
goes on extending itself indefinitely, and the flowers spring
from the sides of the axis of growth, as shown in Fig. 19,
»from the axilla marked 6.

Big. 10.

The Bracts, or floral leaves. These bracts are situated all
along the floral axis at the basis of the peduncle or flower-
stalk, and are simply the ordinary leaves’of the stem reduced
in size in consequence of the absorption of nutriment from
them by the flower. These bracts become smaller in propor-

THE INFLORESCENCE. 115

tion as they approach the upper part of the floral axis. Hence
the leaf gradually passes into the bract in consequence of its
development in the neighborhood of the flower, and the same
proximity doubtless produces the abortive leaves of the calyx.

Sometimes, however, the bracts are as richly colored as the
petals themselves, as in Castilleja euchroma, or the painted
cup, which owes all its beauty to its conspicuous and deep
scarlet bracts. The curious envelope of the Indian turnip,
(Arum triphyllum), and the Kthiopian lily, (Calla Ethiopica),
called a spathe, is nothing but a colored bract; so also the
conspicuous petal-like involucre or bract of the dogwood,
(Cornus florida), is much more showy than the real flowers
which it surrounds.

Bracts are generally distinct from each other; but when the
flowers are brought together and situated on a common recep-.
tacle as in Umbelliferous and Composite plants, the bracts are
also brought together and surround the basis of the general
receptacle in one or more verticils or whorls. In the Umbelli-
ferze, there is usually a whorl of bracts surrounding the general
umbel, which is called an involucre, and in some genera another
whorl of bracts also surrounds the umbellets, termed an
involucel. In the Composite, the involucre consists of several
rows of imbricated bracts which surround the head of flowers,
ag in the Aster, the Solidago and the Helianthemum. Not
unfrequently the separate flowers also are subtended by bracts,
termed paleze or chaff. In the grasses, bracts occupy the place
of both calyx and corolla. They form the cupula or cup of
the acorn, and also the husky covering of the hazel-nut.

The leaf appears to pass by means of the bract into the
sepal or calyx leaf. There is in reality no exact limits between
common leaves and bracts, and the limits between bracts and

116 COMPOUND ORGANS OF PLANTS.

sepals are equally imperceptible, such is the gradual transition
of one into the other. The gradual transition of the bract
into the sepal is well seen in composite flowers such as the
marigold, the involucre or calyx of which is composed of
numerous bracts and sepals more or less soldered together.
The same transition is also visible in the common hollyhock of
the gardens, the leaves of which approximate together, become
modified in size and appearance, and slide as it were insensibly
into a calyx.

As flower buds are produced in the axils of bracts, and as
bracts are only modified leaves, it follows that the arrange-
ment of flower buds follows the same law as the arrangement
of leaf buds, the flower bud being merely the last term of
ramification.

When the flower buds are lateral and the inflorescence
axillary the axis elongates indefinitely, and only ceases when
the terminal bud is suppressed or on the approach of winter.
When the floral axis elongates in this manner, the lower
flowers are the first to expand, whilst those towards its apex
remain closed, and the expansion is said to be centripetal or
from the circumference to the centre. For when a floral axis,
developing indefinitely, is shortened by the non-development
of the floral internodes, so that the flowers are brought together
in clusters at its summit; the outermost flowers, which corres-
pond to the lowest flowers of the lengthened axis, will be the
first to expand, whilst the innermost flowers, which answer to
those at its apex, will remain closed. The expansion of the
flowers will be therefore necessarily centripetal, or from the
circumference to the centre.

When the flower buds are terminal, the elongation of the
floral axis is necessarily arrested; it is nevertheless able to

THE INFLORESCENCE. 117

extend itself by secondary and tertiary axes, which are also
arrested in their growth by the expanding flower at their
summit.

If we take, for example, the inflorescence of Erythraa
centaurium, (Fig. 20,) we shall see at the summit of the

Fig. 20.

primary axis a flower, a, which is truly terminal; but from
either axis of the first pair of leaves or bracts at 4, arises a
secondary axis, each axis being similarly terminated by a
single flower, and bearing also two pairs of bracts, c, c, which
in their turn, give rise to unifloral tertiary axes, and so on.

As the secondary axis arises from leaves below the primary
and central flower, the flower at the apex of the secondary
axis, is consequently farther, removed from the centre of
growth; and the same remark applies to the flower at the

11

118 COMPOUND ORGANS OF PLANTS.

summit of the tertiary or any other succeeding axis which
may be developed. In this case, therefore, since the flower
terminating the growth of the primary axis is the oldest,
and consequently the first to expand, the other flowers
expanding in succession in proportion as they are removed
farther from the centre,—the expansion of the flowers is
necessarily centrifugal, or from the centre to the circum-
ference.

This mode of inflorescence is termed a cyme, and as the
divisions in this case always take place by two, it is called a
dichotomous cyme, (Sézo, two ways, and réuvw, I cut.) If,
instead of two, three whorled leaves or bracts developed floral
axes in a similar manner, the cyme would be trichotomous,
(zpexa, in three ways.)

The inflorescence has received different names according to
the different modes in which the flowers are arranged on the
axis, and the extent to which that axis is developed.

The following are the leading forms assumed by the indefi-
nite or indeterminate inflorescence. If the axillary flowers are
without a peduncle or flower stalk and sessile along the
common axis, they form a spike, as in the Plantain, (Fig.
21.) If, on the contrary, the axillary flowers are supported
on a peduncle under the same circumstances, they form a
raceme, as in the wild cherry, (Fig. 22.) .

If the floral axis of a spike is shortened by the non-develop-
ment of the floral internodes, a capitulum or head is produced,
as in Cephalanthus occidentalis, (Fig. 23.) Sometimes the
capitulum becomes partially elongated into a spike as it grows
older, as in Sanguisorba and many species of Clover. The
shortened axis of a head is called a receptacle.

Frequently, instead of being globular as in Cephalanthus,

THE INFLORESCENCE. 119

Fig. 22.

or prolonged as in Clover, the apex of the floral axis is dilated
horizontally, so as to allow a large number of flowers to grow

120 COMPOUND ORGANS OF PLANTS.

together on its flat or convex surface. What are called com-
pound flowers, as the Helianthemum, Aster and Dandelion, are
heads of this nature, surrounded by a common involucre of bracts. *
This flat, dilated receptacle, is very conspicuous in the Dande-
lion after its ripe pericarps have been removed by the wind.

If the spike be succulent and covered with unisexual flowers,
ordinarily incomplete, that is to say, without floral envelopes,
and if the whole be enclosed in a spathe, the inflorescence is
called a spadix, as in Arum maculatum, (Fig. 24.)

If the spike be covered with unisexual flowers, male or
female, borne in the axils of bracts, the axis of the spike being
articulated at its base so that it is detached and falls off all in
one piece, the inflorescence is termed an amentum or catkin,

(Fig. 25.)

Fig. 24. Fig. 25.

Fig. 25. a Unisexual amentum of the hornbeam (Carpinus betulus.) b. One of the
flowers with its subtending bract magnified.

THE INFLORESCENCE. 121

When the floral axis of a raceme is so shortened, and the
peduncles of the lower flowers are so elongated, as to elevate
them to the same level as the upper flowers, a corymb is
formed, as in Achillea millefolium. If the floral axis of a
raceme be suppressed altogether, so that the peduncles all
start from the same point, we have an umbel, (Fig. 26.)

Fig. 26. Fig. 27.

Fig. 28. Diagrams of a corymb b, and of an umbel ¢.

If the secondary floral axis of a raceme gives rise to tertiary
ones, the raceme is branching, and forms a panicle, (Fig. 27.)
The panicle ordinarily assumes a pyramidal form, that is to say,
the floral axes become shorter in proportion as they approach the
summit. If on the contrary, the floral axes of the middle part
are the longest, the inflorescence takes a more or less ovoid
form, and is denominated a thyrsus, as in the lilac.

The definite and determinate inflorescence. The lowest
developments of this form of inflorescence, is that in which a
single floral axis is terminated by a solitary flower, of which
the Anemone nemorosa furnishes a good example. When such
an inflorescence branches, the branches do not grow in an

11*

122 COMPOUND ORGANS OF PLANTS.

indeterminate manner, but are arrested in their development
by the terminal flowers.

The most common and regular cases of determinate inflo-
rescence occur in opposite-leaved plants. In these plants the
inflorescence is composed of a superposed series of bifurcations
of the primary axis, in the centre of each of which a terminal
flower is situated. This mode of inflorescence, which is termed
a cyme, hag been already explained, and may be studied to
advantage in the chickweeds, (Cerastium and Stellaria,) in
which it is recognizable at once by the solitary flower, destitute
of bracteoles, in each fork of the branches.

Fig. 28. Fig. 29.

Sometimes only one of the two bracts on the primary and
succeeding axes developes a flower, as in Arenaria stricta, (Fig.
28;) or the floral bract is suppressed altogether, so that the
flower appears opposite the remaining bracts, (Fig. 295) or
both bracts are suppressed, and the flowers only are developed,
as in Myosotis palustris, (Fig. 80.) When this is the case,
the cyme assumes a remarkable curvature, turning round in a
peculiar way so as to resemble a snail or the tail of a scorpion,
and hence it is called a helicoid or scorpioid cyme; (émé, a
spiral, and éd0s, form.) This form of inflorescence may be

THE INFLORESCENCE. 123

Fig. 30.

observed in the Heliotropium Peruvianum, in the Sedums, in
the Droseras or sundews, and in most Boraginaceous plants.
The theoretical formation of this inflorescence may be ascer-
tained by consulting the ideal figure placed here by the side of
the scorpioid cyme of Myosotis palustris.

The first flower is situated at 5, and terminates the growth of
the primary axis a, 6; from the axil of the bract, or in the
place where it is suppressed atc, arises a secondary axis, ¢, d,
which by its vigorous development, usurps the place of the
primary axis, which is thus cast to one side. In like manner,
a tertiary axis, e, /, springs from the axis c, d, at e, the ter-
minal flower at d becoming apparently lateral, as before; in
this manner a succession of unifloral axes are produced from

124 COMPOUND ORGANS OF PLANTS.

each other, which have the appearance of a continaous primary
axis; but the flowers which appear lateral are in reality all
terminal.

As might be expected, all these forms of inflorescence pass
into each through endless intermediate gradations. In nature
they are not so absolutely fixed as in our written definitions,
and whether this or that name should be used in a particular
case, is often a matter of fancy. ‘

The manner in which the leaves of the flower are arranged
in the bud, before the expansion of the flower, is called their
eestivation, (estivus belonging to summer,) or preefloration,
(pre, before, and jflos, flower.) These terms bear the same
relation to the flower bud that vernation does to the leaf bud;
and indeed, since the flower bud is only a modified leaf bud, as
might be expected, the corresponding terms applied to vernation
are used in reference to preefloration or eestivatiun. A few

new terms are however added, descriptive of certain peculiar
modifications in the general forms described in vernation.
Fig. 31. Fig. 32.

Fig. 32. The flower bud of Althea rosea, showing the valves of the calyx, ¢, opened,
and the contorted preefloration of the petals of the corolla, p.

The following are the principal forms of eestivation to which
the others are generally reducible.

THE FLORAL ENVELOPES. 125

1. The valvate. When the sepals or petals fit by their
edges, without overlapping each other, as in the mallow.

2. The imbricated. When the petals or sepals cover each
other by a part of their height merely, like the tiles of a roof.
The calyx of the Camelia japonica, (Fig. 31,) is a good illus-
tration of the imbricated preefloration.

3. The contorted. When the petals or sepals exhibit a
tortion of their axis, and overlap each other’s margins, the
whole appearing to be more or less spirally twisted, as in the
flowers of the Althea rosea, (Fig. 31.)

o:
—+@

CHAPTER X.
THE FLORAL ENVELOPES.

In a complete flower we find, without the stamens and pistils,
two whorls of progressively metamorphosed leaves, the calyx, -
which is the exterior whorl, and the corolla placed immedi-
ately within the calyx. The modified leaves of the flower are
brought into close proximity by the non-development of the
floral internodes, in order that the several whorls may the
more readily communicate with each other; which immediate
communication is necessary to the production of the seed.
Let us now examine more particularly the two outermost
whorls of floral leaves, designated as the calyx and corolla.

The calyx, so named from xaavf a cup. This forms the
outermost whorl of the floral leaves, and consists in its usual
state of a leafy green cup more or less divided. The sepals or
leaves of the calyx differ but slightly in structure and appear-
ance from the ordinary leaves of the stem; they are for the

126 COMPOUND ORGANS OF PLANTS.

most part of a greenish hue, chloropbyl being formed in their
cells, and stomata or pores existing on their lower epidermis ;
and in some cases of monstrosity, they are actually converted
into the ordinary leaves of the plant. In proliferous states of
the rose, the calyx. assumes a leafy aspect; whilst in Gentiana
campestris and, Gentiana crinita, it differs in no respect from
the ordinary leaves of the plant.

The sepals of the calyx are sometimes separate from each
other as in the buttercup, at other times they are united toa
greater or less extent, as in the Polyanthus. When the sepals
are separate from each other, whatever may be their number the
calyx is polysepalous (zorvs many, sepala leaves;) but the term,
as currently understood amongst botanists, is simply used to
express the absence of cohesion amongst them, and is equiva-
lent in meaning, to the expression sepals distinct. When the
sepals of the calyx are united to each other by their margins
in a greater or less degree, the calyx is monosepalous (ovos
one, sepala leaf.) The same remarks apply to the petals of the
corolla, which are polypetalous or monopetalous, according as
the petals are separate from each or in a state of cohesion.

It is well known that the parts of plants which grow closely
together are apt to cohere, the parts anastomosing with each
other. Accidental unions of this kind among the leaves of
plants are of frequent occurrence. Now owing to the non-
development of the floral internodes, the metamorphosed leaves
which constitute the flower are necessarily brought into closer
contact, and hence they are more frequently found united with
each other than the leaves of the stem.

The sepals of a monosepalous calyx may cohere together
by their bases, or by their margins, through their inferior half,
or through their entire length, and various terms are employed
to express these different degrees of cohesion.

THE FLORAL ENVELOPES. 127

Fig. 33.

Fig. 33. Thus salto the sepals are coherent by their bases as
in the Pimpernel a, we employ the terms bi-partite, tri-partite,
quadri-partite, according as there are two, three, or four sepals
thus united. When the union of the sepals takes place through
the lower half of their margins, such sepals are bi-fid, tri-fid,
quadri-fid, as in Erythreea, 2. If the sepals are united with
each other by their margins nearly to their summit, they are
bi-dentate, tri-dentate, as in Lychnis, c. Finally, if the union of
the margins is complete through their entire length, the calyx
is said to be entire. It is seldom, however, that the cohesion of
the calycine leaves is complete, and the number of lobes at the
summit of the calyx will in general show the number of sepals
which have cohered together. In the entire monosepalous
calyx, the venation assists in haciamias the number of
cohering sepals.

When the sepals are Senet developed and united, the
‘ealyx is said to be irregular. This takes place in the Labiate,
‘ or mint tribe, where some of the sepals of the calyx unite toa

greater extent than others, thus forming a bi-labiate or two-
lipped calyx, as in the dead nettle, Lamium. (Fig. 34.) The
upper lip is composed of three sepals, the lower of two; the
united parts form the tube; the free portions the lobes or
segments of the limb; and the part where they join one
another the mouth or throat.

128 COMPOUND ORGANS OF PLANTS.

Fig. 34. Fig. 35.

The monopetalous corolla, has corresponding terms applied to
its modifications and to the degrees of cohesion amongst its petals.

When the calyx falls as soon as the corolla expands it is
termed caducous, as in Sanguinaria Canadensis, which is at
first enclosed in a calyx of two leaves, which fall off as soon as
the flower is fully blown. The calyx is deciduous when it
drops off with the corolla, but in many cases, the calyx
remains after the corolla and other floral whorls have faded
and fallen, as a protecting envelope to the fruit, as in the
mallow, (Fig. 35.) In this case it is said to be persistent (per
through, and sisto to remain.)

Sometimes the calyx and fruit cohere together, so that the
calyx appears to arise from the summit of the fruit, as in the
rose; such a calyx is called a superior calyx; if, on the con-
trary, the calyx and fruit do not cohere together, the calyx is
said to be inferior, as in the strawberry. In some plants
the calyx is suppressed altogether, or it may be present and
reduced to a mere rim or border, as in the Umbelliferze; or to
a pappus, as in Composite.

A great many plants, however, have only one floral envelope
exterior to the stamens and pistils, as for example, the hyacinth
and the lily. The early botanists differed amongst themselves
as to the term by which this single floral envelope ought to be
distinguished from the others. Tournefort and Linnzus called
it the calyx when it was green and bore the general character

THE FLORAL ENVELOPES. 129

of a calyx, and gave it the general name of corolla when by its
color and the delicacy of its tissue it approximated to that
organ. But this distinction is utterly worthless, for the same
organ may vary in color without changing its nature. Thus,
in the Fuchsia or lady’s ear drop, and in Salvia splendens, one
of the Mexican sages, the calyx is of the same bright-scarlet
color as the corolla; and in the white water-lily and magnolia,
the sepals gradually approximate in color to the petals. Hence
it is now agreed amongst botanists when a flower has but one
envelope to its stamens and pistils, to consider it as a calyx,
whatever may be its color and form.

The corolla, from “corolla” a garland, is that part of the
flower situated immediately within the calyx, between the calyx
and stamens. It is generally the most showy and beautifully
colored of all the floral organs, and is the part which is popularly
called the flower. Thus the red leaves of the rose, the yellow
leaves of the buttercup, constitute the corolla of these plants.

The divisions of the corolla are called petals from (xerarov a
leaf.) If these petals are united by their margins so as to form
apparently one petal, as in the primrose and Campanula the
corolla is termed monopetalous; if, on the contrary, the petals
do not cohere together, but grow separately and distinctly apart
as in the rose, the corolla is said to be polypetalous. When
the various divisions or petals of the corolla are alike and its
incisions uniform, the corolla is regular; if otherwise, it is
irregular. The lower part of a monopetalous corolla is called
the tube, the upper and expanded portion the limb, and the
part where the two are connected with each other the throat.

The sepals of the polysepalous calyx are usually sessile
leaves, having nothing analogous to a leaf stalk at their base ;

but it is otherwise with the petals of the polypetalous corolla.
12

130 COMPOUND ORGANS OF PLANTS.

These, although sometimes sessile, as in the rose and crowfoot,
have not unfrequently their base tapering into a narrow stalk
analogous to the petiole of the leaf, which is called an unguis
or claw; whilst their upper portion, which corresponds to the
blade of the leaf, is broader and more expanded, and is called
the lamina, as in the wall-flower. (Fig. 36.) Petals organized
in this manner are termed unguiculate.

Fig. 36,

Fig. 36. Cruciform corolla and unguiculate petal of the wall-fiower,. (Cheiranthus.)

The following are some of the leading forms assumed by the
regular polypetalous corollas. The rosaceous, of which the rose
is the type, have spreading petals without claws or with very
short ones. The cruciform, in which there are four petals,
usually with claws, arranged in the form of a cross, as in the
wall-flower. The /liaceous, in which the petals, six in number,
gradually taper from the base to the apex, as in the lily. The
caryophyllaceous, where the petals have long, narrow, tapering
claws, which are enclosed in a tubular calyx, as in the pink.

Irregularities in the form of polypetalous corollas may result
from the unequal development of the petals, as in the violet;
but these are not sufficiently marked as to justify the appli-
cation of any particular term. ‘There is, however, one form

THE FLORAL ENVELOPES. 131

of irregularity amongst polypetalous corollas which usually
receives a special notice, on account of the remarkably anoma-
lous development of its petals, and because it is characteristic
. of an extensive natural order of plants, viz: the papdlionaceous
corolla, from (papilio, a butterfly,) of which the pea-flower
furnishes a good example. (Fig. 37.) This corolla is composed

Fig. 87.

of five unequal and dissimilar petals. One larger than the
rest, a, called the vexillum or standard, which is ‘usually
folded over the other petals in estivation; two lateral petals,
b, which are designated as the ale or wings; and two inferior
petals, usually completely covered by the al, and their lower
margins so united as to form a single keel-like piece, called
the cariza or keel, c. This last piece embraces the essential
_ organs, the stamens and pistils.

The following leading forms may be distinguished amongst
the regular monopetaluus corollas. The campanulate, or bell-
shaped, as in Campanula rotundifolia, (Fig. 38, a,) which is
without a tube, and which enlarges gradually from the base to
the apex. The infundibuliform or funnel-shaped, as in the
Convolvulus purpureus, or morning-glory, in which the tube
is narrow below but widely-expanded towards the summit.
The hypocrateriform or salver-shaped, as in the Phlox (Fig.
38, b,) where the limb spreads out at right-angles with the

182 COMPOUND ORGANS OF PLANTS.

Fig. 38,-

a b

more or less elongated tube of the corolla. The rotate or
wheel-shaped, as in the Myosotis palustris or forget-me-not,
which is a salver-shaped corolla without a tube, or with a very
short one. The tubular or tube-shaped, as in the Caprifolium
or honey-suckle, where the limb is not developed and the
corolla is cylindrical or tubular throughout its entire length.
The urceolate or urn-shaped, as in Vaccinium macrocarpon, the
American cranberry, in which there is scarcely any limb, and
the tube is narrowed at both ends and expanded in thé middle.

Irregularities in the form of monopetalous corollas are
produced by differences in the degrees of cohesion amongst the
petals. The principal forms of irregular monopetalous corollas
are :—the dabiate or lipped, (from labium, a lip,) (Fig. 39, a,)
in which thé tube is more or less elongated, the throat open
and dilated, and the limb divided traversely in such a way as
to produce an upper and lower portion called the labia or lips,
with a hiatus or gap between them, like the mouth of an ani-
mal. The upper lip is usually composed of two petals, as the

THE FLORAL ENVELOPES. 133

Fig. 39. Fig, 40.

little notch at its summit proves; the lower, of three. When the
two lips are thus gaping and the throat open, the corolla is
said to be ringent, as in Lamium amplexicaule; but when the
mouth is closed by the approximation of the two lips, by an
elevated protuberance of the lower called the palate, as in the
snap-dragon or toad-flax 6, the corolla is designated as per-
sonate or masked. When a tubular corolla is split down on
one side in such a way as to form a strap-shaped process with
several tooth-like projections at its apex, it becomes ligulate
(ligula, a little tongue,) or strap-shaped. (Fig. 40, d.) This
kind of corolla is well seen in composite flowers such as the
dandelion, in which all the flowers forming the head are
ligulate. In the Composite there are often two kinds of florets
associated in the same head. Thus the outer florets which form
the white ray of the Ox-eye daisy (Leucanthemum) are ligulate,
whilst those which form the yellow disk are tubular. (c.)

The largest flower in the world is the Rafflesia Arnoldii,

(Fig. 41,) which was discovered by Sir Thomas Stamford
12*

1384 COMPOUND ORGANS OF PLANTS.

Fig. 41.

Raffles in the forests of Sumatra, in the year 1818, growing on
the stems of the Cissus augustifolia, a kind of climbing plant or
grape-vine. In the bud state this flower is nearly a foot in
diameter, and when fully expanded, nine feet in circumference
and three feet over from the tip of one petal to that of another.
Tts substance is about half an inch thick, and the whole plant
weighs fifteen pounds. Its color is light orange mottled with
yellowish-white, and like other parasites, it derives its nutri-
ment from the tree on which it is found. A few other species
of less gigantic size have been discovered in the other islands of
the Eastern archipelago.

Structurally, the petals or leaves of the corolla are composed
of cellular and vascular tissue, the latter consisting of spiral
vessels and delicate tubes. The color of the petals is produced
by the refined and splendidly colored juices elaborated from the
sap by the walls of the cells which form their tissue or substance.
This fact is easily verified by submitting to microscopic exami-
nation a fragment of the petal of a rose or of a camellia, when
it will be seen that the color does not exist in the walls of the
cells of the petal, but it is the result of the colored fluids with
which the cells are filled.

THE FLORAL ENVELOPES. 135

Sometimes, by the mere juxtaposition of the different cells
in the petals, a mechanical admixture of their various contents
takes place ; thus is probably produced that delicate and inimi-
table shading seen in the petals of some flowers; at other
times, the petals are spotted and variegated, as in the tiger
lily and balsam. Such spots result from the peculiar power,
possessed by some of the cells, of attracting from the colorless
sap these particular colors, and of which power the other cells
appear to be deprived. No admixture of color with the neigh-
boring cells takes place in this case. «In the petals of
Impatiens balsamina, the garden balsam,” says Dr. Lindley,
‘Ca single cell is frequently red in the midst of others that are
colorless. Examine the red bladder, and you will find it filled
with a coloring matter of which the rest are destitute.”

Every one must have noticed the regularity with which
these spots are formed in the petals of certain flowers, which
are in fact nevet without them. Such cells appear to have
definite functions assigned them, the exercise of which is pro-
bably as important to the healthy vital action of the plant as
that of the most elaborate organs.

The chromule, or coloring substance of plants, is by no
means confined to their petals, but sometimes pervades the
sepals of the calyx, as we have already shown, and is even
occasionally extended into the tissue of the bracts and ordinary
leaves of the stem. The beautiful wild flowerecalled Castilleja
euchroma, or the painted cup, owes all its beauty to its con-
spicuous and deep scarlet bracts; and in Croton pictum, a
plant which may be frequently met with in conservatories, the
chromule tints the ordinary leaves of the plant. The analogy
of the petals to the leaf is thus clearly traceable; and how-

136 COMPOUND ORGANS OF PLANTS.

ever dissimilar the petals of the corolla may appear to the
ordinary leaves on the stem of some plants, so that we may
feel disposed to regard them as separate organs, yet the
evidence afforded by these transition forms shows the intimate
connection subsisting between the petal, the sepal, and the
bract, and the common origin of the whole of them from the
ordinary stem-leaf, of which they are but modifications.

Functions of the Floral Envelopes.—The calyx and corolla
are by far the most conspicuous and showy parts of the flower,
and are the parts of it which usually attract popular notice.
Yet their functions are entirely of a secondary and subordinate
character. The internodes between the several whorls of floral
leaves are not developed, in order that they may the more
readily act on each other. The calyx and corolla doubtless
foster and protect the two inner whorls of leaves, viz.: the
stamens and pistils, which are more immediately connected
with the process of reproduction than they are.

All must have noticed the folding up of the calyx and
corolla at sunset, or in wet weather. The function exercised
by the two outer whorls of the floral leaves is in this case
clearly protective, and the design of their close proximity to
the stamens is at once apparent; that they may fold over the
stamens and pistils, and thus ward off the injurious effect of
the night dews and falling rain, which would act injuriously
on the pollen contained in the cells of the anther. Thus
safely and beautifully sheltered at every epoch of their
development, the stamens and pistils perform their respective
functions.

The bracts and calyx, when of a green color, doubtless per-
form the same functions as the ordinary leaves of the stem ;
but it is otherwise with the petals of the corolla and with the

THE FLORAL ENVELOPES. 137

other parts of the flower ; these exercise on the atmosphere,
a different kind of influence. Before the appearance of
the flowers, the plant is wholly an apparatus of reduction,
all its parts being concerned in the assimilation of the food.
It decomposes the carbonic acid borrowed from the atmo-
sphere and the soil, fixing the carbon and exhaling the oxygen,
and forming within its green leaves, young shoots, and super-
ficial parts, the substance called chlorophyl. But when the
flowers develope, this part of the plant becomes an apparatus
of combustion. The starch granules which in the leaves were
changed into chlorophyl, in the petals are changed into
chromule, and become wholly oxidized and converted into
saccharine matter. The carbon or sugar accumulated by the
nutritive organs of the plant, is consumed by its reproductive
organs. Hence we see these matters disappear at the epoch
when the flowers expand, and it is therefore necessary to reap
those vegetables, which we cultivate for the sugar which they
contain, before that period. This disappearance of the saccha-
rine store is the result of its slow combustion, or the conver-
sion of the carbon of the sugar into carbonic acid. Oxygen is
therefore necessarily consumed and heat evolved by the flowers,
whilst at the same time carbonic acid rises from them into the
atmosphere. Whilst, therefore, the green leaves of plants
purify the air, their beautiful flowers contaminate it, although
to a degree of course which is relatively insignificant.

The development of heat by flowers was first observed by
Lamarck in the Arum maculatum of Europe. It was afterwards
detected by Saussure, in the Bignonia, Gourd, and Tuberose.
In these cases the heat was measured by a common ther-
mometer. But since the invention of thermo-electric instru-

138 COMPOUND ORGANS OF PLANTS.

ments, heat can be detected in any ordinary cluster of flowers.
The best plants for experiment are the Araceze, where the heat
is confined and reveberated by the hood-like inflorescence. In
some of these plants the temperature rises at times to 20° and
50° Fahrenheit, above that of the surrounding air. The tem-
perature rises from the first opening of the flowers, and reaches
its maximum when they shed their pollen, at which time the
heat developed is so’great as to be perceived by the hand; it
afterwards gradually declines until the flowers fade.

$

CHAPTER XI.

THE ESSENTIAL REPRODUCTIVE ORGANS.
THE ANDRECIUM OR STAMINAL ORGANS.

THe stamens are situated immediately within the corolla, and
form the third verticil of the flower. They constitute, collec-
tively, the androecium (doyp a male, and dcxdov habitation), or
the male sexual organs of the plant.

There is a power given to all plants of developing new plants
out of any of their cells, when these cells are placed in suitable
circumstances. In the cells of plants in general the expression
of this law seldom occurs, since it is only in rare cases that the
necessary conjunction of all the conditions is brought about.
Nevertheless, there are cases in which the ordinary leaves of
the stem may be made to develope new plants, as, for instance,
the leaves of Bryophyllum calycinum which, when placed on
moist earth, develope young plants from the indentations of their

THE ANDRECIUM. 139

margin. So, also, if a notch is made in one of the thick veins
of the leaves of the splendid Gesneria, and if the leaf is placed
on the ground, in about a week a new plant will be pro-
duced on its surface. The same phenomena oecur in the leaves
of the beautiful and scarlet-flowered Echeverias, and in many
other succulent plants. Now these plants could only originate
in the extraordinary development of certain cells in the leaf.

In general, however, those plants which have true leaves and
flowers, have these cells always produced in their terminal
leaves, which at this time take a peculiar form, as, for instance,
in the stamens. These reproductive cells, which are termed
pollen, are always developed in the interior of these metamor-
phosed leaves or stamens.

A stamen, when complete, consists of three parts; the fila-
meut, or thread-like portion, f; the anther, a, which is
situated on the top of the filament, and which usually consists
of two cells placed side by side, and attached to a prolongation
of the filament called the connectivum or connective; and
the pollen, or granular matter, p, contained in the cells of the
anther, by means of which the ovules are impregnated,
(Fig. 42.) The stamens are very conspicuous in the garden

Fig. 42.

lily, an examination of which flower will, in connection with
our engraving, convey a very accurate conception of these
important organs.

~

110 COMPOUND ORGANS OF PLANTS.

A fully developed leaf is composed of two parts, a little stalk
or support called a petiole, and a flat expanded portion called
the blade or limb, which is composed of woody fibre and paren-
chyma. The veins of the leaf constitute its woody fibre and
form its framework or skeleton, whilst the parenchyma is the
green cellular matter which fills up the interstices or intervals
between the veins. Now the petiole of the leaf is represented
in the stamen by the filament; the midrib by the connectivum ;
whilst the anther corresponds to the lamina or blade, each
portion of the lamina, on either side of the connectivum or
midrib, forming an anther lobe. The pollen contained in the
anther-cells results from a peculiar transformation of the
parenchyma._or green cellular matter of the leaf.

When the stamen is destitute of a filament, the anther is
said to be sessile, the filament being no more essential to the
stamen than the petiole to the leaf. When the anther is
imperfect, abortive, or wanting, the stamen is considered to be
sterile, abortive, or rudimentary, its real nature being known
by its situation.

In the stamens, the leaf undergoes such extensive structural
changes that its parts can scarcely be recognized. That the
stamens are only leaves which have undergone a greater
metamorphosis or change of form, nature herself teaches: All
will allow the analogy of the petal to the leaf. Now, the con-
version of stamens into petals is a common occurrence in plants
which have numerous whorls of stamens, especially when such
plants are brought under cultivation, as, for example, in the
rose and peony; but in no plant is it seen more clearly than
in the flower of the Nympheea alba, or white water-lily. In
this flower, perfect stamens are formed in the centre, the
filaments of which gradually enlarge towards the circumfe-

THE ANDRCCITIM. 141

rence, until at length the outer whorls of stamens exactly
resemble petals, except in having their tops developed into
yellow anthers, as seen at a and } in (Fig. 43;) and finally the

Fig. 43.

anther disappears altogether from the summit of the petal, as
at ce, and the metamorphosis is completed.

In this manner, what are called double flowers are produced.
The numerous whorls of colored petals in the rose and peony
result from a metamorphosis of a part, or sometimes of the
whole of their stamens into petals. This metamorphosis is the
effect of cultivation, the normal number of petals in the rose
being five, as is seen in the wild roses. A double flower,
therefore, although an object of admiration to the gardener, is
nevertheless justly regarded, scientifically, as a monstrosity.

If all the stamens are converted into petals, the flower is
13

142 - COMPOUND ORGANS OF PLANTS.

necessarily sterile ; but if some of the stamens are perfect,
even in a double flower, there may be fruit.

The number of stamens which compose the andreecium
varies very considerably. There may be only one, as in Calli-
triche verna, Water star grass, or many hundreds as in the
poppy. ‘The flower, according to the number of its stamens
from one to ten, is said to be monandrous (yéves one, dvyp male,)
diandrous (Ss two,) triandrous (zpecs three,) tetrandrous (rerpas
four,) pentandrous (xévve five,) hexandrous (2 six,) heptan-
drous (7a seven,) octandrous (éx7o eight,) enneandrous (fved
nine,) decandrous (6éxa ten.) Above ten there is no regularity
in the number of the stamens. All flowers having from ttvelve
to twenty stamens, are designated as dodecandrous (Sudexa
twelve ;) and if their number exceeds twenty, Polyandrous
(xows many.)

Proportion of the stamens.—The relative length of the
stamens is not always the same, the filaments being sometimes
more or less developed in the same flower. In some cases
there exists a definite relation as regards number between the
long and the short stamens. When a flower encloses four
stamens of which two are constantly the longest, it is called a
didynamous flower, (ds twice, and Sivayes power;) Fig. 44;
and when there are six stamens in the same flower and four of
them longer than the other two, the flower is said to be tetra-
dynamous, (¢erpas four, and divoprs power;) Fig. 45. The
natural orders Labiatee and Scrophulariaceze furnish us with
samples of the first, and Cruciferse of the last disposition of
the stamens. In the wood sorrel, (Oxalis,) there are ten
stameng, monadelphous at their base, five long and five short,
which alternate with each other.

Connexion of the stamens.—The stamens, in common with

THE ANDRGCIUM. 143

Fig. 44.

the other leaves of the plant, are found in a state of cohesion
in many flowers. When they cohere by their filaments to a
greater or less extent, forming a tube around the pistil, as in
the oxalis and mallow, (Malva,) Fig. 46, they are called mona-

Fig. 46.

Fig. 46. Vertical section of the flower of Mallow (Malva.) The stamens are
monadelphous, being united by their filaments into a cluster round the pistil.

144 COMPOUND ORGANS OF PLANTS.

delphous stamens (vos one, and 482905 brotherhood ;) diadel-
phous, when the filaments are united into two bundles, as in
the pea and fumitory. In the latter instance, the same number
of filaments cohere together in the two bundles, each of them
being composed of three stamens, but in nearly all papiliona-
ceous flowers, out of ten stamens nine are united by their
filaments while one is free. When the filaments cohere into
three bundles, the stamens are triadelphous, as in the St.
John’s-wort, (Hypericum,) Fig. 47; and when they grow
together into many bundles, polyadelphous, as in Ricinus com-
munis, the Castor oil plant, Fig. 48.

Fig. 47.

Fig. 47. Vertical section of St. John’s-wort (Hypericum.) This flower has tria-
delphous stamens, and a triearpellary pistil. Only two of the bundles of stamens
are visible, the third having been removed along with a part of the pistil.

Sometimes the stamens adhere to each other by their anthers,
the filaments being free, they are then said to be syngenesious or
synantherous (civ together, and yeveois origin, or avéqpa anthers).
This kind of union occurs in Composite flowers, of which the
cichory is a sample, (Fig. 49.) Occasionally, however, the
union of the stamens takes place through their entire length,
their filaments as well as their anthers cohering, as in Lobelia,
(Fig. 50.) Atlength the andrcecium, instead of forming a dis-
tinct verticil about the pistil occupying the centre of the flower,

TRE ANDRGCIUM. 145

becomes united with it so as to constitute but one body.
In this last case the stamens are gynandrous ( yw a female,
and dep a male), and the central body or column is called the
gynostemium (ym pistil, and orev a stamen), as in Aristolo-
chia, (Fig. 50.)

Fig. 49.

Fig. 51. Gynandrous stamens of Aristolochia rotunda. u. The ovary. 6. The
gymnostemium. c. The six stamens d. The six lobes of the stigma.

Let us now examine briefly the parts of which the stamen is
composed, having viewed them collectively. We have seen
that a fully developed stamen is composed of a petiole termed
a filament: a limb or blade named an anther, the pulverulent
parenchyma contained in the anther being called pollen.

The filament or petiole of the stamen supports the anther or
metamorphosed lamina of the leaf, and commonly justifies its
name from its form, that is to say, it is generally filiform and
slender. Sometimes, however, it is dilated and petaloid, as in
Ornithogalum umbellatum, the Star of Bethlehem, a white
flower with a bulbous root, quite common in meadows and

pastures about the middle of Spring.
13*

146 COMPOUND ORGANS OF PLANTS.

Filaments are usually of a white color, but occasionally
they take the same hues as the corolla. In Tradescantia Vir-
ginica, the spiderwort, the filaments are blue; in the different
varieties of the Fuchsia or lady’s ear-drop, they are red, and in
Ranunculus acris, yellow.

The anther is generally situated at the summit of the fila-
ment, to which it is attached in a variety of ways. Sometimes
it adheres to the filament by its entire length, when it is said
to be adnate, as in Magnolia glauca; or its base rests directly
on the apex of the filament, when it is innate, as in Sangui-
sorba Canadensis, or burnet; or it may be attached by a point
to the apex of the filament on which it lightly swings, when it
is versatile, as in the grasses.

The anther is the most essential part of the stamen. It con-
tains the pollen or fecundating matter, before the act of fecunda-
tion. Jt is most generally formed of two little pouches or cells
supported against each other by one of their sides, or united
together by an intermediate body, to which the name connec-
tivum or connective has been given. In this case the anther
is bilocular, (bis, twice, uculus, a pouch.) More rarely the
anthers are unilocular, as in the mallow, or quadrilocular, as in
Butomus umbellatus, the flowering rush; a plant occasionally
met with in England in brooks and rivulets.

The pollen or fecundating matter, when artificially removed
from the anther cells, looks to the naked eye like powdery
matter devoid of all organization, and is usually of a yellow
color; but it is also purple, blue, scarlet, black, and various
other shades. Placed beneath the microscope, this powder
resolves itself into a collection of spherical or oval grains, the
surfaces of which are generally smooth, but sometimes fur-
nished with strong points or bristles, as in the hollyhock,

THE ANDROCIUM. 147

(Althea.) In most plants these grains are free amongst them-
selves; but in the Fuchsia and Adnothera biennis, or evening
primrose, they are held together by slender threads, and in
other genera they adhere together.in masses called pollinia.

Pulverulent pollen. This is its most general aspect and
disposition. Pollen cells are ordinarily composed of two
membranes, which are distinguished as external and internal.
The interior of the cells is filled with a mucilaginous fluid
matter, containing granules, named fovilla. The exterior
membrane of the pollen cell, denominated the extine, (exto,
to stand out,) is thick, firm, and is readily ruptured by dis-
tension. It is this membrane which is covered with papillee
or granulations, the surface of the pollen being rarely smooth.
It is applied immediately on the internal membrane, or intine,
(intus, within.) This membrane is thin, transparent, very
extensible, and without any appreciable organization.

The mucilaginous fluid and granular matter in the interior
of the pollen cells has been the object of a great deal of discus-
sion amongst physiologists. The fovilla exhibit very marked
movements in the fluid where they swim. These movements,
-it was at first thought were spontaneous, and the pollenic
granules were supposed to be assimilated by them to the
zoosperms of animals. But the analogy has been completely
destroyed by an examination of the chemical nature of these
bodies, which are nothing but grains of starch, turning blue
with iodine, and showing all the characters of the fecula taken
from the other parts of the plant. This observation is due to
M. Fritsch of Berlin, who published in 1882 and 1833 two
interesting dissertations on pollen.

Solid pollen is that in which the grains instead of being
distinct are united together in masses, which in general take

\

148 COMPOUND ORGANS OF PLANTS.

the form of the cells of the anther which serves as a kind of
mould. The name pollinia has been given to these agglomera-
tions. It is only in the family of Orchidacese amongst
Monocotyledons, and that of Asclepiadacee in Dicotyledons
that we observe solid pollen.

Tn orchideous plants each of the pollen masses is supported
on a stalk called a caudicle (cauda a tail), which carries at its
extremity a glandular body called a retinacula (retinaculum a
band or rein), by means of which it is attached to the stigma.
These masses when bruised divide into grains which are
agglutinated together in fours.

Fig. 52.

Fig. 52. u. Represents one of these pollen masses, with its caudicle. 6. The reti-
nacula. c. fome of the grains separated from a similar mass to show the nature of
their agglomeration.

CHAPTER XII.
THE GYMNGCIUM OR PISTILLINE ORGANS.

Tue pistil occupies the centre of the flower and terminates
the axis of growth. The pistils constitute collectively the
Gymnecium (yv7 pistil, and é:xdov habitation,) or female
sexual organs of the plant.

THE GYMNGCIUM. 149

When fully developed the pistil, like the stamen, consists of
three parts, the stigma, the style, and the ovary (Fig. 53.)

Fig. 53,

The ovary a, is the lower part of the pistil, containing within
its cavity the ovules or rudimentary seeds d, and forms after
the impregnation of the ovules the future seed vessel. The
apex of the ovary usually tapers into a slender column called
the style 6, the summit of which is commonly somewhat
enlarged, denuded of cuticle, and secretes a viscid matter to
which the pollen grains adhere. This denuded and glandular
summit of the style is termed the stigma, c.

The ovary and stigma are never absent, the style sometimes
is ; in which case the top of the ovary itself is called the stigma,
as in the poppy, where it appears like the spokes of a wheel.

Like the other organs of the flower, the pistil is eomposed of
one or more modified leaves, which in this instance are called
carpels, from their connexion with the fruit, (xapzos, fruit.)
These leaves are folded inwardly, and their margins united, so
that their lewer surface forms the outside, and their upper
surface the inside of the carpel, the ovules being developed
along the margin of the leaves. That this is the true nature
of the pistil, the monstrous variety of the garden cherry con-
clusively proves. In this flower, the place of the pistil is

150 COMPOUND ORGANS OF PLANTS.

occupied by a green leaf, somewhat folded together, and similar
to the leaves of the branches, except in its lesser size. If we
compare this leaf, with the perfect pistil of the cherry, we
shall see that the folded lamina answers to the ovary, the
midrib projecting beyond the ovary to the style, and its
slightly dilated apex to the stigma. The analogy of carpels to
leaves may also be deduced from their similarity in texture
and venation, and from the situation of the ovules, which
exactly corresponds to that of the germs or buds found on
the margin of some leaves, as on those of Bryophyllum caly-
cinum.

Fig. 54.

The modified leaves or carpels forming the gymnecium,
cohere together to a greater or less extent, like the parts of
the flower ; and all degrees of union amongst them may be ob-
served from the mere cohesion of the contiguous bases of their
ovaries, (Fig. 54, a) to their perfect consolidation whilst their
styles are distinct, 6. In other species, both the ovaries and
styles of the carpels are consolidated, and the whole gymne-
cium forms an unique body, which may be mistaken for a single
pistil,c. But single pistils are by no means so common as is
usually supposed. If we make a transverse section of the ovary

x THE GYMNGCIUM. 151

of this apparently single pistil, we shall find.a number of cells,
which are in general equal to the number of consolidated car-
pels or pistils. If the ovary of the lily, for example, be cut
in this manner, what appears at first view to be a single pistil
will be found in reality to consist of three united ones.

When the carpel and pistils of the gymneecium are al] distinct
the pistil is termed apocarpous, (dz separate, and xapzos fruit,)
when they are united into one mass it is said to be synearpous,
(ov together or united.) :

Let us now carefully examine the different parts of the pistils.

The ovary is the inferior part of the carpel or pistil, and
contains the ovules within its cavity. It is either simple or
compound. Simple when it is unilocular or one-celled; com-
pound when it is bi-locular, tri-locular, &c.

The partitions which divide the compound ovary into cells
are termed dissepiments (dissepio I separate); and each dissepi-
ment being furmed of the united and contiguous walls of two
carpels, necessarily consists of two layers, one belonging to each
carpel, the ovary containing as many cells as there are carpels
in the compound pistil.

The placenta is the line or ridge to which the ovules are
attached, and corresponds to the ventral suture or line formed
by the union of the margins of the carpellary leaves.

The simple pistil has of course a one-celled ovary, but not
unfrequently the ovary of the compound pistil is also unilo-
cular. For the edges of the carpellary leaves are sometimes
folded inwardly, and form imperfect dissepiments which pro-
ject more or less into the cavity of the ovary but do not divide
it into cells. In this case the ovary is necessarily unilocular,
although it may be connected with a compound pistil.

If we suppose a circle of three carpellary leaves with their

152 COMPOUND ORGANS OF PLANTS.

ty

margins turned inwards, yet not so as to meet in the centre of
the ovary, to cohere merely by their contiguous inflexed por-
Fig. 55,

IO

tions, a one-celled tri-carpellary ovary would result, with three

c

imperfect dissepiments projecting into its cavity, in Fig. 55, a.
If we imagine the margins of three carpellary leaves to cohere,
making only three slight introflexions, it is obvious that there
would be no dissepiments, and the placentas would be truly
parietal (pardes a wall) the ovules being borne directly on the
walls of the ovary, as at 6. If, on the contrary, we suppose
the three carpellary leaves to be so folded inwardly as to carry
the inflexed portions of their united lamina, or in other words,
their dissepiments to the centre, and the dissepiments there
to unite and form a common axis, about which the ovules
develope; and if we then imagine the walls of the dissepiments
to be ruptured by the rapid growth of the ovary, it is obvious
that we shall have what is called a free central placenta, as
shown at c, Fig. 55, and also in Fig. 56. In all these cases the
compound pistil has an unilocular ovary.

All gradations may be observed in nature between strictly
parietal placenta and those which are carried forward so as to
mect in the centre of the ovary and separate its cavity into
distinct cells. d

In the Dog’s-tooth violet (Erythronium) and Campanula the
walls of the dissepiments are not ruptured. Fig. 55 is a tra-

THE GYMNGCIUM. 153

Fig. 55. Fig. 56.

\

Fig. 56. Vertical section of the tricarpellary ovary of Spergularia rubra, a plant
belonging to the chickweed family, showing the attachment of the ovules to a free
central placenta.
verse section of the trilocular or three-celled ovary of the Ery-
thronium. The ovules are attached to a central placenta. In
this instance the compound character of the ovary is sufficiently
evident.

In the chickweed family, Fig. 56, the dissepiments at first
project across the cavity of the ovary and meet in its centre,
but are finally torn asunder by the expansion of the ovary, so
that the several loculi communicate, the ovules remaining
attached to the placentas in the middle. The vestiges of the
dissepiments remain attached to the walls of the ovary, proving
that this is the mode in which free central placentations are
produced. In the blood-root and violet, the placenta are strictly
parietal.

In most cases the compound pistil, provided with a one-celled
ovary, is easily recognized. Thus every time that an unilocular
ovary is surmounted by several free styles and stigmas, or by
the same united amongst themselves and only distinguishable
at their summit by some slight incision, the pistil will be com-
pound. It is only necessary to remember that a pistil is never
without an ovary and stigma, and in most cases possesses a

14

154 COMPOUND ORGANS OF PLANTS.

style; the plurality of styles and stigmas therefore necessa-
rily proves a plurality of pistils.

In general the carpels contract no adhesion with the floral
envelopes. They are simply attached to the receptacle, so that
when they grow and elevate themselves they remain perfectly
intact. We say in this case that the ovary is free and superior,
being situated above the floral envelopes, and that the stamens
are perigynous (wep around, and yw»7 pistil.) But sometimes
the calyx grows to the surface of the ovary carrying with it
the petals and stamens, so that all these organs seem to rise as
it were out of the summit of the ovary, as in the honeysuckle
and dog-wood. The ovary in this instance is inferior, as it is
situated below the floral envelopes, and the stamens epigy-
nous (é¢ upon, yvv7 pistil.) This distinction between the infe-
rior and superior ovary is very important, as it serves to
distinguish certain natural families.

The Style-—The general character of the style in simple
ovaries has been already described. In compound ovaries there
are as many styles as there are carpels; and they either remain
distinct, as in the pink, or become partially united, as in the
geranium, or completely consolidated to their summit, as in the
lily.

When we examine a transverse section of the style with a
sufficient magnifying power, we always find it hollow. The
interior of the style is in fact a canal, extending from the
stigma to the cavity of the ovary. This canal is sometimes
open; but generally it is filled with a humid and lax paren-
chyma, which differs considerably from the other parenchyma
of the style, and which is distinguished as the conducting
tissue. This tissue spread out on the summit of the style

rms that spongy surface called,—

THE GYMNG@CIUM, 155

The Stigma,—This is a glandular body, placed on the sum-
mit of the style, when there is one, or immediately on the
ovary when there is no style. It is denuded of cuticle, and
secretes a viscid fluid which detains the pollen grains, and
causes them to emit tubes. This secretion becomes more
abundant as the period of fecundation approaches.

The stigma is simple when it is connected with a single
pistil; but in the compound pistil there are necessarily as many
stigmas as there are carpels united together. When the
ovaries and styles of all the carpels of a compound pistil are
in a state of complete cohesion and consolidation, the stigma
always presents a number of lobes or divisions more or less
deep, which clearly indicate the number of pistils which have
cohered together.

The lobes of the compound stigma are excessively variable ;
they may be flat and pointed, and hemispherical and blunt,
smooth, or covered with salient papille, or with hairs simple
and glandular, or with branched and plumose hairs, as in the
grasses.

The Ovule is the body which is contained in the cavity of
the ovary and attached to the placenta, and which, after im-
pregnation, is transformed into the seed. It experiences in
this transformation remarkable changes in its structure, form
and position.

In order accurately to trace the development of an ovule,
we must commence our observations as soon as the plant
begins to form flower-buds. We shall then see in the interior
of the ovary, forming on the placenta, a minute excrescence or
tubercle, formed solely of cellular tissue. This gradually en-
larges into a more or less obtuse conical form, constituting
what has been called the nucleus of the ovule. As growth

156 COMPOUND ORGANS OF PLANTS.

progresses, one of the cells towards the apex of the nucleus
expands, forming a cavity in its interior, termed the embryo
sac, because it isin this cavity, after impregnation, that the
rudimentary embryo first makes its appearance.

In the mistletoe the ovule remains in this simple and naked
condition. Fig. 57 is the ovule of the mistletoe entire and in
section with the embryo sac, c. In most plants, however, the
nucleus becomes surrounded by one or more coverings during
the progress of growth. These first appear around the base of
the nucleus in the form of circular swellings, which gradually
spread over its surface.

Fig. 57.

In some cases, as in the ovules of the Walnut, fig. 59,

Fig. 58, Fig. 59.

the nucleus n, has only one covering formed on its surface ;
generally, however, whilst this envelope is increasing, another
envelope is formed outside of it, beginning at its base, and

THE GYMNG@CIUM. 157

overspreading its surface in precisely the same way, as is repre-
sented in Fig. 59.

A fully developed ovule, therefore, consists of a conically-
shaped nucleus of cells containing a cavity or embryo sac in
its interior, with two external coverings. The one s, next the
nucleus n, which is first formed, is termed the secundine, the
other p, the primine. At the apex of the nucleus, both cover-
ings leave an opening which has been termed the foramen or
micropyle, (uixpds little, xvay gate), through which the nucleus
slightly projects when it is not completely covered. The open-
ing or mouth of the primine, ex, is called the exostome, (c&a
outside, and océuo mouth ;) that of the secundine end, the en-
dostome (?Sov within). f is the point where the ovule is
attached to the placenta.

The nucleus and its two external investments have no organic
connexion with each other, excepting at the base of the ovule,
where vessels pass from one into the other and unite the several
parts firmly together. This common point of union is termed
the chalaza.

The ovule is attached to the placenta either directly, when
it is said to be sessile, or by means of a prolongation or umbi-
lical cord termed the funiculus, (funis, a cord.) The point
where this cord is inserted into the ovule is termed the hilum.
The micropyle or foramen is therefore situated at the apex of
the ovule, and the chalaza and hilum at its base.

When all the parts of the ovule develope uniformly, they
maintain the same relative position throughout their entire
growth, as they had at its commencement. Fig. 60. The
chalaza ch, is at the hilum or base of the ovule, and the micro-
pyle, m, at its apex or opposite extremity, so that a straight

14*

158 COMPOUND ORGANS OF PLANTS.

Fig. 60.

Nt!

line passes through their axis. In this instance the ovule is
said to be orthrotropous, (épéos, straight, and zpdémos, mode.)

This is the primitive and most simple form of all ovules,
although not the most common. The ovules of the Urticaceex
or nettle tribe, of the Cistaceze or rock-rose family, and of the
Polygonacez or buckwheat family, are of this character.

When, however, there is an inequality in the development of
the parts of the ovule, either one or the other of the following
modes of growth will generally be the result.

Either the hilum and chalaza will remain together and the
ovule will curve upon itself, so that the micropyle will be
brought near to the hilum, and we shall have a campulitropous
ovule, (xausvnds curved,) as in all cruciferous plants, (Fig. 61 ;)
or else the chalaza will elongate from the hilum and become
transported to the apex of the ovule, whilst that apex by an
inverse movement directs itself to the place which the chalaza
has abandoned. In this case, the ovule is said to be inverted
or anatropous, (dvarpénw I subvert.)

The curvature of the ovule in the first instance, is to be
attributed to an inequality in the development of its sides.
Thus, one of the sides of the primine possesses more energy of
development than the opposite side; the former therefore elon-
gates whilst the latter remains stationary ; and the resistance

THE GYMNGQCIUM. 159

Fig. 61.

“Stina!

Raa,

—S
oy

vesting

~

Fig. 61. Campulotropous ovule of Wall flower. (Chetranthus.) jf. The funiculus by
which the ovule is attached to the placenta, . The primine. s, The secundine. 7.
The nucleus. ch, The chalaza. The ovule is curved on itself, so that the micro-
pyle is brought near to the hilum.

Fig. 62. Anatropous ovule of Dandelion (Leontodon). _f. The foramen or micropyle.

h%. The hilum. ch. The chalaza. x. The nucleus. 7. The raphe connecting the
chalaza or base of the nucleus with hilum /, and placenta.

offered by the inert side necessarily compels the extensible one
to turn round the centre of resistance, and the ovule curves
upon itself.

In the other instance, since the hilum retains its place, the
vascular bundle which brings it into communication with the
chalaza is forced to follow the ovule in its evolution, and forms
by its elongation, a cord more or less prominent within the
thickness of the primine which is called the raphe, (pagy a
line.)

Some botanists think that the anatropous ovule, is simply
an orthotropous ovule inverted on an elongated funiculus or
podosperm, (mois a foot, and onépya a seed,) which is attached
in the form of a raphe to one side of the ovule. But the
raphe 7, Fig. 62, appears to be an elongation of the vascular
bundles which connect the chalaza with the hilum; and this
view is established by the fact that in anatropous ovules, the

160 COMPOUND ORGANS OF PLANTS.

hilum is not seen at s, the part where the raphe joins the
chalaza, but at A, the part where it unites with the placenta.

These three forms of ovules are by no means clearly defined
in nature, but exhibit varieties, among which we must mention
the amphitropous or heterotropous ovule, which is produced by
a partial adhesion of the funiculus or raphe to the ovule,
(Fig. 63.) The funiculus is seen at right angles to the ovule,
and the hilum is placed midway between the micropyle and
chalaza. The Leguminose or pea tribe have generally ovules
of this character.

Anatropous ovules are the most common in plants. The
orthotropous form is considered to be the condition of all ovules
at the commencement of their development, and the other
forms are referable to changes produced during growth. The
anatropous ovule of the celandine and the campulitropous ovule
of the mallow, have been traced from the orthotropous condi-
tion at the commencement of their growth, through all the
intermediate stages of development.

Fig. 63. Fig. 64.

Fig. 64, is a representation of the development of the anatropous ovule of the
Celandine, (Chelidonium majus.) 1 and 2, are the first stages. The primine and secun-
dine investments are marked p and s,and the summit of the nucleus n. 3 is the fully
developed ovule after it has executed its demi-revolution on its funiculus f{ The
reason of this singular change in the position of the ovule will appear in the next
chapter.

All these changes in the structure, form and position of the
ovules, are executed whilst the flower buds are forming. About

FERTILIZATION. 161

the time of the expansion of the flower, the ovules are gene-
rally fully formed and ready to receive the impregnating influ-
ence of the pollen. They have become regularly shaped
usually roundish bodies fixed to the placenta by one side.
They are not yet seeds, but are destined to become seeds at a
future period.

CHAPTER XIII.

THE PROCESS OF FERTILIZATION OR FECUNDATION.

Functions of the stamens and pistils——Fecundation is that
function by which the pollen is brought into contact with the
pistil, so as to produce within the ovule the formation of an
embryo. The results of fecundation are the transformation of
the ovules into seeds and of the carpels into fruits. Let us
consider,—

1. The preparatory or precursory phenomena of fecundation,
or the arrangements made for securing the application of the
pollen to the stigma. Fecundation in general takes place at
the period of anthesis, (dv6n0s, flower opening.) The anthers,
up to this time unruptured, open their cells, and spread the
pollen over the stigma and very frequently over the other
parts of the flower, and it is then that fecundation is effected.

There are however a certain number of plants among which
fecundation takes place before the expansion of the floral
organs. This is the case with many of the Composite and
Aster tribe which have syngenesious stamens, the stigmas and
styles of whose pistils are clothed with what botanists have

162 COMPOUND ORGANS OF PLANTS.

agreed to call collecting hairs. The style of these plants is at
first shorter than the stamens and enclosed by the cohering
anthers; as it developes it pushes its way through them, and
the hairs on its surface brush the pollen out of the anther-
cells, carrying it up along with them. Hence when the
flowers are fully expanded we find the anthers already open
and in part empty, fecundation having been accomplished.

In most cases, however, fecundation does not take place
whilst the perianth encloses the sexual organs, but at the time
of anthesis. When this period arrives, the opening of the floral
envelopes frees the stamens from all confinement and restraint,
and they take a rapid development. Their filaments elongate,’
and the pollen contained in the anther-cells up to this period
succulent, moist, and adherent to the cell walls, becomes dry,
pulverulent, and free within their cavities. About this time
too, the stigma or summit of the pistil, tumefies and excretes
in. great abundance a viscous fluid which lubricates its surface
and causes it to retain the pollen grains.

But before the pollen of the stamens can be applied to the.
stigma of the pistil, it is necessary that it should have some
outlet or means of escape from the anther-cells. In the
greatest number of cases, the cells open longitudinally through
the whole extent of that furrow or groove which may be
readily observed on their surface, as in the gilliflower,
Sometimes, however, the dehiscence, (dehisco, I gape), only
takes place at the upper part of the furrow, by an aperture
resembling a pore, as in Pyrola chlorantha, (Fig. 66.)

In the common barberry, (Berberis,) the cells present no
furrow, but a portion of their anterior surface opens in the
form of valves, (Fig. 67.) In Pyxidanthera barbulata, the

FERTILIZATION. 163

anther-cells open by traverse dehiscence in the form of an
operculum or lid, (Fig. 68.)

Fig. 66. Fig. 67.

The mechanical application of the pollen to the stigma is
sometimes secured by certain relative adjustments of the
organs. Thus when the stamens and pistils are situated in
separate flowers on the same plant, the staminate flowers are
generally situated above the pistillate. The Indian corn
exemplifies this arrangement. It is well known that the
flowering panicle at the summit of the stem does not produce
corn; these are the staminiferous flowers, from whose anthers
descend clouds of pollen on the thread-like pistils, forming the
silky tuft beneath. Without this pollen, the corn in the lower
spike w.uld not ripen; hence the evident design of nature in
placing the pistillate below the staminate spike of flowers.

In pendulous and upright flowers, the filaments of the
stamens and the style of the pistil are so developed as to bring
the anthers and stigma into the most favorable relative position
for communicating with each other. This is beautifully exem-
plified in the ladies ear-drop, (Fuchsia.) Within the pendulous
corolla of this flower, we have an adjustment of the sexual
organs with an evident reference to their mutual action on
each other. The filaments of the stamens are short and the

164 COMPOUND ORGANS OF PLANTS.

style of the pistil is considerably elongated, and its lubricated
and viscid stigma is brought below the anthers ready to receive
the falling pollen. In upright flowers we have a reverse
arrangement of the parts; for the style of the pistil is ina
great measure suppressed, and the filaments of the stamens
are so developed as to place the anthers above the stigmatic
surface.

In many plants fecundation is effected by certain special
movements of the male or female organs of the flower. The
flowers of the mountain laurel (Kalmia) are, in this respect,
especially deserving of examination. The corollas of the
Kalmia are rotate or wheel-shaped, and have ten stamens.
The anthers of these stamens, before the flowers expand, are
contained in ten little cavities or depressions in the side of
each corolla, where they are secured by a viscid secretion ;
when the corollas open, the filaments are bent back by the
confinement of their anthers, like so many springs, in which
condition they remain until the pollen in the anther-cells
becomes ripe, and absorbs the secretion. The anthers becom-
ing suddenly liberated by this means from their confinement,
fly up from their cavities with such force as to eject their
pollen on the stigma of the pistil. The slightest touch with
the point of a needle, or the feet of an insect crawling over
their reflexed filaments, will produce the same effects, if the
pollen is mature.

In the same manner, the stamens of the common barberry
spring to the pistil if the lower part of their filaments is
touched, and seldom fail in making the movement to throw a
quantity of pollen on its stigma. The stamens of the Rue, of
some of the Saxifrages, and of Parnassia palustris, a rare and
beautiful snow-white swamp flower, do this in succession, first

FERTILIZATION, 165

one and then the other approaching the pistil and discharging
upon it the polliniferous contents of their anthers.

When grains of pollen are thrown on water, the absorption
of the fluid is so rapid, that they burst, and a thick liquid
escapes from them which spreads itself over the surface of the
water. This thick liquid, in fig. 69, is seen escaping from one

Fig. 69.

of the pollen grains of Ipomcea hederacea, and is the fecundating
matter of the grain. The action of the pollen is therefore
liable to be frustrated by wet weather. This evil is guarded
against by the property which the anther-cells possess of open-
ing only in fine weather, as well as by the action of the floral
envelopes, which in some plants appear to be exceedingly
hygrometrical, enveloping the sexual organs on the slightest
appearance of any humidity in the atmosphere. The flowers
of the red chickweed (Anagallis) are a very remarkable illus-
tration of this phenomena.

In this view too the economy of various aquatic plants is
exceedingly interesting, as for instance the pondweeds (Pota-
mogeton.) These plants live wholly submerged in the water ;
but at the time of flowering, the peduncles or flower stalks
elongate so as to raise their flowers to the surface on which
they may be seen floating. The act of fertilization is thus
accomplished in the open air, and the ovaries are again drawn
beneath the water, where the seed ripens. The peduncles of

15

166 COMPOUND ORGANS OF PLANTS.

the white water lily, Nelumbium, and Brazenia peltata, some-
times attain the length of from fifteen to twenty feet along the
shores of some of the American lakes, so as to bring their
flowers to the surface; in fact, the length of the peduncle of
these plants appears to be wholly regulated by the depth of
the waters in which they are found floating.

The essential phenomena of fecundation, consist in those
changes which take place in the pollen grains when brought
into contact with the stigma of the pistil, together with the
action of the pollenic tubes on the ovules. We have intimated
that pollen grains discharged from the anther-cells on the
stigma, are retained there by a viscid fluid, which at this time
most plentifully bedews the stigmatic surface. Very soon we
see them swell out, as they absorb this fluid, those which
are elliptical or elongated becoming almost spherical. At the
end of a certain time, consisting of a few hours for some species,
and many days for others, the thin and highly extensible intine
or inner coat of the pollen grain is seen protruding in the form
of a tubular or vermiform appendage.

The mode of dehiscence of the pollen grains is always deter-
mined by their structure. Those which present pores, grooves,
or folds on their exterior surface, usually emit their tubes at
these points. The number of tubes emitted from pollen grains
ig very variable; sometimes we see only one, and occasionally
two or three, as in the triangular pollen of the evening prim-
rose (nothera) Fig. 70. Amici was able to detect from
twenty-six to thirty tubes which were protruded from the same
cell. The number of tubes must necessarily bear some relation
to the number of pores when these exist, and we know that
they are sometimes very numerous. The pollen tubes are
filled with a fecundating fluid termed fovillee, and it is easy to

FERTILIZATION. 167

see through their thin transparent walls the movements of the
microscopical corpuscles which it contains.

As soon as the pollenic tubes have been protruded from the
pollen grain, they penetrate the loose cellular tissue which
constitutes the mass of the stigma, known as the conducting
tissue, and insinuating themselves amongst the interspaces of
its cells where they find an abundance of moisture, they grow
downwards through the central part of the style until they
reach its base, a distance in some cases of several inches.
Hence by making a longitudinal section of the pistil we are
able to find these tubes and to trace their course.

The pollen tubes may be readily inspected under the micro-
scope in many plants, and in none more readily than in the
Asclepias or milkweed. In that family the pollen grains
cohere together in masses termed pollinia, and their united
tubes are protruded, and consequently are of such a size as to
be easily perceived with a very moderate magnifying power.
Fig. 71.

The action of the pollenic tubes on the ovules. At the time
that fecundation is operating, and the pollenic tube is being
elaborated, the ovules are organized into a suitable form for its

168 COMPOUND ORGANS OF PLANTS.

Fig. 71. Pistil of Asclepias a, with pollen masses p, adhering to the stigma s.
b. Separate pollen masses united by a gland like body.
reception. We have seen that the nucleus is covered by two
membranes, called the primine and secundine, and that at the
“apex of the nucleus both coverings leave an opening which
has been termed the foramen or micropyle.”’ Now this open-
ing, or the nucleus projecting beyond it, is the ultimate desti-
nation of the pollen tube. Before its arrival, however, one of
the cells towards the summit of the nucleus expands and thus
creates a cavity in its interior which is called the embryo sac,
because it is in the interior of this sac that the embryonal
vesicle first makes its appearance in the upper part of the
cavity. Itis at first a simple cell which insensibly elongates,
and by the formation of transverse septa forms itself into a sort
of confervoid tube. The terminating cell of this tube enlarges
and forms the embryonal vesicle. The pollen tube, having
arrived at the base of the style, enters the ovary, and’ makes
its way through the micropyle or orifice of the ovule, pene-
trating the tissue of the nucleus till it reaches the embryo sac.
Fecundation appears to be produced by the simple contact of

FERTILIZATION. 169

the pollen tube with the embryo sac, and the imbibition by the
embryonal vesicle of the contents of the pollen grain through
the intervening membranes, the vitally active contents of the
two cells being thus commingled.

The development of the embryo, The embryonic vesicle is
at first simply a spherical cell, developed at the end of the sus-
pensory filament, filled with fluid, and containing granular
matter. A little time after fecundation, a longitudinal septum,
in the same direction as the suspensor, is seen to form’ across
the cavity of the cell, which thus becomes divided into two cells.
Very soon each of these two cells is divided into two others,
and all prove successively the same segmentation. The neces-
sary result of this is, a little mass of cellular tissue limited
exteriorly by the walls of the primitive cell, forming the
embryonal vesicle. It is this mass of cellular tissue which by
degrees organizes itself into an embryo.

In some plants it remains in this primitive and somewhat
amorphous state, being simply a mass of cells without distinc-
tion of organization or of parts. This is its condition in all
Acotyledonous or Cryptogamous plants, where the embryo
bears the special name of spore. In Phanerogamous plants,
however, this mass of cells assumes a more highly developed
state. The cells in the upper part of the mass which are
immediately connected with the suspensory filament, elongate
into a somewhat conoid body, which in the perfect embryo
constitutes the radicle, ‘whilst the cells in the lower part soon
begin to present traces of their future cotyledonary character,
the end farthest from the suspensor becoming two-lobed in
Dicotyledonous, and one-lobed in~ Monocotyledonous embryos.
(xorvandav, the name of a plant having leaves like seed-lobes. )

15*

170 COMPOUND ORGANS OF PLANTS.

Fig. 72,* shows the different stages in the development of

Fig. 72. a. Vertical section of the pistil of Polygonum after fertilization, showing,
the pollen grains adherent to the stigma with their tubes passing down the style, the
erect orthotropous ovule in- the interior of the ovary, and the nascent embryo sac.
A pollen grain detached with its tube. b. The ovule more highly magnified, showing
the embryonal vesicle formed in the interior of the sac at a later period. c. The nas-
cent embryo and.its suspensor removed from the sac, and more magnified. d,e¢ f.
The embryo in succeeding stages of development. g. The embryo as it exists in the
seed.

the Dicotyledonous embryo of a species of Polygonum. Only
one ovule is contained in the ovary of the pistil, and this is
orthotropous. The plant has therefore been very properly
selected for illustration on account of the simplicity of its
pistil. The process is the same, although more complicated
when the ovules are more numerous.

The change of the ovule into the seed. It will be perceived
that as the embryo is developed, the suspensory filament by
which it is attached to the summit of the embryo sac is
gradually absorbed; also that great changes must necessarily

* «Botanical Text-Book,” by Asa Gray, M. D.

i

FERTILIZATION. 171

take place in the structure of the ovule as the embryo forms
in its interior.

In some instances, though rarely, all the parts of the ovule
are visible in the seed; but in general these parts either dis-
appear altogether, as the embryonic mass increases in bulk, or
are very materially altered. In many Dicotyledons, the em-
bryo as it develops absorbs into itself not only the embryo
sac but the tissue which forms the nucleus, so that the seed
at its maturity contains nothing but an embryo of which the
cotyledons are thick and fleshy, by the amount of nutritious
matter which they have absorbed, and the integuments of the
ovule, the primine and secundine, which form its general
covering. This is the case for example in the Leguminous
family. The pea (Pisum) is a good illustration...

But in other Dicotyledonous plants, and in all Monocotyle-
dons, the nutriment which the ovule contained in its interior
is unabsorbed into the embryo, which does not increase much
in bulk, and encroaches very slightly on the cells of the nu-
cleus. These cells therefore become filled with a deposit of
solid matter termed albumen, in the midst of which the embryo
is embedded.

Seeds in which the embryo occupies the entire seed are
called ex-albuminous (ex without), as the Composite, Cruci-
ferze, and Leguminose, whilst others having separate albumen
are albuminous. The larger the quantity of albumen in the
seed, the smaller the embryo.

Soon after fertilization,.the pollen tube withers from above
downwards, the foramen or micropyle of the ovule closes, and
when the embryo is fully developed within it, the ovule
becomes the seed and the ovary the fruit.

The changes which manifest themselves in the flower and in

172 COMPOUND ORGANS OF PLANTS.

the other sexual organs of the plant after fecundation. A plant
in every stage of its existence is a beautiful subject for con-
templation, but particularly at the close of the period of its
life. What, when its leaves are withering and falling from its
stem! when its flowers are losing their brilliant hues and
inimitable coloring! and when the whole vegetable economy
of the plant is languishing! Yes, even then it becomes, if
possible, an object of deeper admiration. Why do the leaves
fall from its stem? Because food is no longer required to be
taken from the atmosphere. Why do the flowers lose their
beauty, the petals detach themselves and fall, and even the
stamens experience the same degradation? It is because these
parts of the plant have fulfilled their allotted functions. No
leaf or flower fades or falls in nature before it has accomplished
the purposes of its creation. You see that the pistil alone
remains in the centre of the flower. But the style and stigma
are now useless to the plant, and therefore they disappear
equally with the other parts. The ovary alone is persistent,
since it is in its bosom that nature has carefully deposited the
embryo or seed which contains in itself the rudiments of future
generations.

A little time after fecundation, we see the ovary increase in
size, the ovules which it encloses being converted into seeds
containing an embryo, and very soon the ovary has acquired
all the characters proper to constitute it a fruit.

MODIFICATIONS OF THE FLORAL ORGANS. 173

CHAPTER XIV.
ON THE VARIOUS MODIFICATIONS OF THE FLORAL ORGANS.

Hirwerto we have studied the flower, in the higher degrees
of its development, in a complete, symmetrical, and regular state.
We have to a certain extent supposed that there was no dis-
turbance of this regularity. Thus we have described the
flower as composed of all its verticils, the calyx, the corolla,
the stamens, and pistils—or in a complete state. We have
supposed the parts of each verticil to be alike in size and
shape, or the flower to be regular, and each verticil to contain
the same number of pieces or a multiple of that number,
separate from each other and alternating among themselves—or
the flower to be symmetrical.

Fig. 73.

(2)
oly

To obtain an exact view of the symmetry of a flower we must
observe it whilst in the bud, and trace it out in the form of a
horizontal section, as if all the verticils had been deprived of
height and sunk down to the same plane. We are thus
enabled to see at a glance the position of the different parts
of the flower. This theoretical section is called a diagram.
Fig. 73 is a diagram of a complete, symmetrical, and regular

174 COMPOUND ORGANS OF PLANTS.

flower, that of a Crassula, showing the alternation of the parts
of each verticil, and also an equality in the number of pieces
of which each verticil is composed. All the verticils are
separate from each other, and the parts of each are equally
distinct.

Assuming this complete and symmetrical flower to be the
normal plan or type on which flowers are constructed, when
we examine the various plants around us we find that most of
them are in an abnormal state, and we are able only to cite a
very small number whose flowers preserve this complete,
symmetrical, and regular condition. In the immense majority
of cases the regularity is destroyed, and the symmetry disguised
by a variety of causes. The following are those which act the
most frequently :—

One or more additional verticils of the same organs have
been developed.—Thus in Ranunculus we have five sepals, five
petals, and numerous stamens and pistils; this is occasioned
by the development of additional whorls of stamens and pistils.
A multiplication of stamens also occurs in other plants, as in
Anemone and Hypericum.

The composition of the flower is somewhat different in
Dicotyledons and Monocotyledons. In the first, it is the num-
ber five or one of its multiples which commonly predominates.
Thus the calyx is generally composed of five sepals, the corolla
of five petals, the androecium of five, ten, or twenty stamens,
and the gymnecium of five pistils or some multiple of that
number, the parts of all the extra verticils alternating with
each other. Fig. 74 is a diagram of the flower of the Ranun-
culus with five sepals, five petals, and numerous stamens and
carpels in alternating rows of five each. This orderly distribu-
tion of a certain number of parts is called symmetry, and a

N

MODIFICATIONS OF THE FLORAL ORGANS. 175

flower in which the parts are arranged in fives is said to be
pentamerous, (éve five, uépos a part.)

In monoeotyledons, on the contrary, we observe more fre-
quently the number three or one of its multiples, or the flower
is trimerous, (mpées three, pépos a part.) Fig. 75 is a good illus-
tration. It is a diagram of the flower of the snow-flake,
(Leucojum), a monocotyledonous plant, having three sepals,
three petals, six stamens in two alternating rows, and three
carpels. This flower is symmetrical, complete, regular, and
trimerous.

The number of extra verticils which are developed is some-
times very considerable, as in the Cactus and white water-lily,
(Nymphzea.) In such circumstances it is easy to perceive
that the disposition by verticils is only apparent and that the
floral leaves are arranged in a spiral about the axis of growth.
The spiral law not only produces the orderly distribution of |
the leaves about the stem, but ensures the same symmetry in
the floral organs, producing that regular alternation of the
parts of each verticil, and in these plants a very perceptible
spiral arrangement of them.

The parts of the floral organs may have been increased by

176 COMPOUND ORGANS OF PLANTS.

deduplication or chorization, (zapiSo, I separate,) that is, by
the splitting of the organs during their development. This
process accounts satisfactorily for the appearance of certain
parts which do not follow the law of alternation. This chorisis
or separation is either collateral, the separated parts being
placed side by side, or transverse, the parts separated being left
one in front of the other.

Of collateral chorisis we have a good example in the tetra-
dynamous stamens of the Crucifere. The stock and wall-
flower belong to this natural order, and are plants with which
all are familiar. Fig. 76 is a diagram of a flower of the com-

Fig. 76. Fig. 77.
&

© .

eee

mon stock, (Matthiola incana,) showing a calyx with four
sepals, a corolla with four petals, but the stamens are six:
four long and two short; the former, placed together in pairs
as shown in the diagram, are supposed to have been originally

MODIFICATIONS OF THE FLORAL ORGANS. 177

one stamen which has been split into two by collateral chorisis,
thus producing the want of symmetry in the stamineal circle.
And that this supposition has some foundation is evident from
what we see in Streptanthus hyacinthoides, (Fig. 76,) one of the
wild flowers of Texas. In this plant the chorization has been
arrested before its completion, so that in the place of two
stamens we see a forked filament bearing two anthers.

The beautiful marsh flower, called by botanists the Elodzea
Virginica, (Fig. 77,) furnishes us with another sample of col-
lateral chorisis. The ground plan of this flower, a; shows it to
be both pentamerous and trimerous in its organization, its
floral envelopes consisting of five sepals and petals, whilst its
andreeciu and gymnecium consist of nine stamens and three
pistils, the nine stamens being triadelphous, and evidently
formed by collateral chorization out of three, as shown at 0.
The three glands which occupy an intermediate position
between the corolla and the andreecium, as shown in the dia-
gram, are probably the rudimentary traces of an exterior circle
of stamens which have been rendered abortive.

Fig. 78. Fig. 79.

Transverse chorization, or the separation of a lamina from

organs already formed, is believed to take place in the case of
16

178 COMPOUND ORGANS OF PLANTS.

appendages to petals. In Ranunculus, transverse chorization
or dilamination of the petals, produces a scale-like body at
their base, (Fig. 78,) and a two-lobed appendage on the inside
of the lamina of the petals of Silene, (Fig. 79 ;) and in Parnas-
sia Caroliniana this accessory structure assumes at the base of
the petal the appearance of abortive stamens, (Fig. 80.) These
bodies, however, are situated opposite to the petals as shown in
the diagram, and the stamens altcrnate with the lobes of the
corolla and are therefore in their normal position, so that
these appendages are certainly not stamens, but are produced
by the transverse chorization or dilamination of the petals
opposite to which they are placed.

One or more pieces of the same verticil may hyve united
among themselves, or the whole of the pieces of the same verti-
cils may have become coherent. These unions are extremely fre-
quent, and may manifest themselves in all the floral verticils.
Thus the sepals may become soldered together and form a
monsepalous calyx, or the petals, a monopetalous corolla; and in
like manner the stamens may unite together by their filaments
and become monadelphous, diadelphous, or polyadelphous, or
by their anthers and become synantherous or syngenesious; and
lastly, the carpels may become united together by their ovaries,
or by their ovaries, styles and stigmas, so as to constitute an
apparently unique pistil. These different kinds of soldering
are very common amongst flowers, and, generally speaking, they
do not alter their symmetry and regularity.

But, in other flowers, it is more difficult to perceive at first
sight what it is that disturbs their regularity and symmetry, as
for instance when two or more pieces of the same verticil
become soldered together. In general, however, with a little
care, the number of petals or sepals which have united, and

MODIFICATIONS OF THE FLORAL ORGANS. 179

the nature and extent of the soldering, may be easily detected.
For example, if the number of petals or divisions of a mono-
petalous corolla do not correspond to those of the calyx, and
this difference is due to the cohesion of one or more of the
petals, the nature of the soldering may be readily detected by
the number of the midribs. Thus when two petals have united
in the place of one nerve, we shall detect two collateral nerves
in the petal, or three, one of which will be in the middle when
the compound petal results from the union of three petals. ._In
general, the number of midribs in the compound petal or sepal
will be sufficient to show the number of separate pieces which
have become soldered together.

Let us. take for illustration and analysis, the flower of the
common Snap-dragon (Linaria,) (Fig. $1.) The calyx is mo-

Fig. 81.

nosepalous, and has five equal divisions. The corolla is mono-
petalous with two unequal lips, of which the superior repre-
sents two petals, the inferior, three, whose midrib is prolonged
into a spur. The stamens are four in number, two long, and
two short; the former being situated between the middle petal
and the two lateral petals of the lower lip, the latter being
placed in the fissures which separate the two lips. At the base
of the superior lip may be detected a little filament represent-
ing the fifth stamen.

180 COMPOUND ORGANS OF PLANTS.

Tn certain circumstances the Linarias develope themselves
with all their petals similar to the middle petal of the lower
lip, and the verticil presents then a perfectly regular figure.
It is a corolla, with five lobes and five spurs, perfectly equal
among themselves. At the same time, the filament placed at
the base of the superior lip developes itself into a stamen
organized like the others, which, although unequal in their
habitual condition, are now absolutely the same in length, after
the manner of a flower provided with five symmetrical stamens.

The name peloria (werdpeos monstrous,) has been given to
this kind of metamorphosis; but modern botanists, far from
regarding this change as a digression of nature, consider it as
a return to the normal state of the flower. To their eyes, an
irregular flower is an habitual alteration, and a pelorious flower
is a flower put into regular order,

The parts or organs of the same verticitl may have been un-
equally developed. This inequality of development is strikingly
shown in the papilionaceous corolla of the pea, the parts of
which are distinguished by separate names. This plant has all
the parts of a symmetrical pentamerous calyx and corolla, only
they are irregular on account of an inequality in their develop-
ment. In certain orders of the papilionacea the corolla has,
however, a tendency to become regular, and in Cassia, the five
petals differ very little from each other either in shape or size.

One or more floral verticils may have united with each other.
Thus the stamens are united to the calyx in the rose and black-
berry, and to all monopetalous corollas. So also the calyx is
often united to the ovary as in the apple, in which case the
sepals, petals, stamens and pistils appear to grow out of its
summit, and the ovary is said to be inferior, as in the honey-
suckle and dog-wood. More rarely, the two interior verticils,

MODIFICATIONS OF THE FLORAL ORGANS. 181

the stamens and carpels, cohere together. This case, neverthe-
less, presents itself in Orchideous plants, which constitute the
true gynandrous plants of Linnzus.

The adherence of the different verticils among themselves is
called their insertion. It is above all essential to study the
insertion of the stamens, as it furnishes for the natural co-ordi-
nation of plants, characters of the first value. Three modes of
insertion have been distinguished, called hypogynous (ino
under, yovz female or pistil), perigynous ( ep: around), and
epigynous (éi upon.)

The hypogynical insertion is that in which the stamens are
inserted upon the ovary, which is therefore necessarily free and
superior, as for instance, in the Ranunculus. This kind of in-
sertion is readily recognized in this, that we are able to remove
the calyx without carrying the stamens away at the same time.

The perigynical insertion takes place when the stamens are
attached to the calyx and surround the ovary, as in the
strawberry (Fragaria.) This is distinguished by this, that
when we remove the calyx, we necessarily remove the stamens
at the same time, which are inserted on it.

The epigynical insertion is that in which the stamens are
inserted upon the superior part of the ovary, which necessarily
happens whenever the ovary is inferior.

One or more organs of the same verticil, may have been
suppressed or rendered abortive. Abortions and suppressions
contribute more than any other cause to destroy the symmetry
and regularity of the floral organs. Abortion is the state of
an organ which, after having commenced to form, is arrested in
its evolution and remains reduced to a species of stump, some-
times a gland ; suppression indicates the total absence of the

organ, which has not even commenced to develope itself.
16*

182 COMPOUND ORGANS OF PLANTS.

The symmetry of the flower is frequently destroyed by the
abortion of one or more organs of the same verticil. In the
natural order Scrophulariaceze we are able to follow, step by
step, the progressive abortion and final suppression of an organ,
as for instance, a stamen, by examining the flowers of its dif-
ferent genera. Thus if we look at the flower of the common
mullein (Verbascum) which is placed at the head of the order,
we shall find that it is symmetrical and pentamerous although
somewhat irregular in its construction, having a calyx of five
sepals, a corolla of five petals soldered together, the lobes
broad, rounded, and a little unequal; the stamens are five, and
alternate with the lobes of the corolla; but one of the stamens
is a great deal less than the others; it has proved already a
certain degree of arrest in its development. In Pentstemon
the anther is abortive, and the stamen appears in the. form of
a bearded filament. In Linaria it may be detected in the
form of a little filament at the base of the superior lip of the
corolla. If now we examine a flower of the Scrophularia we
shall observe no more than four stamens. However, between
the two upper lobes of the corolla, on its interior surface we
shall find a little glandular scale, occupying the very place of
the missing stamen, and of which it is not difficult to recognise
the nature. Lastly, if we open a flower of the Digitalis or
Antirrhinum, we shall find no trace of the fifth stamen, which
has completely disappeared.

The abortion and suppression of the staminal verticil is
carried still further in other genera of the same family. Thus
Gratiola Virginica has a calyx of five sepals, five petals united
almost to their tips, and only two perfect stamens, the three
others having been entirely suppressed. But we can satisfy
ourselves that this abortion and ultimate suppression of the

MODIFIUATIONS OF THE FLORAL ORGANS. 183

organs has been gradual, for in another species of the same
genus, the Gratiola aurea, two minute filaments occupy the
place of two of the stamens, although no trace of the fifth is
observable. In Gerardia 4 pair .of perfect stamens a little
shorter than the other two, occupy the place of these filaments,
and the stamens are thus rendered didynamous.

In the Labiatee or mint tribe, which are all pentamerous
flowers, the fifth stamen is suppressed altogether, and the didy-
namous form prevails; or we have two long and two short
stamens, the filaments of each pair being unequally developed.
That didynamous stamens are the consequence of a defective
development of the organs, is evident from the fact that in
other genera of the same family the development of the organs
has been arrested at an earlier stage, so that the two short
stamens are either reduced to mere filaments, or are absent
altogether from the corolla, the flower being strictly speaking
diandrous.

The symmetry and regularity of the floral organs is more
frequently disturbed by the defective development or entire
suppression of one or more of the organs of a verticil, than by
any other cause. Such deviations from the normal structure
are very common, in fact almost any flower will discover them
to the intelligent student, and the principle when once clearly
understood may be extended almost indefinitely.

We close our illustrations of this topic with the following
analysis of the Spring beauty (Claytonia), a very common but
remarkably unsymmetrical flower. (Fig. 82) isa diagram show-
ing the flower and its ground plan. It will be perceived that
the flower is complete, for the four verticils are present, some
of them being partially but not entirely suppressed; that the
flower is regular, for all the pieces of each floral verticil are

184 COMPOUND ORGANS OF PLANTS.

Fig. 82.

equally developed, but that it is remarkably unsymmetrical, for
only two of the four circles have the same number of members,
and one of them, viz., the staminate circle is in an abnormal
position; for instead of alternating with the petals, we find the
stamens in this flower placed directly opposite the petals.
The diagram shows a calyx with two sepals, but as the normal
construction of the flower is evidently pentamerous, three of
the sepals have been suppressed, and the two which have been
developed, have evidently obtained possession of the place
which the three by their absence had left void. Within the
calycine verticil, are five equally developed petals alternating
with the sepals of the calyx, and which are therefore regular
and normal in their growth. Within and opposite the petals:
we find five stamens. The number of the stamens is normal,
but not their position. Here is an evident departure from
that law of alternation which usually manifests itself in the
relative position of the pieces of the floral verticils, when they
follow each other directly in the flower.

MODIFICATIONS OF THE FLORAL ORGANS. 185

It would appear from this, that a verticil of stamens has
been suppressed, and these stamens belong to a second verticil
and are therefore necessarily opposite the petals. It is true
that the abortive stamens have left no traces of their existence
within the corolla, but this does not disprove this method of
accounting for their absence, because it receives abundant
confirmation from what we see in other orders of plants which
are equally defective in the stamineal circle. Thus in the
natural order Primulacesx, the general disposition of the floral
organs is as follows. A monosepalous calyx of five sepals, a
monopetalous corolla of five petals, the lobes of. which are
situate opposite the sinuses of the calycine lobes, and therefore
in their normal position; and a verticil of five stamens which
are directly opposite the petals. The law of alternation is
therefore defective in the staminical circle. We account for
the phenomena in the same manner as we explain it in Clay-
tonia, on the theoretical supposition that the primary alter-
nating verticil of stamens is generally suppressed in this family.
We examine the other genera of this family for confirmation
of our theory, and we find, that although the primary verticil
of stamens is suppressed in most of the corollas of the order,
yet this very verticil is present’ in some of them in an inter-
mediate stage of development. Thus in Samolus floribundus,
for example, there are found in the sinuses of the corolla, in
the normal position of the absent verticil, five sterile filaments
which are unquestionably these very stamens in a rudimentary
condition. When, therefore, we find in the Claytonia and
other plants, a verticil of stamens opposite the petals of the
corolla, instead of alternating with those petals, we are justified
in supposing that the primary alternating verticil has been
suppressed, although we may not be able to recognize any
traces of its existence in the corolla.

186 COMPOUND ORGANS OF PLANTS.

Lastly, in the centre of the flower of the Claytonia, there
are three pistils; two have therefore been suppressed.

One or more entire verticils may have been wholly sup-
pressed. We have already said that the complete flower
consists of four verticils of metamorphosed leaves, viz., the
calyx, corolla, stamens and pistils: now if any one of these
verticils be absent, the flower is incomplete. Deviations
resulting from the non-production of the verticils are not
uncommon, and may affect any of the floral organs. Thus the
ealyx is reduced to an obscure ring or border in the holly and
dogwood, and is suppressed altogether in the prickly ash,
(Zanthoxyllum.) We infer that it is the corolla which remains
in this flower because the five stamens alternate with it. In
other instances the corolla is suppressed and the calyx remains
as in Anemone and Clematis.

It is proper to remark here, that where there is only one
floral envelope, the law of alternation will enable us to detect
whether it is a calyx or a corolla. Thus if the corolla is sup-
pressed, it is easy to see that the two verticils between which
the corolla is developed will have their parts opposite, that is
to say, the andrcecium and the calyx. This is their natural posi-
tion, since the stamens alternating with the petals are necessa-
rily placed opposite to the sepals; and therefore when we find
them in this position and only one floral envelope, we may con-
clude that envelope to be the calyx and the petals, to have been
suppressed, whatever may be its color and hue. If, on the
contrary, there is a single envelope with which the stamens
alternate, we may conclude that it is the calyx which has been
suppressed, and that the colored envelope is a true corolla.
This appears to be an easy way of settling this question where
other methods fail.

MODIFICATIONS OF THE FLORAL ORGANS. 187

Not unfrequently, however, the floral envelope which has
disappeared, has left traces of its existence sufficient to disclose
the character of that which remains. We know that the pro-
per place of the petals is between the sepals and the stamens,
and if we find anything occupying their position, we are at
once convinced from the laws of development which are so
clearly and beautifully expressed in other genera, that it is
the same organ in an abortive or rudimentary condition. Thus
the flower of ‘Pulsatilla patens has only one floral envelope.
In the place, however, usually occupied by the petals, we find
certain abortive gland-like stamens, which are in fact the rudi-
ments of the suppressed petals; this therefore decides the
envelope to be a calyx.

In some plants, such as nettles and Chenopodiums, the
floral envelopes are green and inconspicuous, and in the grasses
they are suppressed altogether, their places being supplied by
rudimentary leaves or bracts. When this is the case, the flower
ceases to attract popular attention. The world attaches the
idea of a flower to that part of a plant which is usually colored
with tints more or less brilliant, which makes its appearance
generally before the seed, and after delighting our senses with
its fragrance and beauty for a brief space of time, is replaced
by the fruit or seed. But such flowers are only characteristic
of the more perfect races of plants. The botanist stops not at
these appearances, for to him the flower is often deprived of
them. The student must learn to recognize the flower in the
lower degrees of its development.

When both stamens and pistils are present in the same
flower, it is said to be hermaphrodite and complete. When on
the contrary, the flower contains stamens only, or pistils only,
it is denominated unisexual, and is male or female according

188 COMPOUND ORGANS OF PLANTS.

as the former or the latter only are found within the floral
envelopes.

When the stamens and pistils are in separate flowers on the
same plant, as in the castor-oil plant, (Ricinus,) and Indian
corn, (Zea mays,) the flowers are moneecious, (udvos one, éuxcov
habitation.) When the staminate flowers are on one plant and the
pistillate flowers on another, the flowers are dicecious (5¢s twice,)
as in the nettle and hop; and when the same plant developes
both unisexual and hermaphrodite flowers, they are polyga-
mous (moms many, yézos marriage,) asin the maple and Huphor-
bia. In the marginal flowers of Hydrangea arborescens, and
Viburnum opulus, the Snow-ball tree, the essential organs, the
stamens and pistils, are entirely suppressed; these flowers are
therefore necessarily sterile.

The following diagrams will illustrate the several stages in
the suppression of the floral organs of Phanerogamous plants,
until we arrive at their minimum reduction, when any farther
suppression would render the production of an embryo or seed
impossible.

Fig. 83, is a representation of the flower of the Saururus
cernuus or Lizaid’s tail. The flowers of this plant are perfect,
and are developed in racemes or spikes, but destitute of all
floral envelopes, a simple scale or bract supplying their place.

MODIFICATIONS OF THE FLORAL ORGANS. 189

Fig. 84. Fig. 85.

This plant iscommon along the margins of streams and ponds,
and may be found in bloom in the month of June.

Fig. 84, is the stinging nettle, (Urtica,) bearing clusters of:
greenish inconspicuous unisexual flowers a, in the axils of its
leaves. Fig 86, one of the male flowers magnified. Its
perianth is a simple calyx of four sepals, within which are four
stamens opposite to the sepals, situated on the receptacle, and

17

190 COMPOUND ORGANS OF PLANTS.

this relation of parts leads at once to the detection of the fact
that the corolla has been suppressed. The perianth of the
female flower is also a simple calyx, consisting of four very
unequal sepals, the two outer small, the inner foliaceous,
enclosing a single pistil.

Fig. 86.

Fig. 85, shows a compound spike of wheat, (Triticum,) with
numerous spikelets or flowers arranged along the axis ina
zigzag form. Fig. 87, one of these spikelets magnified, and
deprived of its glumes,-showing the three stamens a, hanging
by long thread-like filaments, and the feathery styles of the
pistil within two bracts sg. The flower in this case is herma-
phrodite.

The flower is therefore still farther reduced in the sedges
(Carices,) which are equally without floral envelopes, and are
unisexual.

Fig. 88, shows the moncecious flowers of a species of Carex.
a. One of the staminate flowers, consisting of a single glume
or scale and three stamens. 6. One of the pistillate flowers.

MODIFICATIONS OF THE FLORAL ORGANS. 191

Fig. 88.

a b
This pistil is covered by an urceolate glumaceous bag marked wu,

called a perigynium. There is one style st, with three stigmas

at its summit.
Fig. 89.

Fig. 89, is a representation of a common, though exceed-
ingly interésting aquatic plant, the Callitriche verna, (xands
beautiful, and épé hair, in allusion to its capillary and tufted
stems.) The lower leaves of the stem are immersed and linear,
the upper floating and spatulate. The flowers are polygamous,
unisexual and hermaphrodite flowers growing together on the
same plant. They are without either calyx and corolla, have

192 COMPOUND ORGANS OF PLANTS.

not even a bract, and consist of a single stamen and pistil
placed together in the axil of the leaves, when hermaphrodite
and complete, and when unisexual, placed apart from each
other. In Fig. 89, the male flower consists of a single stamen
and the female flower is represented by a solitary pistil. In
the Callitriche, therefore, the flower is finally reduced to a

minimum.

CHAPTER XV.
THE FRUIT, OR MATURE OVARY.

Tue term fruit, as understood among botanists, has a more
extended signification than its meaning in ordinary language.
It is applied by them to the fecundated and mature ovary
enclosing seeds, capable of germinating and reproducing the
plant, whatever be its form or texture, and whether it be edible
or not. In this respect a grain of wheat or corn, or the peri-
carp of the sun-flower or thistle, is as much a fruit as a peach,
gooseberry, or melon.

Very often, besides the ovary, other parts of the flower, and
especially the calyx, enter into the composition of the fruit;
but these are only accessory parts, the term fruit being strictly
applicable only to the ovary.

The fruit is composed of two parts, the pericarp (wep: around,
xapzds fruit,) and the seed or seeds. The pericarp is formed by
the walls of the ovary itself; the seeds are the ovules fecun-
dated and containing an embryo. Let us consider each of
these parts in succession.

THE PERicARP.—The pericarp is that part of the fruit

THE FRUIT. 193

which is formed by the walls of the ovary, and which deter-
mines the general form of the fruit. Since the walls of the
ovary constitute the pericarp, it must be constantly present in
all fruits. When the fruit is a single cell and contains only one
seed, the pericarp is so thin and is united so completely with
the seed, that they can hardly be distinguished from each
other. Such, for example, are the fruits of the grasses, Cype-
racez, and syngenesious plants, which were formerly regarded
as seeds, but which are in reality pericarps or seed-vessels
enclosing a seed.

A fruit may be usually distinguished from a seed, or other
organ assuming its character, by the presence of some vestige
of the style. Thus the carpels of the Ranunculus, (Fig. 90,)

Fig. 90.

. Fig. 90. Carpels of the Ranunculus with a few stamens, the calyx and corolla having
been removed. One of the carpels magnified, showing it to be a single-seeded vessel
with the pericarp applied close to the seed. Such fruits resemble seeds in appearance,
the style and stigma, s, aid in distinguishing them from seeds.
which are vulgarly regarded as seeds, are at once determined
to be seed-vessels by their apiculate summit, the vestige of the
style. In the same manner we discover that the strawberry is
not a single fruit, but an enlarged fleshy receptacle bearing the
simple fruits at its surface. (Fig. 91.)

The pericarp, like the leaves from which it proceeds, is

composed of two plates of epidermis, between which exists a
17*

194 COMPOUND ORGANS OF PLANTS.

Fig. 91.

Fig. 91. Fruit of strawberry, (Fragaria vesca,) showing the carpels or achenia on the
surface of its enlarged and fleshy receptacle. Each achenium has a style and stigma,
and is thus at once distinguished from a seed. The calyx is seen at the base of the
receptacle,
cellulo-vascular bed of fibres and parenchyma. The exterior
membrane of the pericarp is called the epicarp, (éa upon,
xapnos fruit,) and corresponds to the lower epidermis of the
leaf. This membrane is ordinarily very thin, and is easily
removed, especially in succulent fruits, such as the peach or
plum. The interior membrane of the pericarp immediately
surrounding the seed, is called the endocarp, (%vdo within,)
and is equivalent to the upper epidermis of the leaf. It is
usually thin arid membranaceous, and sometimes appears like
parchment, as in the pea and apple. In the peach and plum
it takes a ligneous consistence, and forms the stone or puta-
men, (putamen a shell,) immediately investing the kernel or
seed of these fruits. The intermediate tissue of the pericarp
between the epicarp and the endocarp, which represents the
parenchyma of the leaf, is called the mesocarp, (uesos middle.)
The mesocarp is more or less succulent, according to the pro-
portionate development of its two constituents, fibres and
parenchyma. It is very much developed in fleshy fruits,
furming their flesh or pulp, as in the peach and plum, and ~
hence it has been sometimes called the sarcocarp, (cap§ flesh.)

THE FRUIT. 195

But sometimes the mesocarp is excessively thin, in dry fruits
for example, such as the pod of the pea, or the fruit of the
gilliflower. In the nut the three parts are blended together ;
in the peach they remain separate. In the latter fruit the
epicarp forms the skin, the mesocarp the fruit or edible part
of the peach, and the endocarp, the stone in its centre which
covers the kernel and seed.

Whatever may be the thickness of the walls of the pericarp,
its anatomical constitution remains the same. It is always
formed of two membranes, the epicarp and the endocarp, and
an intermediate bed of tissue called the mesocarp, sometimes
thin and dry, at other times thick and succulent. Such is the
constitution of the leaf from which it is derived, and of which
it is only a peculiar modification.

This remarkable transformation of the leaves is not peculiar
to fruits, for in more cases than is usually supposed, similar
changes take place in the other floral organs. Thus the calyx is
changed into a hard crustaceous body in Salsola and in Spinage;
and is red and juicy in the Strawberry blite and Winter-
green (Gaultheria), being in both instances commonly mistaken
for the fruit from which it is wholly distinct. In the Yew,
the bracts enveloping the seed become pulpy and berry-like.
Nearly the whole bulk of the apple is a thickened calyx. The
pulp of the Strawberry, as we have already intimated, is
nothing else but the enlarged and juicy extremity of the
flower-stalk or receptacle. Examples might be multiplied
proving that all the appendages of the axophyte are subject to
these transformations, which are erroneously imagined to be
peculiar to the fruit.

The fruit, like the pistil of which it is the final development,
may be either simple or compound. The fruit is simple when

196 COMPOUND ORGANS OF PLANTS.

it proceeds from a simple carpel or pistil. In this case the
pericarp presents constantly one single cell, or it is unilocular.
The compound fruit proceeds from the compound pistil, the
pericarp, like the ovary, containing as many cells as the number
of pistils which have united. Thus the pericarp is bi-locular
in the tobacco, tri-locular in the tulip, quadri-locular in the
epilobium, quinque-locular in flax, &. We have already made
known these particulars in treating of carpels.

It is necessary to observe here, that the number of cells in
the pericarp or ripe fruit, does not always exactly correspond
to the structure of the oyary. It often happens, between the
moment of fecundation and the maturity of the seed, that con-
siderable changes take place in the internal structure of the
ovary, a number of its dissepiments being absorbed during its
progress towards maturity, so that an ovary originally multilo-
cular becomes finally an unilocular fruit or pericarp. A great
many of the Caryophyllaceze and the Cistacese are in this case.
The rapid increase of their ovaries break and efface their disse-
piments, so that they are not found in the mature pericarp.
Alterations take place, not only in the number of cells, but
also in the seeds, the ovules being equally liable to become
obliterated. In the acorn, (Fig. 92,) the young pistil is formed
of three carpels, the ovary consisting of three cells with two
ovules in each cell as represented in the transverse section.
But the walls of the cells and five of the ovules are suppressed in
the progress of development, so that the pericarp ultimately
becomes unilocular and monospermal, or one-seeded. Hence
the acorn or fruit of the oak (Quercus), consists of a one-seeded
pericarp, surrounded by an involucre of bracts forming the cup
or cupula.

Dehiscence of the pericarp. When the fruit has arrived at a

THE FRUIT. 197

Fig. 92.

state of maturity, the pericarp opens to let the seeds escape.
The fruits which open spontaneously in this manner are said to
be dehiscent, (dehisco I gape). However, there are some fruits
which fall to the ground without opening or dehiscing. The
fleshy pericarps of the peach and apple for example, do not
open; their seeds are liberated as the fruit decays. The dry
pericarps of the Composite, the Maize, and the Ranunculus,
remain indehiscent on the soil, enveloping the grain till the
plantule in germinating forces a passage through them.

The pericarp, whether it proceeds from a single pistil, or
from one that is compound, always presents on its outer surface
longitudinal lines which are called sutures. One of these
sutures, formed by the union of ‘the free margin of each carpel-
lary leaf, is called the ventral suture; the other, exactly oppo-
site, and corresponding to its midrib, is named the dorsal
suture; the former is generally connected with the axis, the
latter with the periphery of the fruit. In a simple pericarp,
such as the pod of the pea, for example, both these sutures are
equally visible on the exterior of the fruit. But when the
carpels solder together by their lateral surfaces, and form a

198 COMPOUND ORGANS OF PLANTS.

compound pistil, the ventral sutures are all united in the centre
of the fruit, and we see on the exterior only the dorsal sutures.
From this union of the carpels among themselves it follows,
that new lines will be formed at their points of contact. These
new lines called parietal sutures, are ordinarily seen on the
exterior of the compound ovary between the dorsal sutures,
and indicate the points where the walls of the several carpel-
lary leaves are joined. Finally, when the carpellary leaves,
instead of folding on themselves, uniting by their free margins
and soldering by their lateral surfaces, so as to cause their
ventral sutures to meet in the centre of the pericarp, and each
folded carpellary leaf to form a distinct cell in its cavity; unite
together by their margins, making only a slight introflexion
towards the axis of the pericarp, in such a manner as to form
a unilocular pericarp; the lines which result from this union
are called marginal sutures. The nature and origin of these
different sutures being understood by the student, he will find
no difficulty in comprehending the several varieties of valvular
dehiscence.

Dehiscence usually takes place in simple fruits either by the
ventral or dorsal suture, or by both. Dehiscence takes place
by the ventral suture in the pooony and Wild Columbine
(Aquilegia); by the dorsal suture in the Magnolia; and by
both sutures in the pea and Acacia.

When the fruit consists of several united carpels or is com-
pound, the dehiscence may take place through the parietal
sutures so as to resolve the fruit into its original carpels, as in
the Colchicum, (Fig. 96,) when it is septicidal (septum a wall,
and cedo I cut). This happens when the lamina of the car-
pellary leaves are only slightly united. When, however, these
lamina are firmly soldered together dehiscence takes place by

THE FRUIT. 199

Fig. 94. Fig. 95.

Fig. 98. The five carpellary leaves or follicles of Aquilegia, opening by their ventral
suture, ‘

Fig. 94. The carpels of Magnolia glauca with their dorsal sutures open and the
seeds suspended from them by curious extensile cords.

Fig. 95. The legumen or pod of the pea, opening by both sutures at the same time.
In the former instances the fruit was univalve, in this case it is bivalve. ep. Epicarp.
en. Endocarp. ov. Ovules attached to the placenta pl, by means of the funiculus
J. The legume opens by both ventral and dorsal suture. The placenta pl is double,
and runs along each edge of the ventral suture. At the apex of the pod are seen
the remains of the style and stigma, and at its base the remains of the calyx.
the dorsal suture, and the several lamina are detached from
their midribs. The result of this is, that each of the valves
carries on the middle of its internal surface a double lamina or
dissepiment, which is composed of a portion of the united
laminze of the two different carpels, as in the Martagon lily.
(Fig. 97.) This dehiscence is loculicidal (/oculus a cell, and
ceedo I cut.)

In septicidal dehiscence each valve is a complete’ carpel,
and generally contains the ovules attached to the placenta.
In loculicidal dehiscence, however, sometimes the placenta

accompany the dissepiments, as in the Pansy. Frequently,

200 COMPOUND ORGANS OF PLANTS.

Fig. 96. Fig. 97.

Fig. 97. Dehiscence of the three-celled fruit of the Martagon lily, showing the die-
sepiments in the middle of the valves.
however, the placenta and ovules remain firmly attached
together, so that the dissepiments or united lamine of the
several carpellary leaves separate from their margins instead
of midrib, which margins remain united and persistent in the
centre of he pericarp, forming a sort of central axis or colu-
mella, as in the morning-glory (Convolvulus). Lastly, not only
the margin but a part of the lamina may be persistent about
the central axis, so that when the pericarp opens at the parie-
tal suture, the central column presents as many walls attached
to it as there were dissepiments in the ovary before its dehis-
cence. We call this variety of loculicidal dehiscence, septifra-
gal (septum and frango I break).

The sutures or seams of the pericarp, instead of dehiscing or
splitting through their entire length, are sometimes only rup-
tured for a short distance from the apex as in the chickweeds,

THE FRUIT. 201

(Cerastium.) In the snap-dragon, (Antirrhinum), the sutural
rupture is so slight as to produce only points or pores in the
Upper part of the pericarp.

Besides these regular forms of valvular dehiscence there isa «
somewhat anomalous mode of rupture which takes place in a
few plants, such for instance as the pimpernell (Anagallis), and
henbane (Hyoscyamus), and which is called circumscissile,
(circum around, scindo to cut.) The pericarp of these plants
opens by a transverse circular line, following no sutures what-
ever but cutting directly across them. It is therefore an
anomaly and not a true dehiscence, as we have employed the
term. Fig. 98 is the seed vessel of the Hyoscyamus which is
ruptured in this manner. The upper part of the pericarp
separates like a lid from the lower part. This kind of fruit is
called a pyxidium, (pyzis a chest.) —

The pods of some Leguminous plants formed by a single
carpel, are divided into several cells, either by the formation
of false horizontal partitions, as in some Cassias, or by the
contractions of the legume itself, as in Desmodium. Each of
these cells contains a separate seed, and the pod when ripe
separates by transverse dehiscence at these joints, and falls into
pieces. This kind of pod is called a loment, (Fig. 99). This

18

202 COMPOUND ORGANS OF PLANTS.

Fig. 99.

Fig. 99. Loment of saintfoin (Hedysarum), which separates transversely into single
seeded portions.
transverse disarticulation may be suppoged to have some
relation to a simply pinnate leaf, whose modification in this
instance forms the carpel, the divisions indicating the points
where the different paits of pinnae have united.

Different kinds of pericarps or fruits—Several eminent
botanists have attempted to make a classification of the dif-
ferent kinds of pericarps. We have not space for the enumera-
tion of any more than those which most frequently occur, and
to which reference is most generally made. The principal
indehiscent fruits or pericarps are,

1. The Caryopsis or grain, (xapvaa nut, and ddés appearance.)
This is a dry indehiscent one seeded pericarp, which is so
incorporated with the seed as to be inseparable from it. It
is seen in the cultivated grains such as maize, barley, oats,
which in common language are called seeds, but which con-
sidered botanically are not seeds, but seed vessels containing a
seed. It is only by examining them in their early state and
noticing their styles, that we can become convinced that these
grains are only apparent but not real seeds.

2. The Achenium (a without, and xaiva I open), isa single
seeded indehiscent fruit, the pericarp of which is distinct from
the coats of the seed. The fruit of the Ranunculus consists of
a number of these achenia borne on a convex receptacle. In
the rose, the receptacle which supports them is concave and is

THE FRUIT. 203

invested by the swollen and succulent fruit-like calyx. The
fruits of the dandelion, sun-flower, and all Composite, are
achenia or single-seeded pericarps. Hach has been produced
by a separate flower, and is provided with a persistent calyx
the tube of which is closely united to the fruit, its limb form-
ing a beautiful stellate down at the summit of the style or
ovary, by means of which the achenium or mature ovary is
lifted from off the surface of the broad and dilated receptacle
and wafted by the winds to spots favorable to its germination.
The bottom of the persistent calyces of the Labiate: or mint
family usually contain four achenia which look at first like
seeds, and were actually regarded by Linnzeus as such. He
defines them as “semen tectum epidermide ossea,”’ that is
seed covered with an osseous epidermis, and hence he called
the whole order gymnospermia, (yuuvos naked, omepya a seed.)
The student may however easily satisfy himself that such is
not the case, and ascertain that they are pericarps or seed
vessels by cutting across them, when he will discover the true
seed in their interior.

The Oremocarp.—This fruit is confined to the great
natural order Umbelliferse, of which the carrot and parsley
are familiar examples. The Cremocarp (xpudo, to suspend,)
is composed of two achenia, which are at first united to a
common axis called the carpophore, (xapris, fruit, and gopea, I
bear,) which axis separates at maturity, as in Fig. 100, the
two achenia being placed apart and suspended from its summit.
Each of these achenia is called a mericarp (w2pos, part,) or
hemicarp, (qucovs, half, and xapzos, fruit.)

The Samara, (samera, seed of elm.) This is'an achenium
with a membranaceous appendage attached to its summit or
margin, and forms those peculiar winged fruits suspended in

.

204 COMPOUND ORGANS OF PLANTS.

Fig. 100. Cremocarp of fennel (Ficniculum vulgare,) arrived at maturity, showing
the carpophore and the two suspended mericarps or hemicarps.

bunches from the branches of the ash and maple, commonly
known as keys. The fruit of the maple consists of two united
Samara.

The Pome, (pomum, an apple.) This is a fleshy indehis-
cent fruit with a superior calyx, which is therefore adherent to
the ovary. In the mature pome, the epicarp and calyx are
blended together and form along with the mesocarp the thick
cellular and edible part of the fruit, whilst the endocarp
enveloping the seeds in its interior takes the consistency of
parchment, and usually forms five cavities in the centre of the
fruit.

The Drupe.—This is a thick, fleshy and indchiscent fruit,
containing an unilocular nut, as in the plum and cherry.
This nut is formed by the ossification of that portion of the
pericarp which is called the endocarp, which in this case forms
a strong stony envelope around the seed. In drupaceous
fruits, such as the peach and cherry, the epicarp, mesocarp,
and endocarp are easily distinguished and separated, but in
the nut these parts are all so much ossified and blended
together as to be indistinguishable. The nut only differs from
the drupe in being a less succulent and more coriaceous

THE FRUIT. 205

pericarp. The fruit of the raspberry and blackberry is an
aggregate of little drupes borne on a common receptacle.

The Bacca, or berry. This is q fleshy, compound fruit,
which is pulpy throughout. This name usually distinguishes
such fruits as the gooseberry and currant, in which the calyx
is adherent to the ovary and the parietal placentas. The seeds
are at first attached to the placentas, but as the fruit ripens
they become detached from the placentas, which finally form
that pulp which fills the interior of the berry and in which the
seeds are imbedded. The term berry is in general applied to
all pulpy fruits.

The principal varieties of the dehiscent pericarp are—

1. The follicle (folliculus, a little bag.) This is an unilo-
cular fruit, opening longitudinally by a single suture, the ven-
tral, into one valve, which represents an open carpellary leaf.
‘The seeds are attached to a simple sutural, or bi-partite pla-
centa, and sometimes become free at the moment the valves
separate. Follicles are very seldom solitary fruits. They are
usually aggregated on a short receptacle, and form a verticil, as
in the Columbine,

2. The legume or pod (Legumen, pulse), is a dry fruit,
bi-valve, opening at the same time by the ventral and dorsal
suture, and bearing its seeds on the former. In the bladder
senna (Colutea arborescens) , the legume is inflated, and retains
its leaf-like character. Fig. 90 is a lomentaceous variety of
the legume to which reference has been already made, and
which breaks up at the constrictions. This fruit belongs to
all the family of the Leguminosz of which it forms the prin-
cipal character. Examples—the pea, bean, and the acacia.

3. The capsule (capsula, a little chest.) This is a general
name for all dry and dehiscent fruits which open by valves or

18*

206 COMPOUND ORGANS OF PLANTS.

Fig. 101.

Fig. 101. The Siliqua of the Wall Flower (Cheiranthus cheiri) opening by two valves
from the base upwards. The two placentas bearing the seeds on their surface, remain
in the middle of the fruit, with a replum between them.
pores. Itis easy to imagine from this, that the forms of the
capsule will be exceedingly variable. The porous capsule is
seen in the poppy, which is a seed vessel of a woody texture,
proceeding from a compound ovary, and dehiscing by chinks
which may be seen in the dry fruit, just beneath the over-
hanging surface of its numerous radiating stigmas. Two other
varieties of the capsule are worthy of a particular notice.

4. The Siliqua (sitiqua a husk or pod.) This is a pod-
shaped capsule, the peculiar fruit of Cruciferous plants, com-
posed of two carpels which open as valves from below, upwards.
The parietal placenta, before the period of dehiscence, having
been united together by a plate of cellular matter termed the
replum, which forms a false septum across the cavity of the
ruit, separate from the valves, when these open and remain

THE FRUIT. 207

attached to the replum, in the axis of the fruit. These pla-
centa thus united together by the replum, frequently remain

after the fall of the valves, until the foliage of the plant finally
decays.

Fig. 102.

Fig. 102. The fruiting branch of the Shepherd’s purse (Capsella bursa pastoris,)
supporting siliculea. a. Magnified silicula, opening by two valves from the back

upwards, each valve leaving its placenta covered with seeds; and attached to the
replum in the centre of the fruit,

5. The Silicula. This is simply a short and broad Siliqua
containing sometimes only one or two seeds. It is also peculiar
to Cruciferous plants.

208 COMPOUND OKGANS OF PLANTS.

CHAPTER XVI.
THE STRUCTURE OF THE SEED.

Tue seed of Phanerogamous plants is the fecundated ovule
ripe and ready for germination, enclosing in its interior a plant
in miniature, called the plantule or embryo, which, when there
are the suitable conditions, is capable of reproducing the mother
. plant, and of again passing through precisely the same phases
of development. .

The seed like the ovule is composed of a kernel or nucleus,
usually covered by two cellular integuments, and included
under the general name of episperm.

The episperm or proper tegument of the seed is the coat
which covers it exteriorly. This coat is formed by the two
membranes which we have seen to exist in the ovule at the
moment of fecundation, viz., the primine and secundine. In
a great number of cases these two membranes are so soldered
together that the episperm is thin and constitutes only a simple
membrane. But it sometimes happens that the two superposed
membranes of the episperm are distinct enough; and when
this is the case the exterior membrane is ordinarily more thick
and tough than the interior one, immediately enveloping the
seed. To distinguish them from each other, the former is
called the testa, and the latter the tegmen. These two mem-
branes are perfectly distinct in the episperm or seed coat of the
Castor oil plant (Ricinus.)

The episperm has usually, on its exterior surface, certain
markings which correspond to those mentioned in the ovule.

STRUCTURE OF THE SEED. 209

On one part of the surface of the episperm we see constantly
the hilum, a scar marking the point by which the seed was
attached to the funiculus or placenta whilst in the pericarp
(Fig. 103.) a. The hilum is more or less conspicuous on the epis-

Fig. 103.

Fig. 103, Leguminous seed. a, Tho hilum under the form of a linear cicatrice. 0.
The micropyle.

perm of all seeds, its color being very frequently quite different
from the color of the rest of the surface of the seed. The
hilum is very conspicuous in the bean and pea, being quite
black in the former. It is by the hilum that the nourishing
vessels of the pericarp penetrate the seed. They traverse the
double or single membrane of the episperm, and enter the
nucleus or kernel by the chalaza, a term applied to the fibro-
vascular bottom of the nucleus or kernel where it unites with
the episperm.

On-the surface of the episperm, we perceive frequently very
near to the hilum or in a point diametrically opposite to it a
punctiform opening extremely small which is called the micro-
pyle. (Fig. 103.) 6. The micropyle is simply the foramen or open-
ing of the two membranes of the ovule which is contracted to a
point, so as to become sometimes hardly perceptible. The
micropyle may be readily detected in the pea or bean in the
form of a small hole or point which in this instance is near the
hilum. The micropyle always corresponds to that point of the
nucleus where the embryo sac is formed, and the summit of
which gives birth to the embryonic vesicle. It follows from

210 COMPOUND ORGANS OF PLANTS.

this that the radicle of the embryo always points to the micro-
pyle. This fact the student may readily ascertain by dissect-
ing any seed which has a visible micropyle on the episperm,
and ascertaining the direction of the radicular extremity of the
embryo. He will find it invariably pointing to this very spot.

Sometimes the micropyle entirely disappears from the sur-
face of the episperm. Its place, however, may be readily
ascertained. If the skin of a seed be carefully examined it
will be usually found to be marked with lines or bands which
run upwards from the hilum. These lines always converge
and meet in the micropyle, so that by following them with
the eye, the micropyle may be frequently discovered on the epi-
sperm, when owing to its minuteness it would otherwise escape
detection.

The chalaza is more or less visible in all anatropous seeds,
being often colored and of a denser texture than the surround-
ing tissue. At the apex of the seed of the orange and many
other plants, it may be perceived on the episperm in the form
of a large brown spot. In orthotropous and campulitropous
seeds the chalaza is directly superposed on the hilum, with
which it is immediately confluent, but in all anatropous seeds
it is placed apart from the hilum, and is connected with a vas-
cular bundle called the raphe, which forms a longitudinal pro-
minence more or less conspicuous on the episperm. In most
plants the raphe consists of a single line, as in the castor-oil
plant, but in the orange and lemon it ramifies upon the surface
of the episperm. Fig. 104.

It is proper to remark here, that the terms orthotropous,
campulitropous, and anatropous, employed to designate the
different kinds of ovules, are equally applicable to seeds, all
seeds occurring under one or other of these three leading
forms.

STRUCTURE OF THE SEED. 211

Fig. 104,

Fig. 104. Anatropal seed of the Orange, (Citrus aurantium,) opened to show tho
chalaza, c, which forms a brown spot at one end; r, raphe, or internal funiculus rami-
fying in the rugose or wrinkled testa of the orange.

On the outside of the episperm there is sometimes an addi-
tional envelope formed, after the fertilization of the ovule, by
an expansion of the funiculus at the hilum. This funicular
expansion, which covers more or less of the surface of the
episperm, is termed aa aril. The aril is very conspicuous in
the Spindle tree or Burning bush, (Kuonymus,) where it forms
a beautiful scarlet envelope to the seed. The tough, fleshy
and lacerated body which invests the seed of the nutmeg,
known in commerce under the name of mace, is an aril.

The nucleus, or kernel. This is all that part of the ripe
seed which is enveloped by the episperm. It is formed by the
development of the nucleus of the ovule, and like that organ is
attached to the episperm by its base, which forms the chalaza.
Generally, in the ripe seed this communication is destroyed.
The kernel in a fecundated seed always contains an embryo.

In exalbuminous seeds, after fecundation, the embryo takes a
considerable development, absorbing into its cotyledons the
nutritive matter of the nucleus, so as ultimately to constitute
the entire kernel, as in the pea and bean. In albuminous
seeds, the embryo appears to be arrested in its growth whilst
yet in a minute and rudimentary condition, developing only so

-

212 COMPOUND ORGANS OF PLANTS.

far as just to exhibit its component organs, and remaining
imbedded in the nutritive matter of the nucleus which is unab-
sorbed. The embryo of the Marvel of Peru, (Mirabilis,) of
the maize, buckwheat, and the whole of the cerealia con-
tinues in this rudimentary condition.

The albumen, termed by some authors the perisperm, and
also the endosperm, when present in the kernel varies in its
consistence according to the nature of the deposit and the state
of the cells. It consists of a mass of cells without any appear-
ance of vessels, which may be thin and dry and contain a great
quantity of fecula or starch, as in the corn and the other
grasses; or thick and fleshy, containing juices of various kinds,
as in the cocoa-nut and Euphorbiacex; or finally, the cells
may be of a horny or ligneous nature, as in the coffee and vege-
table ivory, (Phytelephas.) The quantity of albumen in seeds
depends on the extent to which embryonic development is car-
ried. When the embryo is small the albumen is abundant, as in
the seed of the monkshood, (Aconitum,) Fig. 105, where ¢ repre-

Fig. 105. Fig. 106,-

Fig. 106. Vertical section of the achenium of the nettle, (Urtica,) showing the em-
bryo nearly filling the achenium, r radicle; pl plumule; ¢ testa, or integument.

sents the embryo. When the embryo is large, as in the nettle,
Tig. 106, the albumen is very scarce. In the Labiate, the

4
STRUCTURE OF THE SEED. 2138

albumen is reduced to a mere pellicle by the great development
of the embryo.

The embryo is the most important part of the seed, and the
final product of the vegetative functions. When this is
formed, and the seed is fully ripe, a pause in growth takes
place, and the embryo which is the future plant in the first
stage of its development, will sometimes remain for a long time
in an apparently dead condition enveloped in the folds of the
seed, until suitable circumstances arouse its dormant vitality.
The embryo is a complete plant in miniature, and therefore
offers the same general disposition of its parts as that which
we have already noticed in the adult plant. Thus we distin-

Fig. 107.

*

Fig. 107. Embryo of the Pea (Pisum,) laid open to show its different parts. This
embryo occupies the whole interior of the seed. ¢, ¢, its two fleshy cotyledons; p, the
plumule; r, the radicle; g, the gemmule; /, the depression left by the gemmule in
the cotyledon. This embryo is dicotyledonous and bypogeal, the cotyledons remaining
below, during germination.

guish, in every young plantule, an axophyte more or less
developed, with the usual appendages, root, stem and leaves, all
in a rudimentary state, and all manifesting an identity in their
incipient vital action with the same phenomena in the adult
plant. The little embryo axophyte commences to develope at
its two extremities in two opposite directions, and puts forth
laterally its rudimentary leaves; that portion which ascends

is called the plumule, that which descends the radicle; the
19

214 COMPOUND ORGANS OF PLANTS.

rudimentary leaves are named cotyledons, and the little bud
by which the plumule is terminated, is called the gemmule,
(Fig. 107.)

Before we examine in succession these four parts of the
embryo, let us consider their relative positions with respect to
the other parts of the seed.

When albumen is present in the seed along with the embryo,
the embryo may either lie in its midst directly in the axis of
the seed as in the pansy, Viola tricolor, (Fig. 108,) when it is
axial; or it may surround the albumen itself, instead of being
surrounded by it as in the Marvel of Peru, (Fig. 109,) when
it is peripherical. In the grasses, maize, wheat and all the
cerealia, the embryo lies external to the albumen on one of
the sides of the seed, having been apparently forced into this
position by the irregular development of the parts of the seed,
when it is abaxial. (Fig. 110, Indian corn.)

Fig. 108. Fig. 109. Fig.110.

Fig. 108, Vertical section of the seed of the Pansy. The seed is anatropal; the em-
bryo homotrope. ch, chalaza to which co, the cotyledons point; pl, the plumule; A,
the hilum; al, the albumen surrounding the embryo which it will be perceived is
axial; r, the raphe connecting the hilum or base of the seed with the chalaza or base
of the nucleus.

Fig. 110. Vertical section of o grain of Indian corn (Zea Mays.) r, the radicle; p, the
plumule ; ¢c, the cotyledons.

The embryo is sometimes straight but very frequently
curved in a varicty of ways, its curvature depending on that

STRUCTURE OF THE SEED. 215

of the seed. In orthotropous and anatropous seeds the embryo
is usually straight ; in such seeds as are campylotropous it is
curved. Whatever may be the form of the seed, the radicle
always points to the micropyle and the cotyledons to the
chalaza or some point in its vicinity. This important law
being remembered, it is only necessary to ascertain the situa-
tion of the micropyle with respect to the chalaza on the
surface of the episperm, and the character not only of the
seed, but the exact position of the embryo within its folds is
at once determined without any further trouble. Thus in the
orthotropous seed of the nettle, (Fig. 106,) we know that the
micropyle is directly opposite to the hilum and ‘chalaza, which
is the base of the seed, the radicle therefore points to the apex
of the seed, and its plumule to the base, and the embryo is
antitrope (zi, opposite, rpéw, I turn,) or inverted. But in
anatropal seeds, as in the pansies, (Viola tricolor, Fig. 108,) we
see on the surface of the episperm the micropyle close to the
hilum or base of the seed and the chalaza at its apex or opposite
extremity ; the radicle or base of the embryo therefore points
to the base of the seed and its cotyledons to the apex, and the
embryo lies in the seed in its natural position ; that is to say,
it is erect or homotrope, (éyov0s, like, and zpéa, I turn.) In
the campylotropal or curved seed, the base is not displaced, the
seed curves on itself, and the micropyle approaches the hilum
and chalaza, which is still confluent with it; from this we
know that the cotyledonary and radicular extremities of the
embryo also approach each other, or the embryo is amphi-
trope (4upi, around, and zpéxa, I turn), or follows the curva-
ture of the seed, (Fig. 109.)

Let us now examine in particular each of the parts which
constitute the embryo.

216 COMPOUND ORGANS OF PLANTS.

Fig. 111. Vertical section of the campulitropous seed of the red campion (Lychnis,)
showing the curved embryo, -

1. The radicle.—This constitutes the lower extremity of the
embryo, which in developing forms the root, or which gives birth
to it. It appears very often under the form of a little round or
conical teat. This, by germination, sometimes elongates and
becomes the body of the root. Its extremity continues naked
and afterwards divides. This mode of development, which is
characteristic of Phanerogamous plants having two seed-lobes or
cotyledons, is termed exorhizal, (?@ outwards, and ufo a root.)
At other times the radicle in germinating, after having taken
a certain degree of elongation, stops the teat at its extremity,
becomes covered with a cellular layer as with a sheath, through
which breaks forth one or more fibres, which constitute the
true roots of the embryo. This kind of development, which
is peculiar to such phanerogamous plants as have only one seed-
lobe, is termed endorhizal, (?véo within,) and the sheath formed
at the extremity of the radicle teat is called the coleorhiza,
(xonéos a sheath, and jrfaa root. Fig. 112, shows both kinds
of germination.

The plume forms with the radicle the axis of the embryo.
It is developed after the radicle, which it surmounts, and with
which it is united. It exists only in Dicotyledonous embryos,
and is terminated at its summit by the gemmule. It is the
plumule which, by its development, produces the stem. It
commences from the point where the cotyledons are attached

STRUCTURE OF THE SEED. 217
Fig. 112. Fig. 113.

go

aco

Fig. 112. a shows the exorhizal germination of the Dicotyledonous seed of the orange;

¢, the cotyledon; g, the first pair of aerial leaves; 7, the radicle naked and without a
sheath.

Fig. 113. Seed of oats sprouting. 7, roots passing through the sheath, sh, from the
single cotyledone. g, The young leaves and stalk.
to it, and which it raises with it above the earth, when its
elongation operates from its base.

The cotyledons.—These are the lateral appendages of the
embryo axophyte. The cryptogamia have no cotyledons in their
embryos, which are therefore acotyledonous. The embryo in
such cases is called a spore, and as it gives off roots indiffer-
ently from any part of its surface, and from a fixed point, it
is termed heterorizal, (Zvepos diverse.) Plants possessing coty-

ledons in theirembryo are termed cotyledonous. If we examine
19*

218 COMPOUND ORGANS OF PLANTS.

the embryo of the bean, the pea, or the oak, we slrall see a coty-
ledonary body formed of two cotyledons. The embryo which
presents such a conformation is a Dicotyledonousembryo. If,
on the contrary, we examine the embryo of the wheat or the
maize, of the iris or the palm, we shall find a simple coty-
ledonary body, formed by a single cotyledon. The embryo
in this instance is termed a monocotyledonous embryo.
The character drawn from the number of cotyledons is of the
highest importance, because it divides all phenogamous plants,
or those which are provided with flowers properly speaking,
into two grand branches, the Monocotyledons and the Dicoty
ledons, which differ not only in the structure of their embryo,
but in the special organization of all their other parts.

A certain number of Phanerogamous plants are, however,
apparently exceptions in the structure of their embryo to these
two grand divisions. The cone-bearing plants, for example,
such as the spruce, fir, and larch, have not one or two, but
sometimes six, nine, twelve and even fifteen verticillate cotyle-
dons, which resemble in their linear form and verticillate
arrangement, the clustered and fascicled leaves of the larch.
To such embryos the term polycotyledonous has been applied,
but M. Duchatre has proved that these polycotyledonous
embryos are only Dicotyledonous embryos, whose two cotyle-
dons are deeply divided into a number of segments. Therefore,
it is proper toretain them among the Dicotyledonous embryos
of which they are only a variety.

In exalbuminous embryos, that is to say, those which are
immediately covered by the episperm or seed coat, the cotyle-
dons are excessively thick and fleshy, and their albuminous
contents furnish to the young and germinating embryo the
first materials of its nutrition. In such seeds as are albumi-

STRUCTURE OF THE SEED. 219

nous, on the contrary, the cotyledons are thin and membrana-
ceous, retaining in a great measure the. appearance of leaves,
in the midst of the surrounding albumen.

At the period of germination the cotyledons separate from
the integuments of the seed, and either appear above the
ground, different in form from the other leaves of the plant, or
they remain hidden in the earth without showing themselves, as
in the pea and the horse chesnut, until they finally decay. In
the former case, théy are epigeal (én upon or above, and yéa
the earth ;) in the latter case they are hypogeal (#7 under.)

The gemmudle, is the little bud at the summit of the plumule.
Like all other buds, it is composed of a little axis continuous
with that of the embryo, and certain minute rudimentary
leaves which represent the first leaves which the embryo is
going to develope. In general, in Dicotyledons the gemmule
is placed between the two cotyledons, which in being applied
one against the other, cover and hide it completely. It is
therefore necessary to separate the cotyledons in order to see
the gemmule. In Monocotyledons embryos, the plumule is
absent and the gemmule is placed within the sheathing base of
the cotyledonary leaf, and situated as it were, on one of its
sides. In developing, the gemmule gives birth to the aerial
portion of the stem, and its unfolding rudimentary leaves soon
take in succession the form, position and size of those leaves
which are peculiar to the adult plant.

220 COMPOUND ORGANS OF PLANTS.

CHAPTER XVII.

ON THE DISPERSION AND GERMINATION OF SEEDS.

Ir must be obvious that the immense quantity of seed which
plants generally produce, could never germinate in their imme-
diate neighborhood, and therefore, as the seed ripens, the peri-
carp gradually assumes such an organization as is calculated to
effect its dispersion or removal to a more distant locality. The
dissemination of the seeds is the result of the peculiar organi-
zation of their pericarp or seed-vessels, rather than of the seeds
themselves, which in this respect present some of the most
interesting and beautiful contrivances in nature.

Sometimes the pericarp opens elastically with a spring-like
mechanism, and discharges the seed contained in its cavity to
a considerable distance. The seeds of. the castor oil plant, of
the common garden balsam, and of the common furz, or whin-
bush of Europe, are separated from their pericarps in this
manner. In Hura crepitans, a plant which grows in the West
Indies and in South America, the seeds are projected from
the strong bony envelope of the pericarp as soon as it opens,
which it does with immense force and with a report as loud as
a pistol.

The pericarps of the thistle, dandelion, and other species of
Compositz, have attached to them a beautiful stellate down;
contrivances which are evidently intended to catch the wind,
and by means of which they are removed when fully ripe from
off the surface of the receptacle of these plants, and wafted to
a distance to spots favorable to their germination. The peri-
carps to which these appendages are attached, will sometimes

DISPERSION AND GERMINATION OF SEEDS. 221

travel for miles until a shower of rain or a humid atmosphere
causes the tuft to collapse, when the pericarp falls to the
ground. In some instances, as in the thistle, this down pro-
jects directly from the surface of the pericarp like the feathers
of a shuttle-cock; in the dandelion and goatsbeard it is sup-
ported upon a stalk which elevates it above the seed. In the
last plant each fine hair of the tuft is itself a feather, forming
altogether one of the most elegant and perfect objects.

In other species the pericarps are furnished with hooked
hairs, which cover their entire surface, as in galium and bur-
dock, by means of which they cling to. the bodies of men and
animals, and are thus scattered far and wide. In autumn it is
impossible to traverse the woods or marshes without having
such pericarps forced upon our attention. The achenia of
Bidens bipinnata, or the Spanish needles, are especially trou-
blesome. The achenia of this plant are surmounted with three
or four persistent awns, which are downwardly barbed, and by
means of which they very readily adhere to the dress of the
traveller. How little are persons aware when they brush off
these troublesome intruders, in some distant locality to which
they have unwillingly carried them, that they are fulfilling the
grand and secret purposes of nature !

Occasionally, as in the Asclepias or milkweed, and the Epi-
lobium or williow-herb, the seeds themselves are furnished
with the coma or tufts of hairs, by means of which, on the
dehiscence of the pericarp, they are lifted by the wind out of
its cavity and carried away sometimes to a great distance from
the parent plant.

Birds, too, are important agents in the diffusion of seeds.
It is well known that the seeds of numerous berries and small
fruits will grow, though they have passed through the bodies

222 COMPOUND ORGANS OF PLANTS.

of birds. It is in this way that Phytolacca decandra, or the
common pokeweed, appears to have been dispersed over the
whole of North America. The berries of this plant are eaten
by the robin, the thrush, the wild pigeon, and many other
birds, which thus carry them hundreds of miles from the plant
which produced them. In this manner we can account for a
fact which every practical botanist and observer of nature must
have noticed, viz.: the sudden appearance of a single plant in
a place where its species was entirely unknown before.

Some pericarps are conveyed by the rivers into which they
fall, or by the waves of the ocean, many hundreds or thousands
of miles from the countries which originally produce them. In
this manner many of the native plants of France, Spain, and
other adjacent countries, have been naturalized in England ;
and the pericarps of tropical climates are conveyed to the coasts
of Norway and Scotland. The foreign pericarps which are
annually left on the Norway coast, are principally cashew-
nuts, bottle-gourds, cocoa-nuts, and the fruit of the dogwood
tree. These are often in so recent a state, that they would
unquestionably vegetate were the climate favorable to their
growth and existence. When carried to countries better suited
to their nature, they germinate and colonize with a new race of
vegetables the land on which the ocean has cast them. In this
manner it is that the coral islands, as soon as they appear above
the waves of the Pacific, are speedily covered with a crop of
luxuriant vegetation. The cocoa-nut is well adapted for this
purpose, as it grows luxuriantly in salt water, and it is proba-
bly the first arborescent species which vegetates on these newly-
formed lands.

Most of the seeds thus carried abroad never germinate at
all, as they either fall into situations unfavorable to their

DISPERSION AND GERMINATION OF SEEDS. 223

growth, or upon a soil which is already pre-occupied by other
plants. All the plants of a given district may be regarded as
at war with each other. The arborescent species prevent, by
the extent of soil which they occupy, the vegetation of species
of a humbler growth. Each has to struggle into existence
against a host of competitors, for nature, although she has
been prolific of the seeds of life, has limited the supply of
room and food. A number of ferns, for example, which may
be growing on a hill-side, will, by their pre-occupation of the
soil, successfully maintain their ground against all other
intruders for ages, notwithstanding the facilities afforded to
other plants for the dispersion of their seeds. If any chance
seed should be borne to this spot by any of the agencies which
we have enumerated, or by other causes, it cannot germinate
among them, as they absorb all the food from the soil.

The seeds which have been thus unfavorably located, retain
their vitality for a longer or shorter period of time. Such ag
have very thin and delicate integuments, will lose their
germinating power after a few weeks’ exposure; so also oleagi-
nous seeds will in general, decay much sooner than such as
contain albumen. Other seeds, on the contrary, will retain
their vitality for an indefinite period of time. This is the case
with the Leguminous plants, the seeds of which may be kept
for years without any material detriment to their germinating
power. Peas taken from the herbarium of Tournefort, where
they had remained for more than one hundred years, were
made to germinate in the botanical gardens at Paris. Those
changes by which the ovule is changed into the mature seed,
appear to be all made with a special reference to any mishaps
which may befall it when thrown on the charity and care of
nature by the parent plant, as well as to provide it with a

204 COMPOUND ORGANS OF PLANTS. x

store of nutriment on which it may subsist during the early
stages of its development.

When the plant approaches the close of its allotted period of
life, it is surprising with what care provision has been made
for the continuation of the species, as if nature had determined
to secure it, if possible, an immortality of existence upon the
earth’s surface. Hence not only the beautiful contrivances to
effect the removal of the seed to spots favorable for its germi-
nation, but also the immense quantity of seed which the dying
plant produces. On a specimen of the castor oil plant, which
the author cultivated in his garden, he counted ten clusters of
pericarps, or seed-vessels; each cluster produced upwards of
fifty pericarps, and each pericarp contained three seeds. The
total number of seeds produced by the plant was, therefore,
10x50%3=1500. Each of these seeds, be it remembered,
contained within its folds an incipient repetition of the pareut
plant in the form of a young embryo. Supposing each seed to
germinate, and the plants to arrive at maturity, the product of
the next season would be 1500 1500=2,250,000 seeds! In
other plants, the first crop of seeds is still greater. It has
been calculated that the sunflower produces 4000 and a single
thistle 24,000 seeds the first year; therefore the second
year’s crop would amount to 16,000,000 of seeds in the former,
and 576,000,000 of seeds in the latter instance. How immense
the amount of vegetable life which may spring from a single
seed! Happily for mankind, every vegetable embryo is not
destined to give rise to a future progeny. Millions of seeds,
or vegetable embryos, are called into existence, but their
incipient life is speedily destroyed by a variety of causes.
Were it not for the operation of these causes, by which the
species is kept within prescribed limits, such is the fecundity

DISPERSION AND GERMINATION OF SEEDS. 225

of nature, that there can be no doubt that the seed from a
single thistle or dandelion would, in the course of a few years,
be sufficient to cover with plants not only every square inch of
the superficies of our own world, but the entire surface of every
other planet in the solar system.

But although nature has been thus careful to ensure a repe-
tition of their beautiful and evanescent forms, all plants
multiply within prescribed limits, which they cannot pass.
Fecundity is therefore no barrier to the variety which every-
where prevails, which is the principal charm of the vegetable
creation, and from which we derive so much instruction in the
study of their individual forms.

When, however, the seed falls into a soil favorable to its
germination, it will grow and become a plant, running through
all the phases of the vegetation of its predecessor.

We have only now to lay before the student the conditions
which are necessary to germination, and the interesting series
of phenomena connected with the evolution of the young
plantule from its integuments.

The exterior agents indispensable to germination are water,
air, and heat.

Water is necessary in germination as in all the other pheno-
mena of vegetable life. It penetrates into the substance of the
seed, softens its envelopes, and makes the embryo swell. It
therefore places the seed in the conditions which are most
favorable for its development. As soon as germination com-
mences, it dissolves the dextrine and the other soluble prin-
ciples which exist in the seed, and which are formed by the
transformation of the starch, and conveys these nutritive mate-
rials to the young embryo.

Air as well as water is also necessary. Experiments show

20

226 COMPOUND ORGANS OF PLANTS.

that seeds will not germinate in vacuo, or in a space from
which the air has been artificially removed. Hence seeds
buried too deeply in the soil will not germinate, as the air
cannot get access to them, but if by any natural or artificial
causes they arc brought into the superficial beds, germination
very soon commences. It is thus that we see suddenly appear
in a locality certain plants which did not grow there before, as
for instance, when a waste field is cultivated.

It is the oxygen of the air which acts principally in germina-
tion ; for seeds plunged in pure nitrogen or hydrogen neither
germinate nor develope. It is by the absorption of oxygen
that the starch which remains in the nucleus, or which has been
absorbed into the cotyledons, is rendered soluble and nutritive.

It is well known that starch is quite insoluble in cold water.
By what remarkable operation: in vegetation does it become
soluble, so ag to be dissolved and transported to all parts of the
plant? This question we now proceed to answer. Starch is
profusely spread through all the organs of the plant, and is
accumulated especially in the seed, as a store of nutriment on
which the young embryo subsists, until such times as its roots
and leaves are sufficiently developed for the accumulation of its
proper food from the earth and atmosphere. But in order
to its assimilation by the young embryo it is necessary that
the starch in the cotyledons or nucleus should be rendered
soluble. When the temperature and other conditions are favor-
able, a vegetable secretion termed diastase forms itself in all the
cells which contain starch. This diastase possesses the sin-
gular property of transforming starch into a soluble gum
termed dextrine, which the water is able to carry to all parts
of the plant. The action of the oxygen of the air, through
the secretion of the diastase, having thus changed the starch

DISPERSION AND GERMINATION OF SEEDS. ° 227

to dextrine, and the continuation of the same process con-
verting some dextrine into sugar, which being dissolved by
the water, also penetrates all the parts of the embryo, thus
induces the necessary nutritive and germinating processes.

The starch contained within the folds of the seed, is there-
fore at the end of a certain time, completely re-absorbed.
This disappearance of the starch is the result of its combus-
tion by the embryo, or of its slow conversion principally into
carbonic and other vegetable acids, and in part into cellulose.

Heat is therefore evolved during germination, and a certain
amount of it becomes indispensable to the vital action of the
seed. Placed in the midst of a temperature below zero, it
remains benumbed-and stationary even under the influence of
air and humidity. But a mild temperature accelerates the
development of all the phenomena of vegetation. Hence it is
that the gardener is accustomed to hasten the development of
such exotic grains as it is his interest to cultivate, by sowing
them in a hot bed, and by consequence surrounding them with a
humid and artificial heat. But it is necessary that this tem-
perature does not pass certain limits, otherwise, so far from
hastening the development of the seeds, it will dry up and
destroy the principle of life within them.

The degree of heat required to excite the vitality of the
embryo, varies from 50° to 80° (Fahrenheit,) for the plants of
temperate climates. The seeds of tropical plants require a
much higher heat’ to call them into action, varying from 90° to
110° (Fahrenheit,) and occasionally a more elevated tempera-
ture than even this, is found to be necessary.

Light exercises an unfavorable influence on germination.
This must necessarily be its effect on the germinating seed, for
we have shown that the absorption of the carbonic acid of the

228 COMPOUND ORGANS OF PLANTS.

atmosphere, the assimilation of the carbon and the evolution
of the oxygen, are processes which are greatly forwarded by
this agent. Now all these processes are just the reverse in
germination, for oxygen is absorbed and carbonic acid is elimi-
nated. It is not true that seeds will not germinate unless pro-
tected from the influence of light, since every day we see plants
germinating very well and with considerable rapidity, on fine
sponges, on sand, or other bodies from which they can imbibe
water ; but it is nevertheless true that a strong light will greatly
retard whilst darkness will favor the germinating process.

The general phenomena of germination may be thus summed
up. When there are the suitable conditions of temperature,
air and moisture, the first phenomena which we observe in the
germinating seed is the swelling and softening of the envelopes
which covered it. These become distended with moisture, and
ultimately ruptured in a more or less irregular manner, as the
swelling of the seed increases. About the same time that the
seed commences to be distended with moisture, it attracts
oxygen from the atmosphere. This oxygen induces the formation
of the diastase, which acts on the starch contained in the coty-
ledons, converting it into dextrine and sugar, which dissolved
by water, are conveyed to all parts of the youngembryo. The
bulk of this sugar is converted into carbonic acid, whilst the
remainder or dextrine, is organized into cellulose. Therefore,
instead of taking in the materials of nutrition from the earth
and atmosphere, or assimilating externally in germination as in
the process of flowering, the plant consumes these materials or
assimilates its own products. Now all the organs of plants,
whatever be their form, their nature, or their destination, have
for a base the same immediate principle, cellulose ; but starch,

DISPERSION AND GERMINATION OF SEEDS. 229

dextrine and sugar have precisely the same chemical composition
as cellulose. Thus it is, that the store of nutritive, though
unassimilated and insoluble starch, with which the seed is so
copiously provided, is by the forces of nature rendered soluble,
and converted into dextrine, sugar, and finally cellulose, the
substance which constitutes the very basis of all the vegetable
tissues, it becomes the source from whence the embryo derives
the materials of its nutrition and increase.

These chemical changes in the substance of the seed soon
awaken its dormant vitality. We see the radicle of the embryo
descend through its swollen and ruptured integuments into the
earth, whilst at the same time the plumule rises into the atmo-
sphere, carrying up with it the young cotyledons, which soon
unfold in the form of two white and opposite leaves above the
earth’s surface. Exposed to the action of light we see them
gradually change their color, chlorophyl being deposited in
their superficial cells. The cotyledons appear to be only.
indifferently adapted to the aerial medium into which they are
elevated, and hence, as we have seen, they sometimes continue
below the ground without any detriment to the growth of the
young embryo. When, however, the gemmule or bud at the
summit of the plumule elongates and the true and permanent
leaves of the plants appear, they perform the functions of
aerial leaves in a much more perfect manner; at the same
time, from the other extremity of the axophyte, additional
roots are developed, and the organs at both extremities are
beautifully adapted to their respective media. Germination
is now completed, the cotyledons and other appendages of
the embryo decay and disappear, having performed their
respective functions, and the young plantule, rejoicing in all

230 COMPOUND ORGANS OF PLANTS.

the freshness and beauty of vegetable youth, developes into
the earth and atmosphere and depends for its future supplies
of food on its leaves and roots, running through precisely
the same phases of vegetation as its predecessors.

THE END.