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BURSABURSA-PASTORISAND BURSA HEEGERI
BIOTYPES AND HYBRIDS

BY

GEORGE HARRISON SHULL

WASHINGTON, D. C.

PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON.

1909.

CARNEGIE INSTITUTION OF WASHINGTON PUBLICATION No. 112

PAPERS OF THE STATION FOR EXPERIMENTAL EVOLUTION No. 12

THE CORNMAN PRINTING COMPANY
CARLISLE, PENNSYLVANIA

BURSA BURSA-PASTORIS AND BURSA HEEGERI
BIOTYPES AND HYBRIDS.

Bv GEORGE HARRISON SHULL.

INTRODUCTION.

Darwin recognized in the facts of variation a key to the riddle presented
by the multiformity and many obvious interrelations of all living" things.
Since the appearance of the Origin of Species the observation and discus-
sion of variations have assumed a dominant place in biology, and a serious
conflict has recently developed regarding the interpretation of the observed
facts. It is now generally recognized that this conflict can be brought
to a termination only through the application of experimental methods.
Inspection alone can not decide the question as to how an observed varia-
tion originated and what bearing it may have on the future of the race in
which it occurs. Studies in the museum and in the field only discover the
fact, and to some extent the range, of variation occurring under a more or
less limited and inadequately known range of conditions, and can not cer-
tainly determine its cause or causes ; neither can these means supply more
than a suggestion based upon insecure inference as to the hereditary nature
of any variation. The causes of variation can be determined only by
subjecting equivalent material to different controlled conditions, and their
hereditary relations can be learned only through the conduct of pedigree-
cultures.

We already know, as a result of experimental work in these directions,
that variations are of fundamentally different types, having different causes
and obeying different laws of development and heredity. A knowledge of
these facts impresses an important principle, namely, that the range of
applicability of any conclusion reached by the investigation of one class of
material or one characteristic can be determined only by similar experiments
with other material and other characteristics.

This has been one of the guiding principles in the institution of the ex-
perimental work at the Station for Experimental Evolution of the Carnegie
Institution of Washington. Numerous species of plants and animals of
quite wide relationships have been brought under observation, and it may
be expected that each will in time give material assistance in determining
to what extent principles and hypotheses now available have general validity,
and will also lead to the discovery of such new principles, or such modifi-

3

4 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

cations of the old ones, as may be necessary for the interpretation of the
observed facts of evolution. Among- the species now being- studied , one that
has very early yielded results of interest in this connection is the common
shepherd 's-purse of our dooryards, Bursa bursa-pastoris (L.) Britton,

Bursa (Capsella} bursa-pastoris is known to the taxonomist as an exceed-
ingly variable species, which seems to have been brought to this country from
Europe, but which is now naturalized and almost universally distributed
throughout the North Temperate zone. So strikingly different, qualita-
tively, were the characters of different individuals observed growing side
by side in nature that it was difficult to believe that they all belonged to a
single series of fluctuating variations, and when the opportunity offered
to make pedigree-cultures this species at once sug-gested itself as favorable
material. This thought was based not alone upon its apparent polymorph-
ism, but also upon its hardiness, ease of culture, and the impossibility of
its having been subjected to any of the artificial conditions of isolation,
crossing, etc., which are usually thought to render plants of economic
value unfit to give information regarding the behavior of plants in nature.

No one has ever attempted to "improve" the shepherd's-purse, and
although its rapid extension over its present great range is undoubtedly
dependent upon the agency of man, both in supplying suitable habitats and
more directly in the transportation of the seeds, yet in performing these
operations his work has been wholly unintentional, and he is therefore to
be classed with the other accidental agents of nature, thus leaving to Bursa
the solution of its problems of maintenance, extension of range, and evo-
lutionary progress, under conditions which are fundamentally like those
that must be met by any other species in a state of nature.

These cultures were begun in April, 1905, and were continued until the
spring of 1907, when they were temporarily abandoned because other in-
vestigations necessitated my absence from the Station for Experimental
Evolution during rather extended periods, and thus made it impossible to
continue the work advantageously on a species like Bursa, which shows no
dependence upon the seasons, but blooms and fruits whenever external
conditions are such as to make its development possible. It is hoped that
these studies on Bursa may be continued in the not distant future, but as
some of the conclusions arrived at have been already presented before sev-
eral scientific bodies,* it seems desirable to publish a more comprehensive
account of the work than has been done up to this time, even though the
evidence is in many places more or less fragmentary.

When undertaking such cultures with any new class of material much
that is necessary for the most satisfactory and economic conduct of exper-

*Sections F and G, A. A. A. S., New York, December, 1906; Seventh International
Zoological Congress, Boston, August, 1907; Botanical Society of America, Chicago,
January, 1908.

BIOTYPES AND HYBRIDS. 5

iments must be learned by experience, for, as said before, mere inspection
of an individual or of a single generation can not distinguish between im-
portant and unimportant variations until the pedigrees themselves furnish
solutions to the question. For this reason there must be in the beginning
a much more minute analysis of the material than is later found necessary
or desirable. Most of the cultures which I have made thus far have been
directed toward determining what variations are of the fluctuating kind and
what are fully transmissible to the offspring.

In one group I have now had large numbers of plants of the fourth
pedigreed generation and a few of the fifth under observation, and in a
number of other cases the third generation has been extensively grown. I
find that while certain variations which were selected disappear in the first
or second generation, others remain constant, easily recognized differen-
tiating marks which, except in one form, show no transgression of the char-
acteristic features of any other form studied. These forms are, therefore,
distinct elementary species, or biotypes, each characterized by certain con-
stant features and each with its own normal range of fluctuating variability.

The systematist has not yet decided what treatment to give to elementary
species, and any nomenclatorial scheme must be regarded, therefore, as
purely tentative ; but utility can not wait for concerted action on the part
of taxonomists in devising a suitable systematic designation of elementary
species, and I have therefore for the sake of convenience assigned to these
elementary forms of Biirsa simple names which can be attached to the
accepted specific name to form a trinomial. I was at first inclined to use
binomial names which would leave the Linnean specific name, bursa-
pastoris, as the valid name of the aggregation of elementary forms having
the same general habit and the triangular or obcordate capsules. It might,
then, be looked upon as a superspecific name which would remain just as
useful in the everyday conversation and experience of the botanist as when
the aggregation for which it stands was believed to be a unit.

The fact that corresponding series of elementary species or biotypes may
occur in different related species, as will be shown later in the discussion
of the heegeri hybrids, makes the trinomial much to be preferred, for cor-
responding forms may then be given the same name without confusion.
Thus Bursa bursa-pastoris heteris and Bursa heegeri heieris may be used to
denote two forms which are alike in rosette -characters but different in the
capsule-character, the latter character being accepted as of specific value.

That Bursa bursa-pastoris is a composite species was first given public
recognition by Lotsy (1906) about a year after my cultures were begun,
and his statement did not come to my notice until after I had presented
my first account before the American Association for the Advancement of
Science at its New York meeting, December, 1906. His statement is very
brief, and the chief interest of his account consists in the photographs

6 BURSA BURSA-PASTORIS AND BURSA HEEGERI I

which show samples of three families which bred true to rather strikingly
distinct characters in his cultures. To these several forms he assigned
trinomial names, though no adequate description is given. No statement is
made as to the extent of the cultures upon which their standing- as perma-
nent biotypes is based, nor is there any statement as to the extent and nature
of the fluctuating variability of each of the forms figured. Until attention
is given to such matters by the student of pedigree-cultures it will be im-
possible to properly estimate the security of conclusions drawn, or to rec-
ognize with any certainty the identity of biotypes discovered by different
investigators.

To obviate these difficulties, I wrote to Dr. Lotsy for pedigreed seeds of
his several types, in order that they might be grown beside my own for the
purpose of testing- their identity, but received the reply that none were
available. The questions as to whether Lotsy 's names are therefore to be
considered nomina nuda and whether I am justified in assigning new names
to the biotypes to be discussed below, which may or may not be identical
with one or more of Lotsy 's named elementary species, I leave to the tax-
onomist to decide. Indeed, in adopting the names used in this paper I have
been governed entirely by the demands of present utility and not by any
thought that these names will be accepted by taxonomists as having proper
standing in the nomenclatorial system now recognized by them.

Still more recently, Almquist (1907) has published over 70 named " ele-
mentary species" of B. bursa-pastoris , but these are described in rather
general terms, and the cultures were conducted in the open garden un-
guarded, usually for but 2 to 4 generations. Almquist states (p. 5) that
after this length of time the cultures had either died out or were no longer
pure. He assumes that this " loss of purity " is due to vicinism with the
other elementary species growing on neighboring plots, and some of it
doubtless was. In my own cultures I have observed that some of the dif-
ferentiating characters used by Almquist disappear upon continued breed-
ing, even when vicinism is carefully excluded. It seems that members of
the same biotype coming from different habitats may retain certain fluctu-
ating differences, such as differences in texture of leaves, degree of redness
of veins, degree of epinastic growth in the petioles, etc., for several gener-
ations of uniform treatment, but finally merge into identical forms, even
when crossing with other biotypes is entirely precluded by careful guarding.

While I can not say with certainty that any of Almquist's "elementary
species" are not distinct and permanent types, my experience indicates
emphatically that he has not demonstrated that they are. I think it very
probable that a number of the elementary species figured and described by
him will not stand the test of longer culture under more carefully controlled
conditions. However this may be, it seems evident that the number of
elementary species of Bursa may be considerable. At the time of the first

BIOTYPES AND HYBRIDS. 7

presentation of my studies on Bursa before the American Association for
the Advancement of Science (December, 1906), I was pretty sure that I
had demonstrated the existence of 11 biotypes, but the disappearance of one
or two of these since, led me to reduce the number to 4, as published in the
extract from that paper (Shull, 1907).

These four forms, whose distinctness and permanence I have demon-
strated beyond a possible question, have been studied in their hybrid com-
binations, and it is the cultures of these and their hybrids with which the
present paper will mainly deal, though some features of other cultures will
be discussed.

It is doubtful whether any bit of technique that has been recently
added to the tools with which the biologist may operate in unlocking- the
mysteries of protoplasmic organization is of so far-reaching importance
as the process of hybridization. I say has been recently added because,
although the art of producing hybrids is very old among the breeders of
animals, and nearly 200 years old among gardeners, little of scientific value
could be secured by means of hybridization until some of the fundamental
laws involved in the process were recognized, and it is only within the last
eight years that biologists have gained sufficient insight into the behavior
of hybrids to give interpretations of the results of hybridization any value
as indications of protoplasmic structure and behavior.

Two general types of hybrids are readily recognizable, namely, the con-
stant and the splitting. The former may or may not give indication of the
characteristics of their parents, being usually, but not always, intermediate
between the parents in most of their characters. The splitting hybrid
always indicates by its offspring what were the characteristics of its im-
mediate ancestors. It is the latter type of hybrid which is of the greatest
usefulness in giving insight into the structure of the germ-plasm. Hybrid-
ization in such cases does not only serve to unite in the same individual all
of the characteristics of both parents, but as successive generations are
followed, it results in a complete analysis of all the points of difference
existing in the two biotypes between which the cross was made.

The conduct of such analysis by hybridization is particularly simple in
the case of plants which, like Bursa, readily self -fertilize, because once the
first-generation hybrid is secured the process of analysis goes on genera-
tion after generation, until all the allelomorphic differences of the parents
are made manifest by being separated in a pure state in different individuals.

In some respects even more satisfactory evidence of the elementary char-
acter of two forms is to be derived from their behavior when crossed than
from their conduct in straight pedigrees. If, for instance, two forms sup-
posed to be elementary to each other should be, instead, merely extreme
fluctuants of a single biotype, their cross-bred progeny would show a fluc-
tuating series, including perhaps the two parental extremes, but could hardly

8 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

be expected to produce a constant intermediate progeny, nor one which
would show the two parental conditions in an expected Mendelian ratio.
Occasionally in the course of pedigree-cultures some slight difference in
treatment will produce a modification in some progeny, which may lead to
doubt as to its classification, and perhaps even after several generations
this doubt will not be wholly removed. In such cases hybridization is
almost certain to give decisive results, as there often appears to be a more
characteristic development of morphological features in the hybrids than in
the pure-bred strains. This may be due to the fact that the allelomorphs
are brought into different relations with other allelomorphs in the recom-
binations which take place in the second and later generations, thus elimi-
nating any inhibiting or modifying influence which may have resulted from
the constant presence of some particular allelomorph or combination of
allelomorphs, or of some permanent condition of the cytoplasm, in the pure
strain. This elimination of modifying factors would allow a more accurate
definition of the unit-characters involved. Or, on the other hand, the better
development of characters in the hybrids may be due to the fact, long
recognized, that heterozygosis produces a stimulation which increases
vegetative vigor, and this should have the same effect as good cultural
conditions in bringing all characters to their full typical development.

MATERIAL AND METHODS.

For the beginning of the cultures, seeds were secured from a number of
more or less strikingly different individuals growing in the vicinity of Cold
Spring Harbor, Long Island; Chicago, Illinois; New Carlisle, Ohio; Man-
hattan, Kansas; and Tucson, Arizona. Leaves of each of the plants chosen
as parents for the cultures were carefully preserved, in order that the con-
ditions of the offspring might be compared with those of the parent and,
in each pedigreed family raised since, care has been taken to preserve in
the herbarium samples of every variation of sufficient magnitude to strike
the eye.

From 20 lots of seeds of Bursa bursa-pastoris brought in from nature
and 1 lot of B. heegeri received by Dr. MacDougal from Professor Solms-
Laubach, I have now had under observation about 200 pedigreed families,
including something over 26,900 individuals of Bursa bursa-pastoris, 2 small
families of Bursa heegeri, and 5 families representing reciprocal crosses
between these two species and involving over 2,500 individuals.

All of my cultures have been carried on under glass, the first 7 months
in the sky-lighted room of the laboratory at the Station for Experimental
Evolution, and since that time, i. e., after December 7, 1905, in the glass
propagating-house at the same place. It was found that the light was too
dim in the former room to bring out the characteristic features of the plants
and to keep them in a state of vigorous health. The leaves became more

BIOTYPES AND HYBRIDS. 9

elongated than is normal, and the lobing more shallow and therefore less
characteristic, so that it was less, easy to estimate the uniformity and dis-
tinctness of the several pedigrees than when they were grown under more
favorable conditions. Even specimens belonging to the most deeply and
distinctly lobed families, when grown in the darker portions of the room,
retained their unlobed, juvenile type of leaf throvighout life, sending up a
weak flower-stem from the juvenile rosette, and in such cases the relation-
ships could not be recognized, since the early leaves of all the forms studied
are very similar. The same difficulty was also experienced in the propa-
gating-house when the members of certain families were allowed to remain
too long crowded in the seed-pans. The offspring of these juvenile plants
have not been extensively studied, but from several families evidence has
accumulated which indicates that when again given favorable conditions
the offspring of these characterless plants return to the characters of the
family from which they sprang.

In all cases the seeds have been sown in soil sterilized in an autoclave
in the manner usually adopted by students of pedigree-cultures, and the
efficiency of the method is inferable from the fact that in all these cultures
only 3 seedlings occurred whose origin was unknown. These unexpected
seedlings were an Oxalis, a Mollugo, and an Erechtites, and it does not seem
likely that any of these withstood the long-continued high temperature of
the autoclave, but rather that they were blown through the ventilators of
the propagating-house in a heavy wind-storm.

It was soon demonstrated that Bursa has many features which make it
advantageous material from a technical point of view for pedigree-culture
work. The habit of the plant, consisting as it does of a moderately lax
rosette and nearly naked, erect flower-stem, allows the preservation of the
rosettes as herbarium specimens whose characters are almost as easily
studied as are those of the living plants, and the inflorescence may be cov-
ered with paraffin-paper bags to prevent chance crosses with other speci-
mens without appreciably interfering with the photosynthetic work upon
which the healthy development of the plant depends.

The small size of the plants makes it possible to raise them to maturity
in 3-inch pots in many of the forms, thotigh it is usually found advanta-
geous to repot to 4-inch pots those which it is desired to keep for seed.
This quality allows a large number of specimens to be raised in a small
compass.

In most instances the life-cycle is short, requiring only 3 to 4 months
between the sowing of the seed and the gathering of the ripe seed of the
earliest matured individuals. In several of the forms of B. bursa-pastoris ,
and in B. heegeri, however, 8 to 9 months were needed. The seeds ger-
minate in 5 to 8 days without a period of rest, thus making it possible to
accumulate data from a number of successive generations in a short time.

10

BURSA BURSA-PASTORIS AND BURSA HEEGERI :

The flowers are adapted to both cross- and self-fertilization, all the evi-
dence now at hand indicating- that the latter method is normally by far the
more efficient. Leaving- out of account 2 of the 21 original cultures which
showed by their constitution that their parents were hybrids, less than 1
per cent of the plants raised from seed collected in nature showed evidence
of being- the result of cross-fertilization between different biotypes . Crosses
between different flowers of the same plant and between different indi-
viduals of the same biotype may take place somewhat more frequently than
crosses between the flowers of plants belong-ing- to different biotypes,
though nothing in the resultant offspring gives any clue to the frequency
of such crosses.

FIG. 1. — Enlarged sections of buds and flower of Bursa bur sa pastor is, show-
ing three stages in anthesis. A. Exposure of the stigmatic surface for the
reception of foreign pollen. B. Anthers opening in contact with the stigma,
thus insuring self-pollination. C. The flower fully open, allowing the access
of the visiting insects to the pollen. All magnified 20 diameters.

The conditions which favor cross-fertilization are : (<z) Slight prote-
rogyny, which allows the stigma to receive foreign pollen some hours before
the anthers of the same flowers dehisce. The distal portion of the globular
or disk-shaped stigma is exposed between the tips of the unopened sepals and
petals (fig. 1 , A) . (6) Although the fully developed inflorescence is a typical
raceme, the flowers and buds are arranged in a nearly flat-topped corymb
having the flowers at the circumference, giving this part of the inflores-
cence a condition quite analogous to that of the head of the Compositae, in
which the whole inflorescence appears to act the part of a single flower in
the attraction of insects. Small insects, particularly flies and small bees,
visit the flowers freely. These rest upon the top of the inflorescence as a
whole while visiting the several individual open flowers about the circum-
ference. In these fully open flowers forming the exterior circle of the inflo-
rescence the anthers have opened, while in the second circle the summits
of the stigmas are exposed on the same general level as the rest of the
corymb. As the insect walks about over the top of the inflorescence, the

BIOTYPES AND HYBRIDS. 11

chances appear favorable for the lodgment of pollen from the outer circle
of flowers, or from the flowers of other plants, upon the exposed stigma-tops
of the second circle.

Adaptation to self-fertilization is found in the facts: (a) that the stigma
is also receptive on its under (proximal) surface as well as on the upper
(distal) surface, and (£) that the anthers dehisce while they are still held
in contact with this under surface of the stigma by the erect segments
of the perianth (fig. 1, B). Soon after the anthers open the petals begin
to spread, thus exposing the pollen to be carried away to other flowers by
visiting insects (fig. 1, c).

At the beginning of these experiments paraffin-paper bags of suitable
size were not available, and as insects were not abundant in the room used
for the cultures, the only precaution taken to guard against cross-pollination
was to set the flowering specimens intended as seed-plants somewhat apart
from each other and from other flowering specimens. As will appear
later, a few individuals, of unexpected character, are doubtless to be attrib-
uted to this unguarded condition of the earlier cultures, but it will also be
seen that the percentage of such chance crosses is extremely small. As
soon as possible, suitable paper bags were secured, and since then the cul-
tures, with few exceptions, have been carefully guarded.

Although crossing among the unguarded cultures in the greenhouse has
been of rare occurrence, it was evidently more frequent in the material
secured from nature. Of the 21 original cultures 2 proved to be of hybrid
origin, while a fraction of 1 per cent of the rest indicated by their atypic
condition that they were probably the result of cross-pollination. Only
when the pollen comes from some form which is dominant to the pistil-
parent is the fact that a cross has taken place obvious in the first genera-
tion. It seems fair to assume that on the average as many crosses take
place with a recessive pollen-parent as with a dominant, and this assump-
tion would require that hybridizations occur with twice the frequency with
which they become obvious in the Fl offspring. On this basis the fre-
quency of cross-pollination between different biotypes of Bursa in nature,
as indicated by these cultures, is about 1 to 65 as compared with the fre-
quency of self-pollination and crosses between flowers of the same plant
or between plants of the same biotype. Of course this ratio is based upon
a very limited number of specimens and can be expected to vary greatly
in different lots of material of the same magnitude, but at least the great
preponderance of self-fertilization may be safely inferred in this species
when in a state of nature.

These characteristics of Bursa which make the production of pure self-
fertilized lines easy are opposed to the ease with which cross-fertilization
may be controlled. The fact that self-fertilization takes place before the
petals spread makes it necessary to carefully remove the stamens about a

12 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

day before the buds open. As the buds are at that time quite small, the
technique of cross-pollination is somewhat delicate. However, with a
needle-pointed forceps the calyx, corolla, and stamens may be readily cut
away from about the young pistil, and the eye and hand soon become so
skilled that the work can be done swiftly and with as much accuracy as
may be attained in the castration of a lily.

PURE CULTURES OF BURSA BURSA-PASTORIS.

The four biotypes of Bursa bursa-pastoris with which this paper mainly
deals are distinguished from each other by certain characteristic lobings of
the leaves. For convenience I have assigned to them the names Bursa
bursa-pastoris heteris, B. dp. tenuis, B. bp. rhomboidea, and B. bp. simplex.
It has been impossible to determine which of Almquist's "elementary
species" agree with these, but it is almost certain that he has assigned
names to several fluctuations of the same biotype. If this is true my four
types are more inclusive than his and under my names will need to be
grouped a number of forms which he has considered distinct elementary
species.

Bursa bursa-pastoris heteris n. sp. element.

Plants belonging to this biotype have the leaves divided to the midrib,
the terminal lobe being usually separated from the nearest lateral lobes by
deep, clean-cut incisions. The lateral lobes consist essentially of two fea-
tures— an elongated, attenuate portion which I call the "primary lobe," and
a more or less rounded or angular portion which forms a ' ' secondary lobe ' '
in the distal axil of the primary lobe (fig. 2). As in the characterizations
of all the following forms, this description refers to the climax-leaves of the
rosettes in properly grown specimens, since the juvenile leaves of all the
elementary species of Bursa are entirely unlobed, and starved or crowded
specimens of all the forms may reach maturity with only juvenile leaves,
as already stated. Several of Almquist's recently described forms would
obviously range themselves under this description, though it is always pos-
sible, of course, that forms which possess the described characteristics may
have still other permanent characteristics which would render them distinct
elementary species.

The first of the forms recognized by Almquist which can certainly be
placed here is his Capsella bp. rubella (see his figs. 5 and 6). His C. bp.
angustiloba (his figs. 56 and 57) represents the most pronounced develop-
ment of this type, and others which probably belong here are reuteri (fig.
11), grandiflora (figs. 12 and 13), hiemalis (fig. 20), grossa (fig. 22), au-
tumnalis (fig. 24), segetum (fig. 28), wittrockii (fig. 44), rhombea (fig. 48),
rhombella (fig. 53), ellipsoidea (fig. 55), and/ucontm (fig. 60).

My first pure-bred family of B. bp. heteris (pedigree-number 040.3) was
grown from seeds collected by J. Marion Shull at Edgewood, New Carlisle,

BIOTYPES AND HYBRIDS.

13

Fig. 2

FIG. 2. — Bursa bursa-pastoris heteris. A typical specimen of my first pure cul-
ture of this biotype.

FIG. 3. — Bursa bursa-pastoris heteris. Climax leaves of the rosette of a speci-
men growing in nature in Jackson Park, Chicago.

14 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

Ohio, May 28 and June 2, 1905. The climax leaves of the parent were of
the most pronounced heteris form. From seeds sown June 26, 1905, 39
specimens were grown, all of which agreed with the parent and with each
other in the character of lobes as described, though there was some fluctu-
ation in general aspect due to difference in elongation of the rachis by
means of which the lobes were more or less crowded, some variation in the
attenuation of the primary lobes, in the degree of lobation of the upper
rosette leaves, in the angle which the rosette leaves made with the hori-
zontal, and in the prominence of the rounded secondary lobe. This sec-
ondary lobe was characteristic, however, and in not a single individual was
it absent in the climax leaves of the rosette.

The significance of several of these variations was tested by using their
best representatives as parents of a second generation. From 3 of these
(053.30, 053.31, 053.32) have been raised 839 specimens, all but the
54 offspring- of one individual being strictly like the grandparental and
parental form. The one aberrant family (053.30), the parent of which had
the upper leaves of the rosette less deeply lobed than usual, showed com-
plete suppression of lobes in 5 individuals, and in the remaining' 33 which
were noted the primary lobes were broader than usual and the secondary
lobes less prominent. It is desirable to test the possibility that these char-
acteristic secondary lobes may be rendered completely latent by selection,
and, if this is possible, to discover by what means they may be again made
manifest.

At least two other pure-bred cultures coming in from nature belong- to
B. bp. heteris, but were considered for two generations to represent two
very distinct elementary forms. It has been the striking modifications ob-
served in these two pedigrees which have led me to suspend judgment on
all my cultures except those whose behavior in hybrid combinations has
left no possible question as to their distinctness and permanence.

The basis for the assumption that these two pedigrees represented dis-
tinct elementary species was the facts that the aspect of each of these fam-
ilies was very different from that of any other culture and that there was
great uniformity among the individuals belonging to either single family,
i. e. , while there was a strong break between the families there was almost
no variation within the family. This was presumably the basis of Alm-
quist's estimates as to the distinctness of his forms, and the following ac-
count of these two families supports my attitude of doubt as to the soundness
of his results.

040.15: This specimen was found growing in Jackson Park, Chicago,
by Charles A. Shull, who collected the seeds in the summer of 1905. Climax
leaves taken from the rosette show only a slight development of the heteris
characters as described above (fig. 3 ) . Though the sinuses reached the mid-
rib, the primary lobes were not sharply attenuate, and the secondary lobes,

BIOTYPES AND HYBRIDS.

15

Fig. 4.

Fig. 5.

FIG. \.-Bursa bursa-pastoris heteris. Typical specimen from offspring of plant
shown in fig. 3.

FIG. 5. — Bursa bursa-pastoris heteris. Offspring of sib of plant shown in fig. 4.

16 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

though present, were not strongly marked. The seeds were sown in the
propagating-house December 27, 1905, and produced 106 offspring, all of
which had a peculiar grayish aspect, owing- to the fact that the leaves were
freely dotted with small specks of red. The rosettes were very lax, the
relatively few leaves made a rather wide angle with the horizontal , and the
leaves, while possessing both the primary and secondary lobes, had the
latter small and angular and little more marked than serrations borne by
the primary lobe, which all experience up to the present time indicates are
of only fluctuating value. (See fig. 4.) All had simple slender stems
which came to bloom in 3 to 4 months from the time the seeds were sown
as compared with 8 months required by the above-described family of
typical B. bp. heteris.

Self-fertilized seeds of two of these plants (0515.95 and 0515.96) were
sown in May, 1906, and gave uniform progenies, but having an aspect quite
different from that of the parent generation just described. Although the
rosettes in this second generation were strongly ascending and were few-
leaved, as in the preceding generation, the texture of the leaves was much
less firm, the grayish aspect was wholly lost, owing to the absence of the
red specks, and secondary lobing was also much reduced, giving a condition
resembling the grandparent as it grew in nature (cf . figs . 3 and 5 ) . The con-
ditions in the propagating- house during the development of these families
were unsatisfactory, and their great change from the characteristics of the
preceding generation did not shake my faith in their distinctness from my
other types, since within the family there was still great uniformity, seem-
ing thus to demonstrate that whatever differences were observed were due
to the differences in environment during the time of development of these
two generations.

An average specimen from one of these two families was chosen as the
parent of a third generation. The pollination of this plant (0695.158) was
unguarded, but it was grown well separated from all other Bursa cultures.
The seeds were sown on November 1, 1906, and produced 213 plants, all
of which possessed well-marked the characteristics of B. bp. heteris , having
completely lost all the peculiarities which had led me to believe that this
pedigree belonged to a distinct biotype. (See fig. 6.) Other proofs that
040.15 was a specimen of B. bp. heteris were derived from hybrid families
formed by crossing its offspring with plants belonging to other biotypes.
Except in one case only the Fx hybrids from these crosses have been
studied and they will not be considered in detail at this time, but these first-
generation hybrids showed the characteristics which they should have pos-
sessed if typical B. bp. heteris had been used . The one F2 family (0693 . 203 )
which has been reared from these hybrids will be considered later. (See
p. 42.)

BIOTYPES AND HYBRIDS.

17

050.80: Another original culture which now seems to belong to B. bp.
heteris was the offspring of a very robust plant collected in a dooryard near
Cold Spring Harbor, Long Island, April, 1906. This plant was taken up
and potted in the greenhouse, where its pollination was guarded. The
earlier leaves of this plant were recognized as resembling B. bp. heteris, ex-
cept in the less sharp attenuation of the primary lobes, but later leaves—
the climax-leaves — were large, and, in addition to the secondary lobes
characteristic of B. bp. heteris, they had somewhat quadrangular secondary

FIG. &.—Bursa bur sa-pastoris heteris. Offspring of sib of plant shown in fig. 5.

lobes in the proximal axils of the primary lobes, and these square lobes
were sometimes almost cut off from the primary lobes, giving the leaf the
peculiar form usually described as interruptedly pinnate (fig. 7). The
seeds of this plant were sown May 23, 1906, and produced 352 offspring.
These were badly damaged by the thrips, 81 being killed and 57 so stunted
as to make an estimation of their characters uncertain. The remaining
214 formed a consistent group unlike its parent and also unlike any other

18

BURSA BURSA-PASTORIS AND BURSA HEEGERI :

of my cultures. Few showed even a suggestion of the squarish lobes
which were so conspicuous in the parent, thus indicating that the strong
development of that characteristic in the parent was in all probability
merely a fluctuation. These plants differed from those considered typical
of B. bp. heteris in having the primary lobes of the climax-leaves oblong
and blunt, not attenuate. However, in some specimens the later leaves
showed the attenuate lobes of typical B. bp. heteris, and this fact leaves
little doubt that another generation would have completely demonstrated
that this family belongs to B. bp. heteris.

Whether a culture (0645) produced from seeds sent by Dr. D. T. Mac-
Dougal, from Tucson, Arizona, is likewise identical with the B. bp. heteris
grown from seeds collected in Illinois, Ohio, and Long Island, has not
been sufficiently tested. While the Tucson plants had in a most strongly

marked way the primary and secondary lobes
described above, there were striking differences
in the texture and color of the leaves, the form
of the lobes, and the form, size, and texture of
the stem-leaves or bracts (fig. 8). As only one
generation of plants has been grown from these
seeds, no proper grounds exist for attempting
to decide as to the permanence of the differences
exhibited, but it might be expected that plants
from the hot, dry, intensely lighted plains of
Arizona would display considerable changes of
a purely transitory nature on being transferred
to the moist atmosphere and relatively dim light

of a more northern propagating-house.
JIG. 1.— Bursa bursa-pastons _. , ..
heteris. Climax leaves of a Field observation indicates that the type of

specimen growing near Cold rosette possessed by Bursa at Tucson, of which
Spring Harbor, Long Island. ... ,. £ . i • ^

this culture was a fair example, is the common

type if not the only type of rosette displayed by Bursa in that locality and
westward. Many facts now at hand suggest that the heteris type is the prim-
itive type of rosette from which the forms to be described below have been
derived, and the relatively wide geographical distribution of this type is
in strong support of this view.

Bursa bursa-pastoris tenuis n. sp. element.

This differs from the preceding type in several important features. The
sinuses are relatively shallow, rarely extending nearly to the midrib in
very strongly developed individuals. The terminal lobe is not separated
from the nearest lateral lobes by deep, clean-cut sinuses, but these more
distal sinuses are relatively shallow, so that one can with but scant pro-
priety speak of the terminal lobe as a definite morphological structure;

<J

BIOTYPES AND HYBRIDS.

19

there is no incision on either margin of the lateral lobes and hence no
rounded lobe corresponding" with what I have called the "secondary lobe "
in B. bp. keteris, though there may be a slight expansion of leaf -tissue in
that region, especially in strongly developed specimens, which no doubt
corresponds to the secondary lobe (fig. 9). All the lateral lobes tend to
be more or less slender, elongated, and acute. In the more robust speci-
mens there is apt to be some secondary lobation, but in all cases these
secondary lobes are also attenuate. A secondary spur directed outward

FIG. 8. — Bursa bursa-pastoris heteris (?) grown from seed received from

Tucson, Arizona.

and proximad from near the base of the lateral lobes is often noted in
well-developed plants belonging to this biotype (fig. 10). In many in-
stances the long, slender lobes are somewhat recurved at the tips, but in
other cases all lobes are practically straight. Under this type may belong
Almquist's C. bp. pedemontana (his fig". 17), leontodon (fig. 31), dentata
(fig. 35), lacerata (fig. 37), bergiana (fig. 38), laxa (fig. 39), querceti (fig.
40), ramselensis (fig-. 41), and linearis (fig. 58).

20

BURSA BURSA-PASTORIS AND BURSA HEEGERI

Fig. 11.

Fig. 10.

FIG. 9. — Bursa bursa-pastoris tennis from my first pure culture of this biotype.

FIG. 10. — Bursa bursa-pastoris tennis from ;i hybrid progeny.

FlG. 11. — Bursa bursa-pastoris tenuis. A stunted sib of plant shown in fig. 9.

BIOTYPES AND HYBRIDS.

Fig. 13.

21

Fig. 12.
FIG. 12. — Bursa bursa-pastoris rhomboidea. Grown in best light-relations pos.

sible in sky-lighted room. Characteristic incisions not well developed.
FIG. 13. — Bursa Bursa-pastoris rhomboidea modified by growing in dim light.

22 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

The two specimens (040.19 and 040.26) which form the basis of all my
pure-bred cultures of this biotype, germinated in vessels of earth over
which were unpacked Bursa seed which had been sent by Prof. H. F. Rob-
erts, of Manhattan, Kansas, and as the soil of one of these vessels had been
sterilized it seems almost certain that these plants came from seed sent
from Kansas. Seeds of these two plants were sown September 22, 1905,
and January 31, 1906, and tog-ether gave a progeny of 361, all but 4 of
which were referable without question to the type of the parents. These
4 specimens in family 0519 which were not quite in agreement with the rest
had the lobes comparatively broad and short, though still strongly acute,
and the margins were slightly crenulate or denticulate (fig. 11). As these
specimens were somewhat stunted, it is thought that their differences may
have been due to the causes producing the stunting, but this point has not
yet been tested.

Besides these two pure-bred families of B. bp. tennis coming presumably
from Manhattan, Kansas, this form appeared as a component of a hybrid
family received from Edge wood, New Carlisle, Ohio, and also as a mem-
ber of a hybrid family from Chicago, Illinois. These hybrid families will
be discussed later.

Bursa bursa-pastoris rhomboidea n. sp. element.

This, like B. bp. heteris, has the leaves divided to the midrib and pos-
sesses a similar, more or less rhombic terminal lobe, set off by deep sinuses
from the nearest lateral lobes. Each lateral lobe of the climax-leaves
usually shows a prominent incision on its distal margin, by which a lobe
is formed next to the rachis, corresponding to the secondary lobe of B. bp.
heteris, and in well-developed specimens there are usually one or two sim-
ilar incisions on the proximal margin. All lobes formed by these incisions
are usually obtuse or broadly angular. The terminal portion of the lateral
lobes has in the best-marked examples a nearly rhombic form, which sug-
gested the name (see figs. 18 and 20, and plates 2 and 4). When grown
under unfavorable conditions the characteristic incisions may be lost, the
prominent incisions setting off the rounded secondary lobe being the most
persistent. Almquist's C. bp. subalpina (his fig. 46), dens a (fig. 51), polyedra
(fig. 52), and perhaps two or three others may belong here.

Three pure cultures demonstrated to belong to B. bp. rhomboidea have
been reared from specimens or seeds brought in from nature, and the same
elementary species has been included as hybrids in two other original
families. Complete proof of its elementary character will appear below in
the section on hybrids.

Cultures derived from two plants growing side by side near the Brooklyn
Institute's Marine Biological Laboratory at Cold Spring Harbor, Long
Island, were thought for a time to belong to a biotype distinct from the

BIOTYPES AND HYBRIDS.

23

more characteristic specimens of B. dp. rhomboidea secured in hybrid fam-
ilies. These two individuals were growing in a situation which received
only the morning sun. The leaves were divided to the midrib, but the
oblong, obtuse lateral lobes frequently had no incisions, though in some
cases the middle lobes of the leaves had the more characteristic incision on
the distal margin and less frequently a similar incision occurred on the
proximal margin. The lobes were rather distant from each other, giving
a very unique appearance to these plants. Seeds of these two supposed
sibs were sown on June 6 and 14, 1905. The first of these (040.1) pro-
duced 65 offspring, which were studied in their relation to different condi-
tions of environment in order
to get a clue to the suscepti-
bility of Bursa to immediate
modification by variations in
the chemical and physical con-
ditions of the soil, differences
in soil-moisture, atmospheric*
humidity, intensity of light,
etc.

Specimens which were grown
under as favorable conditions
as the sky-lighted room pro-
vided were essentially identi-
cal with the parent (fig. 12).
Aside from the complete sup-
pression of lobes and long
delay of the flowering period
in dim light (figs. 13 and 14), FIG. 14. — Bursa bursa-pastoris rhomboidea. A sib

the most marked effect of en- of Plants shown in fiss- 12 and 13> and of the same

, . , age, showing the complete suppression of lobes.

vironment was noted m plants Thig p,ant wag gmwn f of fiye months in a poor]y

kept in a nearly saturated illuminated corner of the sky-lighted room,
atmosphere attained by cov-
ering with glass jars. The leaves became long, of very thin membranous
texture, crinkled and otherwise distorted, and with very shallow sinuses
(fig. 15) . Plants which had been grown in the dim light of the sky-lighted
room of the laboratory and which showed a consequent reduction of the
lobes returned to the fully lobed condition upon being removed to the better-
lighted propa.gating-house. Seeds of one of these (051.19) were sown
March 6, 1906, and produced a large progeny, only 100 of which were potted
up and studied. These were uniform throughout, but as they were stunted
by unfavorable conditions in the propagating-house it was impossible to
determine with certainty their relation to other cultures which had been
grown under better conditions.

24 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

The second of two original plants of B. bp. rhomboidea (040.8) produced
330 offspring, all but 5 of which possessed characters recognized as prop-
erly belonging" to the parental type, namely, distinct obtuse lateral lobes,
usually with an incision on the distal margin and sometimes with a similar
incision on the proximal margin (fig". 16). The 5 individuals which were
aberrant from the type of the other 325 had the essential characters of B.
bp. heteris. As will be seen later, heteris is dominant over rhomboidea, and
the simplest explanation of the presence of these 5 specimens of heteris in

FIG. 15. — Bursa bursa-pastoris rhomboidea. Grown in nearly saturated air
and moderate illumination. Leaves very thin and translucent.

a family of rhomboidea is the supposition that they represent chance crosses
between these two biotypes.

Another culture which was not at first thought to be referable to B. bp.
rhomboidea was raised from seeds of a plant (050.82) growing in a door-
yard near Cold Spring Harbor, Long Island. The parent had rather small
leaves with crowded lobes and coriaceous texture. The lobes had a strongly
marked distal incision and rarely a slight proximal incision (fig. 17) . This
plant was removed to the greenhouse April 18, 1906, and the flowers were

BIOTYPES AND HYBRIDS.

25

guarded against cross-pollination. The 150 offspring1 (06190) were some-
what diseased and stunted for a time, but subsequently developed un-
doubted characters of B. bp. rhomboidea (fig1. 18).

Bursa bursa-pastoris simplex n. sp. element.

This biotype is like B. bp. tennis in that the sinuses never reach the mid-
rib, but it differs in having mostly simple rounded or triangular acutish
lobes, not attenuate. No incisions are seen in the lobes and there is no
secondary lobing, even in the most vigorous specimens, except some slight

FIG. 16. — Bursa bursa-pastoris rhomboidea from the second original family.

denticulation (fig. 19). I am in doubt as to whether any of Almquist's
named forms can be referred to this biotype, though it is possible that his
C. bp. hauniensis (his figs. 33 and 34) is identical with my B. bp. simplex.
His C. bp. gallica (fig. 62) and several others in which lobes are almost or
quite absent might be placed here, but in most cases his figures show evi-
dence that the plants had lost their characteristic lobing through some un-
favorable cultural conditions, and if this was the cause of the reduction of
the lobes, such specimens might belong to any other biotype.

26 BURSA BURSA-PASTORIS AND BURSA HEEGERI:

Two original cultures of B. bp. simplex were grown from seeds col-
lected at Edgewood, New Carlisle, Ohio, by J. Marion Shull, June 2 to 17,

1905. In the parents of both these cultures (040.5 and 040.6) the lobes
were undivided and not elongated, though they were somewhat triangular-
acutish. One of these (040.6) had a more tapering apex than the other
and its lateral lobes were more divaricate and more acute, but their prog-
enies were generally indistinguishable from each other. Only one speci-
men among the offspring of 040.6 had the long, tapering apex of the parent,
this fact apparently showing the difference between the two parents in this
regard to be a simple fluctuation.

These two lots of seed were sown June 26, 1905. The first (040.5)
produced 170 plants which, except for some slight fluctuations, were evi-
dently of a single type. A second generation of 44 plants raised from the
seeds of an average specimen (055.24) kept- strictly to the same type.
The other original culture (056) consisted of 425 specimens, all but 5 of

which were typical B. bp. simplex. The 5
aberrant specimens belonged to B. bp. rhom-
boidea (fig. 20), whose presence in this family
was assumed to be due to chance crosses in
nature. This assumption was tested by rear-
ing a family from seeds of one of these
oso.82 (056.130). Its hybrid character was fully

demonstrated, and the results are given in

FIG, ll'-^rsa bursa-pastoris ^^ inthe gection deyoted to hybrids (p. 42).
rhomboiaea. Climax-leaves ol „

a plant growing in a dooryard Some fluctuation was observed among the

near Cold Spring Harbor, Long specimens of B. bp. simplex. A very few

Island. somewhat stunted specimens had the leaves

smaller than normal, somewhat shining, and with the lobes more crowded.

One of these (055.103) was tested. Fully guarded seeds were sown July 18,

1906, and produced 48 specimens, all but one of which were typical B. bp. sim-
plex, the one slightly aberrant specimen having a more coriaceous texture
and slightly more distant lobes than the others. It is probable that this
also represents a mere fluctuation. Other variations among the members
of the original families have not been fully tested as yet. Thus in one the
leaves were broader than in the usual form, in another the sinuses were
deep and the lobes rather long, strongly divaricate, and acutish. There
can be little doubt that these are fluctuations which would be but slightly
if at all apparent in their offspring.

BIOTYPES AND HYBRIDS. 27

A VARIABLE SERIES OF DOUBTFUL SIGNIFICANCE.

Besides the four forms named and described above, which have been
shown to breed true to type with only slight fluctuations, most of my at-
tention has been given to a series of cultures whose behavior has been up
to the present time quite baffling. Some time I hope to understand this
group better, and I shall then have more to say about it, but its behavior is in
such striking contrast to that of the biotypes already described that it
seems only fair to give a short epitome of my results as they now appear.

FIG. 18. — Bursabursa-pastorisrhomboidea grown from guarded seed
of the plant shown in fig. 17.

Two specimens (040.2 and 040.7) were taken into the sky-lighted room
from different habitats near Cold Spring Harbor, Long Island, in the very
beginning of these cultures, April 15 to 20, 1905, and allowed to ripen seed.
The aspect of these two specimens was very diverse. One (040.2) was
robust and had rather firm, thickish leaves with 4 or 5 pairs of oblong, ob-
tuse, wavy lobes, while the other (040.7) had a small rosette with thin,
flat leaves and few triangular lobes. Notwithstanding these differences,
the offspring of the two plants, as followed in numerous cultures through
several successive generations, were indistinguishable from each other.

28

BURSA BURSA-PASTORIS AND BURSA HEEGERI :

The seed of the first of these (040.2) was sown June 6, 1905, and gave
a progeny of 262 individuals not quite equal to each other and not then well
understood, because these were my first cultures of Burs a. These plants
were nearly uniform in their membranous texture, smooth surface, and
rather light-green color, but in the character of the lobation considerable
differences were noted. The most common form, and that considered in
consequence to be typical, had only obtuse lobes resembling B. bp. simplex.
Besides these obtuse-lobed specimens there were about 4 which had the
lobes decidedly acute or elongated.

FIG. 19. — Bursa bursa-pastoris simplex grown from seeds received from
Edgewood, New Carlisle, Ohio.

Seeds of the other original specimen (040.7) were sown May 8, 1905, and
produced 70 offspring, about 40 of which were obtuse-lobcd. The rest
varied through acute lobes not elongated, somewhat elongated acute lobes
with slight serration on both margins to forms with well-marked attenua-
tion of the lobes similar to those of B. bp. tennis described above. Most of
my cultures of Btirsa have been made to determine the status of the varia-
tions in the offspring of these two plants and their succeeding generations.

Before taking up the discussion of particular cultures, it may be stated
that these variations from obtuse lobes and shallow sinuses to more or less

BIOTYPES AND HYBRIDS.

29

attenuate lobes, and many other variations, have behaved in many respects
as if they were the normal fluctuations of a single biotype. While in cer-
tain cases there appeared to be a marked capacity of one or the other ex-
treme to transmit its character to its offspring1, the usual result of breeding
any one of these variations was a progeny giving again the whole range of
fluctuation, or a considerable portion of it. Time and again forms were
picked out so different from their sibs that they were thought to represent
distinct elementary species, but breeding-tests showed that their offspring
return completely to the usual condition of the other related families.

FIG. 20. — Bursa bursa-pastoris rhomboidea from a family of B. bp. simplex.
A half sib of the plant shown in fig. 19.

Over 100 pedigreed families, including more than 15,500 individuals
derived from the two plants (040.2 and 040.7), have now been studied, and
of these families considerably more than half ranged between forms with
very obtuse lobes and others having greater or less attenuation of the lobes,
the extreme developments in the latter direction being scarcely distinguish-
able from pure-bred B. bp. tenuis; 17 of the remaining families, in which no
attenuate-lobed element was noted, had been injured by too long crowding

30 BURSA BURSA-PASTORIS AND BURSA-HEEGERI :

in the seed-pans. This cause, with the ravages of thrips, in many cases
so affected the development of the plants during several months of the
summer of 1906 that it was often very doubtful just how much depend-
ence was to be placed upon the observed results. However, enough of
the families developed healthily to render it not improbable that all degrees
of variation between completely unlobed individuals and those with highly
developed attenuate lobes may be normal fluctuations of a single unit-form.

Of the healthy cultures several may be taken as examples of the pecu-
liar behavior of these plants. Several noteworthy instances were presented
in which the conditions of the parent were transmitted to the offspring
with only a narrow range of fluctuation. Thus, in the second of the two
original families from which all of these variable cultures sprang, there
were among 70 specimens 13 which had attenuate lobes. Seeds of 1 of
these attenuate-lobed specimens (057.20) were sown October 9, 1905, and
produced 681 plants, all but 1 of which showed almost uniformly strong
development of attenuate lobes. One specimen was entirely free from
lobation of any kind, but it produced no good seeds and therefore it was
impossible to test the significance of this unlobed condition. One of the
attenuate-lobed plants of this family (0520.196) produced 35 more or less
attenuate-lobed offspring, and some which were unlobed, but the latter
were so crowded in the seed-pan that stunting might be considered the
cause of the suppression of attenuate lobes. Progenies of 4 of these
stunted individuals (05196.134, 05196.136, 05196.137, and 05196.147) have
since been examined. Two of these 4 families (06136 and 06137) had
the wide range of variation usually found in the related families, and the
other 2 (06134 and 06147) contained unlobed or slightly obtuse-lobed
specimens. Since these last 2 families were badly crowded, little de-
pendence is to be placed upon this behavior.

Leaving out of account all the families which may have been injured by
crowding or otherwise, in 3 other instances the offspring were uniform
and in fair agreement with the parent. All 3 of these belonged to the
first of the 2 original families. Seeds of a well-developed obtuse-lobed
individual (052.24) sown January 16, 1906, produced 100 specimens
of uniform aspect, with only well-developed obtuse lobes. An obtuse-
lobed sib of the last (052.179) was the parent of 130 plants of the same
uniform character, having well-marked sinuses, but acutish, not elongated
lobes. One of these (05179.170) produced about 30 obtuse-lobed offspring,
but thrips injured them so much that their characterization was unsafe.
Another obtuse-lobed plant (052.193), a sib of 052.24 and 052.179 just
described, gave a progeny of 125 plants of very uniform aspect throughout
and always with well -developed sinuses and obtuse lobes.

In all these exceptional cases there seems to be a consistent behavior
in that the parental character dominates the entire progeny, but in each

BIOTYPES AND HYBRIDS. 31

such case there were sibs having1 the same external characteristics, which
behaved in an altogether different way when bred. Thus, a sib (057.25)
of the attenuate-lobed specimen (057.20) noted above as producing off-
spring- uniformly of the parental type, was, like that plant, attenuate-lobed,
but its offspring-, instead of agreeing- with the parent, consisted of 1 plant
entirely without lobation, 168 with obtuse lobes, 186 with some of the lobes
slightly elongated, and 415 with attenuate lobes. This result looks very
much like a case of Mendelian inheritance if we assume that the parent
was a DR, but it is not at all in accord with such an assumption that in
like manner obtuse-lobed sibs of the plants whose entire progenies were
characteristically obtuse-lobed as described above have produced offspring
ranging from the totally unlobed condition to the well-marked attenuate-
lobed extreme. For example, one of these obtuse-lobed plants (052.182)
whose seeds were sown February 12, 1906, produced a family of 471, of
which 337 were observed to have the following composition : 11 were wholly
unlobed, 43 were unlobed in the distal half of the leaf, but had small trian-
gular lobes in the proximal half, 89 were obtuse-lobed throughout, 102 had
some lobes slightly elongated, and 192 had some lobes strongly attenuate.

Besides a few families that were left crowded too long in the seed-pans
to allow of a satisfactory estimation of the foliar characteristics, 15 addi-
tional families were reared from parents which had attenuate lobes and all
gave uniformly the same result, namely, progenies showing the complete
range of variation from a wholly unlobed condition to the attenuate condi-
tion of the parent; 17 families from obtuse-lobed parents had the same
composition, as did also 7 families from unlobed parents.

Besides these variations, which can be easily arranged in a simple linear
series, there were noted several variations of so definite a character as to
lead to the attempt to segregate them as distinct forms. Thus one form
which appeared in several families (e. g-., 0527, 05182, etc.) was charac-
terized by a distal unlobed half of the leaf and a proximal half with tri-
angular lobes. Another group of specimens (0623) of strikingly uniform
appearance had very robust rosettes with leaves broad, obtuse, and entirely
unlobed except for the presence, occasionally, of a few shallow triangular
lobes at the very base.

If further breeding should confirm the conclusion that these families
belong to a single biotype, including within its normal range of fluctuation
individuals with obtuse lobes or no lobes at all and others with strongly
marked attenuate lobes, this form would be indistinguishable in some of
its phases from B. bp. simplex, and in other phases it would closely re-
semble B. bp. tennis. Only breeding-tests could safely distinguish these
several types, and so long as the practical taxonomist's work consists en-
tirely in the classification of individuals as they occur in nature, he would
be justified no doubt in refusing to recognize such elementary species as

32 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

of working: value, but in the taxonomy of the future they must be reckoned
with, becaiise they are the real natural entities with which all students of
biology must deal. The old name and the old delimitation of Bursa bursa-
pastoris may remain as the only thing" practicable for the amateur collector of
plants, but the morphologist, the physiologist, the ecologist, and the evolu-
tionist must be more discriminating. It appears to me that systematic
botany stands at the parting of the ways. Either it is to be left stranded
as a caterer to the amateur or it must adopt the cultural method in lieu of
the herbarium method, which has until recently held almost supreme sway
among systematists.

I am not fully convinced, however, that the variable families here de-
scribed belong to a single biotype. The fact that both extremes have in
certain cases bred true leads to the question whether there is not some way
of accounting for the anomalous behavior of the other families on the
ground of hybridization. I believe that such an explanation can be found
on the principle of latent characters which I have recently discussed else-
where (Shull, 1908). At certain times the cultures have become very
unhealthy on account of the attacks of thrips and other causes, such as
overheating during the summer, crowding in the seed-pans, etc., and the
specimens weakened in this way may very well have failed to show their
distinctive characters, owing to what I have called "latency due to fluc-
tuation," thus destroying completely the ratios by which hybridization
phenomena would have been rendered evident.

Whether these plants belong to a single biotype of wide variability or
to a hybrid group whose nature has been obscured by latency will have to
await further cultures. Lotsy (1906) also found that while some forms
bred true with very slight fluctuation, the variability of others was consid-
erable. If hybridization with latency accounts for the behavior of my
variable families it may also perhaps account for a similar condition in
Lotsy 's material. Whatever the situation may be in these highly variable
cultures, it is perfectly certain that the true-breeding forms are distinct
and elementary, as will be more fully demonstrated in the next section.

HYBRIDS BETWEEN BIOTYPES OF BURSA BURSA-PASTORIS.

Although artificially produced hybrids between the several biotypes above
described have been studied as yet only in the first generation, except in
the case of one F2 family to be described hereafter (see page 42), I have
had under observation 31 hybrid families belonging to a second or later
generations. My good fortune in being able to report on these second-
generation hybrids is due to the facts that two of the original families were
the offspring of hybrid individuals growing in nature and several other
individuals in the original cultures were obviously produced by pollination
from another biotype. The first of these natural hybrids (040.4) was col-

BIOTYPES AND HYBRIDS. 33

lected by J. Marion Shull at Edgewood, New Carlisle, Ohio, May 26-28,
1905. This specimen was robust and the lobes of the leaf were lanceolate,
with prominent dentations on both proximal and distal margins .

The seeds were sown June 26, 1905, and produced a progeny of 284
individuals, among which there was such a variety of form and aspect as
to make them at that time entirely inexplicable, because none of the unit-
differences between the forms was then known and there was no possibility
of discriminating between minor fluctuations and the distinguishing char-
acteristics of the several elementary species. Efforts were made to arrange
these plants into groups that would be strictly homogeneous, and in this
way no fewer than 25 categories were established. Even these were not
sufficient, and finally a considerable number of individuals that could not
be classed with any of these were preserved in order that they might be
studied at any subsequent time at which it should be desired to work out
their variations and their relationship to each other and to the rest of the
family.

Fortunately the distinguishing marks of each of these several groups
had been noted with sufficient thoroughness that later, when the differen-
tiating characteristics of the several elementary species were better under-
stood, it was possible to go back and reclassify the several categories in
such a manner as to determine approximately the proportions in which the
different elementary species were present. The maze of different forms
had proved so baffling at every attempt at a satisfactory classification that
the result of the redistribution of the several forms in the light of knowl-
edge subsequently gained occasioned much surprise. There were among
the 284 individuals composing the family, 114 B. bp. heteris, 47 B. dp. tennis,
53 B. bp. rhomboidea, 16 B. bp. simplex, and 54 which, because of certain
deficiencies in the original notes, could not be reclassified with certainty,
but nearly all of which were certainly fluctuations of B. bp. heteris.

It need only be assumed that of these 54 doubtful individuals 46 were
B. bp. heteris, 6 B. bp. tennis, and 2 B. bp. simplex to make the ratios of
these several elementary forms agree exactly with a frequently observed
Mendelian ratio, 9:3:3:1. Fortunately most of these doubtful specimens
were preserved in the herbarium and were thus available for study. Of 51
thus preserved, 37 were B. bp. heteris, 9 B. bp. tennis, 1 B. bp. rhomboidea,
and 4 of doubtful affinity, these latter probably belonging to the heteris
group also, but representing cases of incomplete dominance.

The ratio 9:3:3:1 is the normal one for the second generation of
typical Mendelian di-hybrids, i. e., hybrids between forms that differ from
each other in two unit-characters. All of these different elementary species
were supposed to differ from each other by single units until this family
was worked out, because in nearly all the other hybrid families the simple
ratio of 3 to 1 appeared.

34 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

As soon as it was demonstrated that there are two unit-difference s
involved in these forms it was not difficult to discover in what these two
units consist. One of them is the elongation of the primary lobes, the
other the extension of the sinuses to the rachis and the presence of rounded
secondary lobes in the distal axils of the primary lobes. If we represent
the first by A and its absence by a, and the second by B and its absence
by b, then we may distinguish the four members of the ratio as
AB : Ab : aB : ab. In the first, AB, is denoted the presence both of the
elongated primary and the rounded secondary lobe, the distinguishing
characteristics of B. bp. heteris ; in the next, Ab, the primary lobes are
attenuate, but the rounded secondary is not present, this being the char-
acter of B. bp. tennis; in the third, aB, the rounded secondary is present,
but the primary is not elongated, giving B. bp. rhomboidea; while in the
fourth, ab, both the secondary and the elongation of the primary are absent,
resulting in B. bp. simplex.

The most remarkable corollary of this composition of these forms is the
possible resolution and recomposition of the several elementary species.
Thus, a cross between pure-bred B. bp. heteris and pure-bred B. bp. sim-
plex, or a cross between pure-bred B. bp. tennis and pure-bred B. bp.
rhomboidea should again give in the second generation all four forms in the
ratio 9:3:3:1. The correctness of this prediction awaits the test of
further experimentation.

Of the families representing the third generation of this hybrid 20 have
now been studied, and the results have agreed well with the interpretation
here given of the constitution of the original family. These 20 families
are briefly described below in the order of the pedigree-numbers by which
they were designated in the cultures and original notes.

054.184: This plant was described as of the same general type as B.
bp. heteris, with long, slender primary lobes and a rounded secondary lobe
in the distal axil of the primary. The secondary lobe usually bore 1 or 2
sharp, erect lobes. Seeds of this plant were sown February 23, 1906, and
produced about 200 offspring. Owing to crowding of the propagating-house
at that season of the year, only 100 plants were potted and kept for obser-
vation, the remainder being discarded with the seed-pan. The specimens
potted up were taken from the pan in such manner as to prevent possible
selection of individuals. Of 82 specimens noted, 80 were regarded as B.
bp. heteris, and 2 as near B. bp. tennis, but many of the specimens were
stunted and it appears certain now that some specimens of B. bp. tennis
were looked upon as stunted B. bp. heteris, because the importance of the
secondary lobe was not fully appreciated at that time. That the parent
plant (054.184) was really a hybrid between B. bp. heteris and B. bp. tennis
is further indicated by the behavior of one of its offspring, which was

BIOTYPES AND HYBRIDS. 35

saved as a seed-plant under the number 05184.145, and whose progeny is
next described.

05184.145 : This plant had the climax leaves of B. bp. heteris, but in the
later rosette-leaves the rounded secondary was lost and the lobes then had
one to several rather sharp incisions on both proximal and distal margins.
Seeds of this plant were sown June 20, 1906, and gave a progeny consist-
ing of 158 B. bp. heteris and 58 B. bp. tennis, or 2.72 : 1.

054.185 : This plant was selected as nearly the equivalent of its parent
(040.4). It had the lobes about evenly tapering and equally serrated on
both margins. Its seeds were sown February 23, 1906, and gave a large
progeny, which at one time suffered greatly from unsatisfactory conditions
for growth during the summer months. Of these stunted specimens 236
were discarded as modifications of B. bp. tennis, but 50 equivalent speci-
mens transferred to larger pots showed that this determination was wholly
unsatisfactory. Well-grown specimens belonging to this family were later
shown to consist of 33 B. bp. heteris, 48 B. bp. tennis, 5 B. bp. rhomboidea,
and 14 B. bp. simplex. It is plain that the manipulation of this family was
such that nothing of value can be derived from the ratios, but it is worth
noting that this plant, which was considered the equivalent of its parent,
produced a family composed of the same four forms which had appeared in
the parental family.

054.186 : This plant had the lobes divided into rather narrow secondary
lobes by deep incisions on both distal and proximal margins. It had some
of the characters of both B. bp. heteris and of B. bp. rhomboidea, but the
resemblance to neither was very striking. The seeds were sown February
23, 1906, and produced a progeny which was judged to consist of 30 B. bp.
heteris, 22 B. bp. rhomboidea, and 45 intermediate between these two.
These intermediate specimens differed from B. bp. heteris in the prominent
incisions on the proximal margin of the primary lobes. As these interme-
diate forms pretty completely bridged the gap between the two parental
biotypes, the wide departure from the ratio 1:2:1 was doubtless only ap-
parent, being due probably to the uncertainty of the judgment in separating
the several classes. This shows that B. bp. heteris\s> not always completely
dominant over B. bp. rhomboidea, and the same fact will be noted in several
other cases.

054.190: This was a well-marked specimen of B. bp. rhomboidea. Its
seeds were sown February 28, 1906, and gave a progeny of nearly a thou-
sand. Owing to the limitations of space in the propagating-house, only
123 of these plants were potted up for observation. One of these died. Of
the remainder, all but 1 were B. bp. rhomboidea; 1 differed from the rest
of the family in the elongation of the terminal portion of the lobes, this
being recognized as the character of the heterozygous condition of the

36 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

hybrid between B. bp. heteris and B. bp. rhomboidea. Had B. dp. heteris
been a normal component of this family it would have made up a large
portion of it, instead of less than 1 per cent. As none of the earlier cul-
tures were guarded against cross-pollination, it seemed more reasonable
to consider this one specimen the result of a chance cross produced by
pollen from B. bp. heteris, which was abundantly represented in the house
at the same time. To test further the character of the normal components
of this family as well as of this one atypic individual, the following two
cultures were made :

05190.167: A typical B. bp. rhomboidea, the type of the last family,
whose seeds were sown July 4, 1906, gave 29 plants, all B. bp. rhomboidea,
showing the expected purity of this extracted form. The pollination of
this plant had been carefully guarded.

05190.178: This was the atypic plant described under the last family
but one. The pollination was guarded, and the seeds sown July 11, 1906,
resulted in a progeny which contained 29 B. bp. heteris, 29 B. bp. rhom-
boidea, and 37 intermediate between the two. The deficiency of the inter-
mediate or heterozygous class is again evidence of the difficulty of distin-
guishing between the extreme variations of these and either pure-bred
parent. The result here secured is sufficient demonstration of the simple
hybrid character assumed for the parent.

054.208 : This plant had the attenuate primary lobes rather distant and
entirely lacked rounded secondary lobes. The sinuses of the earlier climax
leaves did not nearly reach the midrib, but later leaves were cut more
deeply. Seeds of this typical B. bp. tennis were sown March 2, 1906, and
gave a progeny consisting of 24 B. bp. tennis and 1 B. bp. heteris. As B.
bp. heteris is dominant over B. bp. tennis, it seems likely that the specimen
of the former biotype in this family represents another instance of chance
crossing, since the pollination was unguarded, as in the other early cul-
tures. If this assumption is true, this family represents another instance
in which an extracted form breeds true.

054.209 : In the climax leaves this plant had the typical form of B. bp.
heteris. Later leaves of the rosette showed a nearly or quite complete sup-
pression of the rounded secondary lobe. Seeds were sown March 2, 1906,
but very few plants were secured. These were not well studied, but were
observed to be heterogeneous, though most were related to B. bp. heteris,
the parent form.

054.210 : This plant was a marked example of B. bp. tennis, remarkable
for the extreme attenuation of its lobes. The seeds were sown March 2,
1906, and of the 31 plants capable of classification, 27 were B. bp. tennis of
strongly marked type, and 4 were B. bp. heteris. Unless these 4 were

BIOTYPES AND HYBRIDS. 37

again the result of chance crosses, their appearance here seems at present
inexplicable. A small number of specimens in this family were too stunted
by unfavorable conditions to render classification possible, and some of
these may also have been B. bp. heteris, but it is probable that most of these
stunted specimens were B. bp. tenuis.

054.3 : The lobes of this plant were attenuate and rather distant, the
terminal lobe spatulate or obovate, obtusish, dentate. Seeds of this B. bp.
tenuis were sown March 3, 1906, and the 18 offspring" observed were doubt-
less all B. bp. tenuis, though there was considerable variation in the degree
of lobation, the least-lobed being stunted and not greatly different from
B. bp. simplex.

054.4: This was a robust specimen which had the earlier climax-leaves
similar to B. bp. heteris and in the later rosette-leaves had long, attenuate,
acute lobes, deeply cut on both margins. The seeds of this plant were
sown March 3, 1906, and none of the specimens were potted, but 47 were
cut from the seed-pan. It was found that classification of Bursa from the
seed-pan is very difficult and uncertain — in some cases quite impossible —
owing to crowding and consequent stunting and suppression of characters.
The specimens of this family removed from the seed-pan were classified
thus: 30 B. bp. heteris, 6 B. bp. tenuis, 4 B. bp. rhomboidea, and 7 interme-
diate between B. bp. heteris, and B. bp. rhomboidea. It is obvious that this
was a complex hybrid, and the number of specimens available was too small
for the satisfactory working out of ratios. The excessive proportion of
B. bp. heteris is easily explainable, because this form has its distinguishing
marks less easily rendered latent by bad cultural conditions.

054.6: This plant had all the lobes simple and mostly obtuse. It should
be an extracted recessive, and therefore incapable of producing any other
than its own normal characteristics. Seeds were sown March 3, 1906.
All of the plants produced seemed to be B. bp. simplex, though only 1
was potted. 120 specimens cut from the seed-pan agreed with the char-
acters of the parent.

054.7: The lobes of this plant were rather broadly compound, with all
the lobes rounded, agreeing with the characters of B. bp. rhomboidea. The
seeds were sown March 3, 1906. None of these were potted, but 43 speci-
mens cut from the seed-pan were typical B. bp. rhomboidea; some specimens
more nearly resembled B. bp. simplex, but opportunity was wanting to
test this point, and a doubt remains whether these were true B. bp. sim-
plex, or whether they were specimens of B. bp. rhomboidea in which the
characteristic incisions were wanting because of crowding in the seed-pan.

054.20: The plant to which this number was assigned had the long,
clean-cut, primary lobe and well-marked rounded secondary characteristic

38 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

of the best examples of B. bp. heteris. The seeds were sown March 6, 1906,
and produced a uniform progeny of B. bp. heteris, some fluctuation being-
noted in the denticulation of the primary lobe. None of these were potted,
but 103 specimens were cut from the seed-pan, all being of the same type
as the parent.

054.21: This was a specimen of B. bp. rhomboidea. The seeds were
sown March 6, 1906. The plants were allowed to become too crowded in
the seed-pan, and did not reach their best development. Their general
aspect was homogeneous, but examination of the lobes showed some similar
to B. bp. simplex, others more like B. bp. rhomboidea. The status of the
simplex-\\\nQ specimens in this progeny is not known, but it seems probable
that many of them were really B. bp. rhomboidea, in which the characteristic
incisions failed to develop because of the crowding of the plants.

054.25: This plant was a well-developed specimen of B. bp. tennis, with
occasional secondary spurs on the proximal margin of the primary lobes,
but with no trace of a rounded secondary lobe in the distal axils. The
seeds were sown April 18, 1906, and produced 136 young plants. When
examined in Aiigust, after my return from 2 months' absence in California ,
54 were dead. The rest were all B. bp. tennis, or modifications of it, except
3 which were B. bp. heteris. Unless the latter were the result of chance
crosses their origin is not understood.

054.26 : This specimen belonged to the most distinct type QiB.bp. heteris .
The seeds were sown April 18, 1906, and produced 173 offspring. About
20 died and the remainder belonged to B. bp. heteris, and B. bp. tennis in
the ratio 119 : 34 or 3.5 : 1. Some of the specimens classed as B. bp. heteris
had the primary lobe rather strongly incised on both margins. This is
a character frequently seen in robust specimens of B. bp. tennis, and it is
possible that this represents a slight lack of complete dominance of B. bp.
heteris over B. bp. tennis.

054.27: This plant had large, wide-spreading, rather thickish, stiff,
strongly bipinnatifid leaves, with all the lobes tapering and acute, agreeing
thus, in an essential way, with the parent (040.4). The seeds were sown
April 18, 1906, and produced about 270 offspring. These are seen now to
indicate that the parent was a di -hybrid, but at the time the family was being
studied the distinctness and the limitations of the several forms were not
sufficiently appreciated, and the notes made at that time were in the terms of
a dominant and a recessive group, the dominant group having the sinuses
extending to the rachis and the terminal lobe more evenly rounded and
more cuneate than in the recessive group, which had the sinuses much less
deep. If these distinctions were consistently made throughout, the result
should be the same as that of a simple Mendelian hybrid, since the former

BIOTYPES AND HYBRIDS. 39

group, as described, would include B. bp. heteris and B. bp. rhomboidea, while
the latter would contain B. bp. temds and B. bp. simplex, and the ratio of the
two groups should be 3 : 1. The observed ratio was 187 : 66 or 2.83 : 1, a
fair agreement with expectation. The quantitative relations of the sub-
groups can not be derived from the notes, but among~146 of the dominant
group discarded at one time, 25 were considered B. bp. rhomboidea, and the
most of the remainder were intermediate between B. bp. rhomboidea and#.
bp. heteris. According1 to theoretical considerations there should have been
36 B. bp. rhomboidea in that number. I have no doubt that this discrepancy
was due to the fact that too narrow a view was taken of the fluctuations
normal to the several recognized forms, so that some which were consid-
ered intermediate between B. bp. heteris and B. bp. rhomboidea were in
reality extracted B. bp. rhomboidea. This is even more certainly true with
respect to B. bp. heteris, for in the same group of 146 dominants, only 2 or
3 were recorded as B. bp. heteris. All those having some incisions on
the proximal margin of the primary lobe were considered intermediate,
but pure-bred B. bp. heteris has since been observed to possess these incisions
frequently as a fluctuating character. No B. bp. simplex was recognized,
as this elementary species would have been thrown with B. bp. temds with-
out question on the basis of depth of sinus and form of the terminal lobe,
which characters alone were used in the classification.

054.28 : This plant was a good specimen of B. bp. rhomboidea, having
the incisions on both proximal and distal margins of the primary lobes,
and all the lobes on the earlier climax leaves rounded or rhomboidal.
The later rosette-leaves had some of the secondary lobes acutish, but not
elongated. The seeds were sown on April 18, 1906, and 325 plants were
potted. Of 302 which were studied later, 202 were classified as B. bp.
rhomboidea and 100 as B. bp. simplex. This is a rather large departure
from the expected ratio of 3 : 1, but here again the excess of B. bp. simplex
may be due to the fact that the characteristic marks of B. bp. rhomboidea
tend to disappear under unfavorable conditions of culture, so that some
that were classed as B. bp. simplex may have been modified specimens of
B. bp. rhomboidea.

054.29 : This plant was a well-marked specimen of B. bp. heteris,
having slight denticulations on the primary lobes. It differed from the
usual habit of Bursa bursa-pastoris in having the leafy portion of the stem
absent. The seeds were sown April 18, 1906, and produced over 400
young plants. These were readily divisible into two groups — B. bp. heteris
and B. bp. tennis— -in the ratio 319 : 95 or 3.36 : 1. A few of the speci-
mens of this family which were classified with the dominant form differed
from B. bp. heteris in the lack of the rounded secondary. They were like
it, however, in that the sinuses extended completely to the midrib, thus

40 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

giving more nearly the aspect of B. bp. heteris than of B. bp. tennis. I now
suspect that these should have been classed with B. bp. tennis.

In summing- up these 20 hybrid families belonging- to the third or later
generations, it appears that 8 of the 9 possible combinations are included.
Two of the seed-plants proved to be extracted B. bp. heteris , 4 were extracted
B. bp. tennis, 2 were extracted B. bp. rhomboidea, 1 was extracted B. bp.
simplex; 3 were again like the parent in being di-hybrids which gave rise to
all 4 biotypes; 3 were B. bp. heteris X tennis, 2 were B. bp. heteris X rhom-
boidea, 1 was B. bp. rhomboidea X simplex, and the characters of 2 were in
doubt. These 2 doubtful families (067 and 0621) were either extracted B.
bp. rhomboidea or hybrids between B. bp. rhomboidea and B. bp. simplex,
the doubt being caused by the fact that stunted or juvenile specimens of
B. bp. rhomboidea may be practically indistinguishable from B. bp. simplex.
In a few families the conditions of the culture made the determination of
ratios impracticable, but in the majority the approximation to Mendelian
expectation is fairly close. Small numbers of B. bp. heteris occurred in 3
families where large numbers would have been expected if that elementary
form had been a normal component of these hybrid families, since B. bp.
heteris dominates both B. bp. tennis and B. bp. simplex, and to a slight degree
B. bp. rhomboidea also. The 8 specimens of B. bp. heteris which seemed
out of place among more than 2,600 individuals included in these fami-
lies—less than one-third of 1 per cent of the whole — were probably the
result of chance crosses, as all of these occurred in families whose polli-
nation had not been guarded.

040.14 : The second hybrid brought in from nature was collected by C.
A. Shull, in Jackson Park, Chicago, in the summer of 1905. This plant
had long, acutish lobes, serrated on both margins, and the sinuses extended
practically to the rachis. There was a very faint indication of a rounded
secondary lobe in some leaves. Seeds were sown December 27, 1905, and
produced 99 B. bp. heteris and 26 B. bp. tennis, or in the ratio 3.8 : 1. The
two components of this hybrid were at first considered distinct biotypes
until another family bearing the same characters as the dominant form
proved in the third controlled generation to be B. bp. heteris. Subsequent
breeding has demonstrated that these hybrids also present the characters
of typical B. bp. heteris and typical B. bp. tennis when grown for several
generations under favorable conditions. The peculiarities of the original
plant and of the first generation under culture are thus shown to have
been fluctuations of these two types. Eight families of the third genera-
tion have been studied, with the following results :

0514.128: This was a specimen of B. bp. tennis, the recessive form,
and as the pollination was carefully guarded, it should have been expected
to produce nothing but the parental form among the offspring. The seeds

BIOTYPES AND HYBRIDS. 41

were sown June 5, 1906, and produced a uniform progeny of 145 plants,
all like the parent.

0514.131 : This specimen was, like the last, B. bp. tenuis. Purely fer-
tilized seeds were sown June 6, 1906, and the 52 plants produced were all
alike and like the parent.

0514.146 : This was of the dominant type, B. bp. heteris. The pollina-
tion was probably not guarded, as no note was made regarding- it. The
seeds were sown June 20, 1906, and produced 444 offspring belonging to
the dominant and recessive forms in the ratio 317 : 127 or 2.5 : 1. The
dominant group appeared to belong to two distinct types, with the pri-
mary lobes broader and less sharp in the one than in the other in the ratio
110 : 32 or 3.44 : 1. The significance of the ratio thus formed by this
family, 9 : 3 : 4, is well known, as it represents the simplest modification
of the ratio for the second generation of a di-hybrid, but without further
data in support of this composition for the family, it appears advisable not
to consider its significance at this time.

0514.161 : This was a typical specimen of the recessive form, B. bp.
tenuis, of which the pollination was guarded. The seeds were sown July
4, 1906, and produced 104 specimens, all of which were like the parent.

0514.162 : This was also an extracted recessive. Guarded seeds were
sown July 4, 1906, and gave a progeny of 15 specimens, all like the parent.

0514.164: A member of the dominant group, B. bp. heteris. Purely
fertilized seeds sown July 4, 1906, produced 134 plants. The family was
badly damaged by aphis, so that 23 were killed and about 15 more so stunted
as to make classification uncertain. The remaining 93 belonged to B. bp.
heteris and B. bp. tenuis in the ratio 66 : 27 or 2.44 : 1. In this family
the dominant group was not scrutinized with sufficient care to determine
whether it was heterogeneous like 0514.146, but it was considered of a
single type.

0514.165 : This was a typical specimen of the dominant type, B. bp.
heteris. Purely fertilized seeds were sown July 4, 1906, and produced 42
specimens of B. bp. heteris and 13 of B. bp. tenuis, the ratio being 3.23 : 1.

0514.182 : Although the notes are defective regarding the character of
this plant, the result of the breeding test leaves no doubt that it was a re-
cessive. Guarded and purely fertilized seeds were probably sown in July,
1906, though no record can be found to that effect. On August 18, 1906,
the entire progeny, consisting of 158 specimens, were potted, and without
exception these belonged to B. bp. tenuis.

These 8 families belonging to the third or later hybrid generation of B.
bp. heteris X tenuis represent the extracted recessive in 5 cases and the
heterozygous form in 3. The extracted recessives have bred true to the

42 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

characters of B. bp. tennis without exception in 330 individuals. In the
offspring- of 3 heterozygous plants that have been tested, the ratios 2.44 : 1,
2.5 : 1, 3.23 : 1, and 3.44 : 1 have appeared, and these are no doubt as
near 3 : 1 as the smallness of the families should have led us to expect.

056.130: Besides these 2 original hybrid families, it will be recalled that
certain unexpected individuals of B. bp. rhomboidea in one of the original
families of B. bp. simplex (056) were supposed to be due to chance crosses
(see p. 26). Seeds of one of these (056.130) were sown June 6, 1906, and
produced 217 B. bp. rhomboidea and 72 B. bp. simplex^ that is, in the ratio of
3.01 : 1, thus showing1 the assumption that they were of hybrid origin to
be correct. (See plate 2).

In addition to the hybrids brought in from nature and their self -fertilized
offspring- as represented by the 31 families which are described above, 7
first-generation hybrid families between different biotypes of Bursa bursa-
pastoris have been produced by artificial crossing. Only one F2 family has
been reared from these to the present time.

0515.93 : The mother of this cross was a specimen belonging to the
second original family of B. bp. heteris (0515) described above (see p. 14).
This plant was castrated and pollinated with pollen from a specimen of the
first original culture of B. bp. tenuis (0519, see p. 22). The seeds were
sown May 2, 1906, and produced 222 plants, all resembling the mother and
possessing the essential features of B. bp. heteris.

0693.203 : Self -fertilized seeds of 3 guarded specimens belonging to the
last-described family were sown together October 12, 1906, and gave a
progeny of 111 specimens, 84 of which were typical B. bp. heteris and 27
B. bp. tenuis, or in the ratio 3.1 : 1. (See plate 1.)

As no other artificially-produced hybrids between biotypes of B. bursa-
pastoris have been studied beyond the first generation and only facts bear-
ing upon the question of dominance can be derived from these cultures as
yet, it seems best to postpone their discussion until the second generation
has been studied. This is the more important, since the specimens chosen
for crossing in a number of instances belonged to families whose relations
to the 4 biotypes which have been involved in the above-described natural
hybrids are still in doubt.

The occurrence of supposed hybrids among the biotypes of Bursa bursa-
pastoris has been noted by Almquist (1907, pp. 22 and 88-89), who also
refers to descriptions of similar cases by Von Borbas and by Grenier. The
actual hybrid character of the plants mentioned is very doubtful, however.
The assumption of their hybridity is based wholly upon the facts of inter-
mediacy and sterility, together with the vegetative vigor and longevity
which are certainly correlated with sterility. I have occasionally observed
such sterile or nearly sterile plants in progenies produced from seeds fully

BIOTYPES AND HYBRIDS. 43

guarded, so that hybridity was excluded with such care as is possible.
Intermediacy in the size of flowers is not at all strange in the case described
by Almquist, for the culture in which the supposed hybrids occurred was
that of a very large-flowered form. Intermediacy in such a case simply
means a reduction in the size of the flowers. In all of my hybrids there
has been no apparent decrease in the number or viability of seeds produced,
and it will be recalled that there is only one clear case of intermediacy,
namely, in the hybrids between B. bp. heteris and B. bp, rhomboidea, in
which it is due to the incomplete dominance of the former. Perhaps in
other cases also dominance is not quite complete, but it is so nearly so that
it is impossible to distinguish certainly between the heterozyg-ote and one
of its parents.

HISTORY OF BURSA HEEGERI.

Eleven years ago Professor Heeger found some specimens of a crucifer-
ous plant growing in the market-place at Landau, Germany, which he could
not identify. In general habit these plants resembled the almost cosmo-
politan species Bursa bursa-pastoris . They differed from the latter species,
however, in having the seed-capsules elliptical in longitudinal section and
circular in cross-section instead of flat and triangular or obcordate, as is
characteristic of Bursa bursa-pastoris.

The specimens were submitted to Solms-Laubach, who was inclined at
first to refer them to the genus Came Una, which is characterized by nearly
spherical capsules, and Professor Ascherson, to whom he showed them, was
of the same opinion. Cultures made by Solms-Laubach from seeds secured
from Professor Heeger soon indicated, however, the near relationship of
the new form to Bursa biirsa-pastoris , when in 1898 several apparent rever-
sions to the capsule-form of B. bursa-pastoris were noted. Solms-Laubach
(1900) published an account of the new form, assigning to it the name
Capsella heegeri, which becomes, according to the rule of priority, Bursa
heegeri (Solms-Laubach).

This very distinct species of Bursa has attracted considerable attention,
for the reason that its occurrence as a component of the flora of a region
so well known systematically has left little doubt of its very recent origin
from B. bursa-pastoris by mutation, and it is mentioned by De Vries (1901,
pp. 477-478; 1905, pp. 582-584) as an instance of mutation in nature.
Shortly after the publication of the original account, Bursa heegeri disap-
peared from the type locality at Landau, owing to the destruction of its
habitat by covering the market-place with gravel, and it has been reported
from nature only once since that time, though it has been widely grown in
botanical gardens.

The second report of the discovery of Bursa heegeri in nature was made
by Laubert (1905), who found it along the Dahlem turnpike in 1905, but

-!4 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

here the likelihood of a new origin is certainly exceeded by the probability
that a seed was carried to this spot by some agency from a nearby culture ,
for it had been grown for several years at Dahlem in an unprotected bed
several hundred meters from the place in which Laubert discovered it.
Hus (1908) takes the alternative view, however, and considers this a case
of repeated mutation.* Laubert points out that in addition to the capsule
character noted by Solms-Laubach, there are other characters of the stem
and inflorescence which serve to distinguish B. heegeri from its supposed
parent and which would suggest a more distant relationship with that form
than had been supposed; but, on the other hand, he found that in both
species there occur frequent instances of abnormal pistillate but sterile
flowers in the lower portion of the flower-stem, and he takes this fact,
together with the occurrence of capsules occasionally simulating those of
B. bursa-pastoris ', formed when B. heegeri is attacked by Albugo and Pero-
nospora, as additional proofs that B. heegeri is a derivative from B. bursa-
pastoris. Of the manner of its origination from B. bursa-pastoris nothing
is known, of course, but Potonie (1906) suggests that it is a reversion
induced by some pathological condition.

More recently Noll (1907) has investigated some plants resembling B.
heegeri, which had already been found by Melsheimer in 1882 in hundreds
in a field of Dattenberger Flur and again in 1884 on a height at Linz. Mels-
heimert considered these plants hybrids, but could not suggest the prob-
able parents, while Kornicke and Wirtgent stated that they are doubtless
identical with Bursa heegeri. Specimens of Melsheimer 's plants were
placed in Petry's herbarium bearing the label "Capsella bursa-pastoris
forma caps, ovatis. ' ' Noll received this material from Petry, together with
living specimens collected by the latter at Didenhofen, Metz, Hagendin-
gen, and Kreuznach. A careful comparison of the anatomical features of
these plants with those of Bursa heegeri and B. bursa-pastoris led Noll
to the conclusion that the Melsheimer plants are not Bursa heegeri, but
a sterile form of B. bursa-pastoris, to which he gives the name Capsella
pseudo-heegeri . The finding of these plants in considerable numbers tends
to weaken the argument that the discovery of B. heegeri in a region so well
.known proves it to be a recent mutation. Perhaps Bursa heegeri will yet
be discovered in some abundance in some locality where it has hitherto
escaped notice.

*I have called the attention of Dr. Hus to this matter and he concedes in a letter
that my explanation of the occurrence of B. heegeri in nature at Dahlem is probably the
correct one.

t Mentioned by Noll, but not verified by me.

BIOTYPES AND HYBRIDS. 45

HYBRIDS BETWEEN BURSA BURSA-PASTORIS AND
BURSA HEEGERI.

The prominent part which mutation may have taken in the production
of new species makes it of great interest to know, in each case, just what
will be the result when the supposed or the demonstrated mutant is self-
fertilized and when it is crossed with the parental form; for its behavior in
these two cases is the first important factor in determining the power of
the mutant to maintain itself at the time of its origin and its capacity to
give rise to a successful series of genetically related individuals belonging
to an independent type.

The result of self-fertilization was investigated by Solms-Laubach before
the publication of his original account, and he showed that the characters
that differentiate B. heegeri from B. bursa-pastoris are fully heritable in a
self -fertilized line. As we have seen, the Bursas normally self -fertilize to
a predominant extent, and this habit, coupled with a vigorous constitution,
would seem to constitute all the factors necessary to successful mainte-
nance. I have now determined what will be the result of intercrossing B.
heegeri and B. bursa-pastoris.

The aspects of Bursa heegeri and B. bursa-pastoris as they appeared in
my cultures were so different that at the first I was skeptical concerning
the near relationship which has been assumed to exist between them.
Bursa heegeri was much more vigorous than B. bursa-pastoris, and the dif-
ferences observed in the inflorescence by Laubert (1905) were strikingly
apparent, the pedicels of the capsules being shorter, more crowded on the
rachis, and diverging from the latter at a wider angle (fig. 21). While
the leaves of the rosette of Bursa heegeri are of the heteris type, having
the primary and the rounded secondary lobes readily distinguishable, the
sinus which sets off the latter is comparatively shallow, and in consequence
the secondary lobe appears low and less well-marked. The primary lobe
is usually broader and less strongly attenuate than in B. bursa-pastoris
heteris. Despite these considerable differences, however, it was found that
B. heegeri may be crossed with B. bursa-pastoris with perfect ease in either
direction and without any apparent decrease in fertility, though I crossed
it with B. bp. simplex, the biotype of the latter species which is most unlike
B. heegeri.

The families of pure-bred B. heegeri and its hybrids with B. bp. simplex
may be briefly considered under the pedigree-numbers used during their
culture :

040.9 : Seeds of Bursa heegeri received through Dr. D. T. MacDougal
from Professor Solms-Laubach were sown July 31, 1905, and produced
26 plants, all of which agreed with the above description, the fluctuating
variations being extremely slight. The unguarded seeds of one of these

•16

BURSA BURSA-PASTORIS AND BURSA HEEGERI :

FIG. 21. — Bursa heegeri (Solms-Laubach). From photographs taken
at three different stages of development.

BIOTYPES AND HYBRIDS. 47

(059.56) were sown November 1, 1906, and produced 24 plants, all agree-
ing" perfectly with the original type as described (fig. 22). These became
diseased later, however, and produced no seed.

056.88 : This plant was considered a typical specimen of B. bp. simplex,
though a little more vigorous and broader-leafed than usual. It was care-
fully castrated and pollinated with pollen from a plant belonging' to my
first culture of B. heegeri (059). The seeds were sown April 25, 1906, and

FIG. 22. — Bursa heegeri. Second controlled generation.

produced 108 offspring:, all resembling B. heegeri more closely than B. bp.
simplex, but they differed markedly from the former because of the imper-
fect dominance of the heteris characteristics . On this account these plants
had some of the characteristics of B. bp. rhomboidea and could be properly
described as intermediate between B. bp. rhomboidea and B. bp. heteris (fig.
23). A few of these died without seeding, but all that came to maturity
had the triangular capsules typical of Bursa bursa-pastoris . One family
was raised from unguarded seeds of one of these plants (0688.212), as
described below.

48 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

059.89 : This typical specimen of Bursa heegeri was castrated and polli-
nated with pollen from a specimen of B. dp. simplex which differed from
the usual condition of that biotype in having the apex of the leaves taper-
ing: and acutish. This condition has been shown to be merely a fluctua-
tion (see p. 26), so that the plant used in this cross is to be considered a

FIG. 23. — Bursa bursa-pastoris simplex X heegeri, Ft. Incomplete domi-
nance of the heteris characteristics produces a form resembling B. bp.
rhomboidea, but mostly with longer, sharper primary lobes.

typical specimen of B. bp. simplex. The pollen-parent of this family was
a sib of the pistil-parent of the family last described, and the two speci-
mens of B. heegeri involved in these two families were likewise sibs, so
that these two crosses were essentially reciprocal. The seeds were sown
April 25, 1906, and produced 23 offspring, which were intermediate be-
tween B. bp. heteris and B. bp. rhomboidea as to the rosette characters, and
in which they were indistinguishable from the reciprocal hybrid family,

BIOTYPES AND HYBRIDS. 49

0688. Most of these plants produced fruit, and in all cases this was flat
and triangular to obcordate, like that of B. burs a-pas torts . Unguarded
seeds of 3 of these and guarded seeds of 1 were used for the production of
second-generation families, 1 as 0689.196 and 3 collected together as
0689.197. The descriptions of the second-generation families follow :

0689.196 : Seeds of this plant, which had been carefully guarded against
cross-pollination, were sown October 12, 1906, and 217 plants were raised,
of which 188 lived to produce seed. These had the following composition:
98 were B. bursa-pastoris heteris, 36 B. dp. tennis, 32 B. bp. rhomboidea, 13
B. bp. simplex, 5 B. heegeri heteris, 1 B . h. tennis, 2 B. h. rhomboidea, and
1 B. h. simplex, giving, so far as the rosette -characters are concerned, a
very close agreement with the ratio 9:3:3:1, but in the form of capsule
showing a very great preponderance of the btirsa-pas torts type.

0689.197: Seeds of 3 unguarded sibs of the parent of the last family
were sown October 15, 1906, under this number. Of 2,014 offspring, 1,815
came to maturity, and were recorded as having the following composition :
1,032 were B. bursa-pastoris heteris, 331 B. bp. tennis, 302 B. bp. rhomboidea,
78 B. bp. simplex, 45 B. heegeri heteris, 13 B. h. temiis, 13 B. h. rhomboidea,
and 1 B. h. simplex (plates 3 and 4). Again there is a close agreement in
the leaf-characters with the typical dihybrid ratio, 9:3:3:1, and a notable
deficiency in the occurrence of the heegeri type of capsule.

0688.212 : Seeds of this unguarded plant, which was a reciprocal of the
parents of the last two families described, were sown October 12, 1906, and
produced a large progeny, of which 621 unselected plants were potted for
study and the rest discarded. Of these 621 plants, 537 reached maturity
and were classified thus : 317 B. bursa-pastoris heteris, 102 B. bp. tennis,
67 B. bp. rhomboidea, 21 B. bp. simplex, 19 B. heegeri heteris, 7 B. h. tennis,
and 4 B. h. rhomboidea, noB. h. stmp/exbeing observed. The same general
relations of the rosette-characters and capsule-characters are obvious here
as appeared in the other two F2 families described, but there is not quite
as close agreement with the ratio 9 : 3 : 3 : 1 as in the other families, prob-
ably because this family became somewhat diseased and the distinguishing
of the several types of rosette became consequently more difficult.

Reviewing the results of crossing- Bursa bursa-pastoris simplex and B.
heegeri, it is seen that the Fj hybrids are essentially uniform, no matter in
which direction the cross is made, and that the rosette in either case is of
the heteris-rhomboidea type, owing- to the incomplete dominance of heteris,
while the capsule is always of the bursa-pastoris type. In F2 there appear
the 4 types of rosette already described, in combination with each type
of capsule. The rosette presented many instances of the best- developed
examples of the 4 described forms, particularly interesting being the fact

50

BURSA BURSA-PASTORIS AND BURSA HEEGERI :

that much better-developed heteris rosettes occur in the F2 than are seen in
pure-bred B. heegeri, though these heteris characteristics must have come
directly from the heegeri side of the cross.

The numerical results of these crosses may be tabulated thus :

Bursa bp. simplex (abC) X Bur so. h. heteris (A Be}

(056) (059)

Bursa bp. heteris-rkomboidea (ABCabc)
(0688 and 0689)

06196

06197

06212

Total.

Expected.

Bursa-pastoris series (C):
heteris

08

10^2

•U7

1447

1-168

tenuis

36

•J-I I

I O2

460

4^6

rhomboidea

12

7O2

67

4OI

4c6

simplex

11

~> ~
78

21

1 12

I f>2

Heegeri series (c):
heteris

C

«

IQ

6q

6l

tenuis

I

11

7

21

2 I

rhomboidea

2

11

A

IQ

2 I

I

I

o

2

7

C • c

IQ.Q : i

24.2 : i

16.0 : i

2 1 .o : i

•5.O ' I

In this table the pedigree-numbers 06196 and 06197 represent families
in which B. heegeri was the mother, while 06212 is the reciprocal cross.
It will be seen that there is no essential difference between these two
crosses. In the rosette-characters these numbers are very close to the
Mendelian ratios, among both those having bursa-pastoris capsules and
those having heegeri capsules, showing by these facts that the heegeri
rosette has the same allelomorphic composition as the rosette of B. bursa-
pastoris heteris, and that these rosette-characters are not coupled in any
but a chance way with the form of the capsule. This independence of
characters is thus a matter of great importance in the production of new
elementary forms ; for before this cross was made there existed, so far as
is known, but one elementary species of B. heegeri, while out of the cross
came 4 distinct elementary forms of this species.

While there is a perfect agreement with Mendelian ratios in rosette-
characters, the capsules give a surprising departure. The capsule-form is
perfectly alternative; there has never appeared any intermediate condition
in the plants in my cultures, and the "reversions" observed by Solms-
Laubach and Laubert were pathological. As the bursa-pastoris type is
dominant, simple Mendelian expectation would require the appearance of
1 B. heegeri in every 4 F2 individuals. Out of 2,540 plants of the F2 gen-
eration observed, only 111 were B. heegeri, or approximately 1 in 23. That
this result should be consistently given in 3 different pedigrees represent-
ing reciprocal crosses adds greatly to the weight that is to be attached to

BIOTYPES AND HYBRIDS. 51

it. The greatest frequency in any pedigree was 1 in nearly 18, and the
least frequency was 1 in 25.*

I take the fact that the reciprocal crosses give similar ratios to indicate
that the heegeri type of capsule is dependent upon something carried by the
germ-cells, and can not be a pathological condition transmissible from
mother to offspring somatically, a possibility which might account for
apparent heritability of characters in a self -fertilized line, but could not
well account for equal results in reciprocal crosses. Normal Mendelian
phenomena are believed to rest pretty securely on the method of formation
of the chromosomes during the reduction division, but no scheme of
behavior occurs to me which would result in the production of a heegeri
homozygote in only 1 individual in 23.

It is conceivable that the union of heegeri germ-cells in the hybrids forms
a less successful combination than that into which the B. burs a-pas torts
determiner enters, and that therefore fewer successful zygotes are formed
by such unions, and it is also conceivable that only a small percentage of
the B. heegeri succeeded in reaching sexual maturity, since in each of these
families a considerable number of individuals failed to fruit, but the assump-
tion that every individual that failed to seed was a B. heegeri would not
nearly bring that form up to one -fourth of the entire progeny. There was
no evidence in my cultures that B. heegeri is in any way inferior to B.
bursa-pastoris or that it is any less likely to mature; neither have I observed
any indication of the material lessening of fertility which would obtain if
almost all of the heegeri homozygotes should fail in the initial stages of
development.

*Dr. Correns tells me that he also made the cross between Bursa bursa-pastoris and
B. heegeri several years ago and likewise found a deficiency in the number of specimens
having the heegeri type of capsule, but as his numbers were small he considered the
deficiency due alone to the inadequate numbers.

52 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

DISCUSSION OF RESULTS.

The occurrence of elementary species within the limits of recognized
systematic or Linnean species is undoubtedly very general, as De Vries
has maintained, and much of the ordinary conception as to the variability
of certain species is attributable to this fact. Thus in the specific case with
which we are dealing- here, Bursa is recognized by all taxonomists as
exceedingly variable, but each single biotype of Bursa is much less vari-
able than the Linnean species taken as a whole, for when grown under
favorable conditions there are certain characteristics which are found in
every individual of the given biotype which are not present in the members
of other biotypes.

All the apparent evidence for the permanent change of species through
selection finds a ready explanation on the assumption that the selection
has merely eliminated certain biotypes from the original mixture with
which the selection started, thus leaving the theory that fluctuations are
inherited or are capable of leading either directly or indirectly to the modi-
fication of any biotype wholly unsupported. This is not to say, of course,
that such modification is impossible or that it does not take place, but merely
that such a proposition must rest upon experience still to be gained.

There is some variation among the members of the single biotype. This
variation is of the fluctuating kind by definition. The most usual varia-
tions of this kind are those which result from crowding, shading, poor soil,
drought, the attacks of insects, or other conditions which decrease the vigor
of the plants, the effect being to arrest differentiation in more or less juve-
nile stages of development. This feature has been very troublesome at
times in my cultures, since it is impossible under such circumstances to
determine by inspection to which biotype a given specimen belongs. The
breeding-test is the only method by which such a determination can be
made and when many specimens have their distinguishing characteristics
rendered latent in this way the labor, time, and patience required for com-
plete classification become unduly increased.

This suppression of characteristics through fluctuation also stands in the
way of the classification of specimens observed in nature in various habi-
tats where they have grown under different conditions which are in a large
degree unknown, and on this account there can be no question as to the
advisability of retaining the Linnean names in practical taxonomy for the
designation of such complex groups of biotypes. Workers in all other
botanical fields must bear in mind, however, that conclusions reached with
one biotype may not hold in some other biotype of the same Linnean species.

The demonstration of the elementary character of these biotypes of Bursa
is made complete by the fact that they Mendelize on being crossed, for such

BIOTYPES AND HYBRIDS. 53

behavior can only rest upon the presence of an internal factor, of whatever
nature, capable of independent movement at the time of the reduction-
division in the formation of the germ-cells. It may be assumed, perhaps,
that this factor or determiner is incapable of division at the time of
the reduction-division ; but however it is explained, the result is the pro-
duction of certain individuals (homozygotes) which produce germ-cells all
of one kind with respect to a particular character and which must there-
fore breed true with respect to that character, and other individuals (het-
erozygotes) which produce germ-cells of two kinds with respect to the same
character and which consequently can not breed true. Individuals which
possess the same characteristics and which are homozygous with respect
to all of these characteristics, collectively form a biotype ; hence the state-
ment that the Mendelian inheritance of the several forms of Bursa demon-
strate beyond a possible question that they are distinct biotypes. I will
not be understood, however, to imply that biotypes must Mendelize on
being crossed, for many are known which do not, as, for example, most of
the various biotypes of Oenothera.

The importance of Mendel's discoveries for our conception of the signifi-
cance of different kinds of variations in the evolution of plants and animals
is now generally recognized. It is seen that in the presence of Mendelian
inheritance the ' ' swamping " of a new character by crossing with the
parent form does not take place, and that the very kind of variation which
was supposed to be swamped by crossing is just the kind which is most
certain to be preserved. The new form may completely disappear when
crossed, but it is only hidden momentarily, not destroyed. Thus when
Bursa heegeri crosses with its parent Bursa bursa-pastoris , all of the offspring
are indistinguishable from the latter, but in the next generation a portion
of the offspring are just as typical B. heegeri as the original specimen, and
no transitional forms occur. If the dominance of the triangular capsules
were incomplete there would be some transitional stages, but this would
not modify the situation in any essential manner, as there would still be
just as many typical homozygotes as before which would breed true when-
ever they chanced to be fertilized by their own kind, and if fertilized again
by the parent form they would form heterozygotes which would produce
as large proportion of typical B. heegeri as was produced by the heterozy-
gotes of the previous generations.

I have shown elsewhere (Shull, 1907 a) that this capacity of a mutant
to disappear upon crossing with its parent may be a great advantage in the
struggle for existence under certain conditions, and if we grant that evolu-
tion is in the main retrogressive (Shull, 1907^), the new forms will be
generally recessive to the parent and will thus be in position to profit by
whatever advantage recessiveness gives. From the data I have as to the
geographic distribution of the several biotypes of B. bursa-pastoris it

54 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

appears that B. bp. heteris is much more generally distributed than any of
the others, and this favors the view that this is the original type from which
B. bp. rhomboidea, tenuis, and simplex were derived by retrogressive muta-
tion. Recessiveness of the last three to the first is probably rarely if ever
any advantage to them, however, since all of these forms appear to be
about equally adapted to the range of habitats in which they grow.

Mendelian inheritance also has an important bearing upon the distribu-
tion of the various biotypes of a species; for the transportation of a single
hybrid seed may carry all of the biotypes which are related to one another
in 'the Mendelian way, however many there may be,* and two pure-bred
seeds landed in the same vicinity may lead to the same result. Thus any
seed of B. bp. heteris which has been produced by pollination with pollen
from B. bp. simplex, or vice versa, or any seed of B. bp. rhomboidea that
has resulted from pollination with pollen from B. bp. tennis, or vice versa,
will give rise to a progeny in the F2 which will include all four of these
biotypes, and self -evidently two pure-bred seeds representing either of
these two pairs of elementary species will carry the capacity to produce the
same four types in the third generation from the time they find themselves
in juxtaposition in a new locality.

The same principle would hold if there were 3 pairs of Mendelian char-
acters involved, but then 8 biotypes might be carried by a single hybrid seed.
This situation would be realized by the material dealt with in this paper
if a single seed of B. bp. simplex pollinated by B. heegeri, or vice versa ,
were taken to a new locality. Transeau (1907) has pointed out how on
the same principle any number of biotypes might be introduced by means
of a single pollen grain into a new locality where a single biotype had
existed before, and to which heavy seeds might find much greater diffi-
culty in being transported.

Still another important effect of Mendelian inheritance in the promotion
of organic evolution is brought out by my crosses between B. burs a-pas torts
and B. heegeri, namely, the production of parallel series of biotypes in
nearly related species. Up to the time this cross was made B. heegeri was
known only in the heteris form, but among the hybrid offspring were 4
distinct pure-breeding biotypes of B. heegeri. It is thus seen that the
single mutation by which B. heegeri originated from B. bursa-pastoris
doubled the number of possible biotypes of Bursa in the world, provided
all such other biotypes behave as do the four under discussion in this paper.

It may be added that the facts here shown that the rosette of B. heegeri
represents the same Mendelian units that are present in B. bursa-pastoris
and that there is only a single fundamental difference between these two
species is the best possible proof of the origin of B. heegeri from B. bursa-

*Except in the presence of "spurious allelomorphism."

BIOTYPES AND HYBRIDS. 55

pastoris, despite the considerable differences of general aspect both of foliage
and of inflorescence. It is possible that these differences of aspect may
rest upon the presence of still other units which have not been taken into
account in these studies, but if such should prove to be the case, the gen-
eral conclusions must be the same.

I wish to acknowledge my appreciation of the facilities which have been
placed at my disposal by the Station for Experimental Evolution of the
Carnegie Institution of Washington, and of the faithfulness of my assistants,
who have greatly aided me with the technical side of this investigation.
My thanks are due to my brother, J. Marion Shull, dendrological artist of.
the United States Forest Service, for the drawings reproduced in figs. 1
to 23. I particularly desire to acknowledge my indebtedness to Dr. E. N.
Transeau, in whose charge the cultures of B. bursa-paslbris X heegeri were
left during my absence in California in the spring of 1907. Much of
the work of grouping the F2 into the appropriate classes was done by him.

SUMMARY.

(1) Bursa {Capsella) bursa-pastoris is a composite species made up of
at least 4 and possibly many distinct elementary species or biotypes ;
4 of these are described under the names Bursa bursa-pastoris heteris,
B. bp. tennis, B. bp. rhomboidea, and B. bp. simplex. Except for the sup-
pression of characteristics due to bad treatment, the fluctuations of these
forms are slight.

(2) These 4 biotypes cross readily, giving in each case a uniform
F! and typical Mendelian splitting in F2 . They are differentiated from
each other by 2 unit-characters, namely, elongated primary lobes of the
climax-leaves and a rounded secondary lobe in the distal axil of the primary
lobes. When B. bp. heteris is crossed with B. bp. simplex, and when B.
bp. tennis is crossed with B. bp. rhomboidea, the Ft is intermediate between
B. bp. heteris and B. bp. rhomboidea because of the imperfect dominance of
B. bp. heteris, and the F2 consists of heteris, tennis, rhomboidea, and simplex
in the ratio 9:3:3:1.

(3) The sterile or nearly sterile plants supposed by Almquist to be
hybrids were probably not hybrids, as similar forms were noted in guarded
cultures, and in all of my hybrids no decrease in fertility was apparent.

(4) Bursa heegeri differs from B. bursa-pastoris in the aspect of its
rosette and inflorescence, but most notably in the form of the capsule. Its
climax-leaves are of the same general type as those of B. bp. heteris.

(5) Bursa heegeri may be readily crossed with B. bursa-pastoris. When
crossed with B. bp. simplex, an F! is obtained which more nearly resem-
bles B. heegeri in rosette characters, but has the capsule-form of B. bursa-
pastoris. In F2 there appear the 4 forms of rosette already mentioned, in

56 BURSA BURSA-PASTORIS AND BURSA HEEGERI :

the ratio 9:3:3:1, in association with each type of capsule. The ratio
of the biirsa-pastoris type of capsule to the heegeri type is about 22 : 1.

(6) Bursa heegeri possesses the same unit-characteristics in the rosette
as B. bp. heteris, which serves to further confirm its direct derivation from
that species.

(7) The fact that the various forms of Bursa show Mendelian inherit-
ance on crossing" is conclusive proof that they are distinct biotypes, not-
withstanding the fact that their distinguishing" characteristics are readily
rendered latent by fluctuation.

(8) In the presence of Mendelian inheritance the swamping of a new
characteristic does not take place, and the kind of variation which has been
supposed to be swamped by crossing is just the kind that is most certain to
be preserved.

(9) The recessiveness of a newly arisen form is to be considered an
advantage in its struggle with the parent, whenever the former is in any
way less adapted to its environment than the latter. This principle appar-
ently has no bearing upon these biotypes of jBursa, however, as all appear
to be equally adapted to the range of habitats in which they live.

(10) Bursa bursa-pastoris heteris appears to have a more general distri-
bution than any of the other forms. This is probably the primitive type
from which the other biotypes have been derived by retrogressive mutation.

(11) Mendelian inheritance also assists in the distribution of the vari-
ous biotypes, since a single hybrid seed or two pure-bred seeds may carry
into a new locality as many distinct biotypes as are related to each other
by Mendelian characters. A single pollen-grain may do the same provided
one biotype is already present in the locality in question.

(12) Crosses between nearly related species may give rise to parallel
series of biotypes in the two species concerned. Before my crosses between
Bursa heegeri and B. bursa-pastoris were made, the former was known only
in 1 form, but out of the crosses came 4 distinct biotypes of B. heegeri,
corresponding with the 4 biotypes which have been demonstrated in B.
bursa-pastoris.

(13) Each mutation which results in the appearance of a new Mende-
lian unit-character doubles the possible number of Mendelian biotypes
belonging to the species in question, however numerous they may be
already, except as limited by "spurious allelomorphism."

STATION FOR EXPERIMENTAL EVOLUTION, July 11, 1908.

BIOTYPES AND HYBRIDS. 57

LITERATURE CITED.

ALMQUIST, E.

1907. Studien iiber die Capsella bursapastoris (L.). Acta Horti Bergiani, 4 :

No. 6, pp. 92, figs 66, May 15, 1907.
DE VRIES, H.

1901. Die Mutationstheorie. I. Die Entstehung der Arten durch Mutation, pp.

xii + 648, pis. 8, figs. 181, 1901. Leipzig. (See pp. 477-478.)
1905. Species and varieties: their origin by mutation, pp. xviii +847, 1905.

Chicago. (See pp. 582-584.)
Hus, H.

1908. Fasciations of known causation. Amer. Nat., 42: 81-97, 2 figs., Feb., 1908.
LAUBERT, R.

1905. Notizen iiber Capsella heegeri Solms. Verh. Bot. Vereins Provinz Branden-

burg, 47: 197-199, 4 figs., 1905.
LOTSY, J. P.

1906. Vorlesungen iiber Deszendenztheorien mit besonderer Beriicksichtigung der

botanischen Seite der Frage. Erster Teil, pp. 384, pis. 2, figs. 124, 1906,
Jena. (See pp. 179-181.)
NOLL, F.

1907. Ueber eine Heegeri-ahnliche Form der Capsella bursa-pastoris Moench. Sitz-

ungsber. Niederrhein. Gesells. Nat.-u. Heilkunde, 5 pp., 1907, Bonn.
POTONIE', H.

1906. Capsella heegeri eine pathologische Erscheinung mit atavistischen Momen-

ten? Naturwis. Wochenschr., 21 (n. s., 5): 788-791, 2 figs., Dec. 9, 1906.
SHULL, G. H.

1907*2. Elementary species and hybrids of Bursa. Science, n. s., 25: 590-591, Apr.

12, 1907.
1907^. The significance of latent characters. Science, n. s., 25: 792-794, May 17,

1907.

1908. A new Mendelian ratio and several types of latency. Amer. Nat., 42: 433-

451, July, 1908.

SOLMS-LAUBACH, H.

1900. Cruciferienstudien. I. Capsella heegeri Solms eine neuentstandene Form

der deutschen Flora. Bot. Zeit., 58: 167-190, pi. vii, 1900.
TRANSEAU, E. N.

1907. Hybridization a factor in migration and competition. Science, n. s., 25:

269-270, Feb. 15, 1907.

Shull

Plate I

06203

Fig. 1. — Bursa bursa-pastoris heteris. Dominant form from the F$ of an artificial cross of B. bp.

heteris X tenuis. The grandparents were sibs of the plants shown in text-figures 4 and 9.
Fig. 2. — Bursa bursa-pastoris tenuis. Recessive form from the same family as the plant shown in

figure 1.

Shull

Plate 2

06130

06130

Fig. 1. — Bursa bursa-pastoris rhomboidea. Dominant form in the p2 of a hybrid family repre-
senting the natural cross, B. bp. simplex X rhomboidea. The parent was a sib of the plant
shown in text-figure 20 and had the same characteristics.

Fig. 2. — Bursa bursa-pastoris simplex. Recessive form in the same hybrid family whose dominant
form is represented by figure 1.

Shull

Plate 3

06197

06197

Fig. 1. Bursa heegeri X bursa-pastoris simplex. A heteris rosette in the F2.
Fig. 2. Bursa heegeri X bursa-pastoris simplex. A tenuis rosette in the F2.

Shull

Plate 4

06197

06197

Fig. 1. Bursa heegeri X bursa-pastoris simplex. A rhomboidea rosette in the
Fig. 2. Bursa heegeri X bursa-pastoris simplex. A simplex rosette in the F2.

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