Defective inheritance-ratios in Bursa hybrids

QH

Defective inheritance-ratio
| in Bursa hybrids.

by
Dr. George H./Shull.

IC-NRLF

B 3 IDfl 357

GIFT OF
Author

* L.tBKAKY-AG«'CUCTU

,

MA4N

DEPT

Defective inheritance-ratios in Bursa hybrids.

By Dr. George Harrison Shull'r

Station for Experimental Evolution, Cold Spring Barber, J

New York.

(Sonderabdruck aus dem XLJX. Bande der Verhandlungeii des naturforscheoden Vereines in Brunn.)

Several years ago (1905 — 1907) I was making extensive
[cultures of .Bwrsa (Capsella) bursa-pastoris, of Bursa Heegeri, and
of hybrids between these two species. Bursa Heegeri is generally
conceded to have originated from B. bursa-pastoris by a recent
mutation and has been found in nature only once in a situation
rhich would warrant the belief that it had not been derived
from a near - by experimental culture. Although a number of
[specimens were found by Professor H e e g e r at the original locality
[on the market-place at Landau, Germany, in 1897, so far as is
low known these represented a single elementary form or biotype .
Sfll bursa-pastoris, on the other hand, is of almost world- wide
distribution, and presents an unknown number, but certainly a
|large number, of distinct biotypes.

In my cultures of the latter species I found four forms of
-osette which were related to one another as the four terms of
Mendelian di-hybrid. These four biotypes were named and
| described as follows:1)

Type (a). Bursa bursa-pastoris heieris has the leaves divided
to the mid-rib, the terminal lobe being usually separated from
[the nearest lateral lobes by clean, deep incisions. The lateral
lobes consist essentially of two features, an elongated proximal
[portion, the „ primary lobe", and a more or less rounded or

J) ^Results of crossing Bursa bursa-pastoris and Bursa Heegeriu.
pp. Proceedings Seventh International Zoological Congress, Boston Meeting,
LUgust 19—24 1907. nAdvanced reprint" issued in 1910.

nBursa bursa-pastoris and Bursa Heegeri: Biotypes and hybrids",
pp., 23 text^figg., 4 pis., Publ. No. 112, Carnegie Institution of
[Washington, 1909.

Yerbandlungen des natarf. Vereines in Brunn. XUX. Band. *

646272

angular portion, the „ secondary lobe", in the distal axil of the
primary lobe.

Type (b). Bursa bursa-pastoris rhomboidea has the leaves
divided to the mid-rib as in B. bp. heteris. The lateral lobes have
an incision on the distal margin setting off the secondary lobe
from an; utoeldngated primary lobe. There is usually a corre-
sponding incision «9n the proximal margin of the primary lobe.
In Jke'« test jl#y«loped specimens these incisions set off a small
terminal portion of each lateral lobe, which is rather blunt or
angular at the apex, being generally of rhomboidal form. Less
perfectly developed specimens have the incisions very shallow or
nearly wanting, but retain the characteristic deep sinuses extending
to the mid-rib.

Type (c). Bursa bursa-pastoris tenuis differs from both (a)
and (b) in that the sinuses do not usually reach the mid-rib.
The terminal lobe is not separated from the nearest lateral lobes
by deep, clean-cut sinuses, these more distal sinuses being rela-
tively shallow, so that one can with but scant propriety speak
of the terminal lobe as a definite morphological structure. All the
lateral lobes tend to be elongated and sharp, and no incisions
are present to set off a secondary lobe, though in particularly
vigorous specimens there may be a slight expansion of leaf-tissue
in the region occupied by the secondary lobe in B. bp. heteris
and B. bp. rhomboidea.

Type (d). Bursa bursa-pastoris simplex, like B. bp. tenuis is
scarcely ever divided nearly to the mid-rib, and the lateral lobes
are mostly obtuse, sometimes more or less acute, but never long
and attenuated. No secondary lobing appears except occasionally
a slight denticulation on the margins.

These descriptions refer to the characters of the climax-
leaves in well-grown specimens, as all of these types have juve-
nile leaves entirely unlobed and indistinguishable from the juve-
nile leaves of the other forhis, and the late rosette-leaves and
stem-leaves likewise generally lack distinctive features. Under
poor cultural conditions plants frequently complete their development,
flower, and ripen their seeds, without exhibiting any but these
juvenile and senescent characters. The complete procession of
leaf-forms in typical specimens of the four described biotypes are
shown in the plates I — IV.

The production of a considerable number of hybrid families
representing the various possible combinations of these four
biotypes has demonstrated that their gametic formulae may be aptly
represented by the conventional Mendelian symbols, as follows :

B. bp. heteris . . AE

B. bp. rhomboidea . aB

B. bp. tennis . . Ab

B. bp. simplex . . ab

By comparing these formulae with the descriptions of the differ-
ent forms it will be seen that A is responsible for the long,
sharp character of the primary lobe in B. bp. heteris, and the
attenuation of the lobes in B. bp. tenuis, while B produces the
division of the leaf to the mid-rib and the resultant definiteness
of the terminal lobe in both B. bp. heteris and B. bp. rhomboidea,
the rounded secondary lobe of B. bp. heteris, and the proximal
and distal incisions of B. bp. rhomboidea.

Bursa Heegeri has the totem-form of rosette, *'. e. with the
gametic formula AB, but differs so fundamentally from Bursa
bursa-pastons in the characters of the capsules, that if its rela-
tionship to the latter species were not so obvious on other
grounds, its capsules would cause it to be placed unquestionably
in a separate genus. The flat, triangular or cordate form of the
bursa-pastoris capsule is too familiar to need description. The
spur-like valves are firm, and fall readily at the slightest touch
when the seeds are ripe. The capsules of B. Heegeri are of oval
form, surmounted by the short, strong, persistent style, and the
valves are thin membranous, not the least inflated, and do not
fall when the seeds are mature. The seeds are liberated by an
irregular break in the central region of each valve.

When this original genotype of B. Heegeri was crossed
reciprocally with B. bursa-pastoris simplex, the rosette-characters
segregated in the F2, into the four types described above, but
the Heegeri-tjpe of capsule appeared in only 111 specimens
among 2540. The distribution of the several characters in the
F2 are shown in the following table :

Table 1.

Bursa bp. simplex (abC) X Bursa H. heteris (ABc}

Fed. No. 056.64

Fed. No. 059.

Bursa bp. heteris (ABC)

Fed. No. 0564.88

Fed. No. 059.89

Bursa-pastoris Series (C)

Heegeri Series (c)

III!

1 1

Fed. No.

heteris rhomboidea tennis simplex

heteris rhomboidea tennis simplex

C: c

06196

98 32 36 13

5

2 1 1

19.9 : 1

06197

1032 302 331 78

45 13 13 1

24.2 : 1

06212

317 67 102 21

19

470

16.9 : 1

Total

1447 : 401 : 469 : 112

69 : 19 : 21 : 2

21.9:1

Expected

1368 : 456 : 456 : 152

63 : 21 : 21 : 7

3.0:1

The deficiencies seen to be consistently present in all these
pedigrees in the number of rhomboidea and simplex were pro-
bably due at least in part to an error of classification, those rhom-
boideas having the greatest elongation of the terminal segment
of the lobes having doubtless permitted their classification as
minus-fluctuations of heteris, and the sharpest-lobed simplex as
minus-fluctuations of tenuis. The greatest surprise was occasioned
by a ratio of about 22 : 1 in the form of the capsules, as it was
thought probable that the difference between the two forms
would be found to be dependent upon the presence and absence
of a single gene, the expected ratio on this assumption being 3:1.

At the close of the second generation the cultures were
necessarily discontinued, and could not be resumed until in the
autumn of 1910. The appearance of a paper by Nilsson-Ehle 1),
showing that certain characters of wheat and oats are independ-
ently determined by two or more distinct units or genes, gave
the suggestion that the capsule-character of B. bur sa- pastor is
might be determined in like manner by two genes, the absence
of both of which produces the Heegeri-type of capsule, although the
observed ratio 22 : 1 is a bad approximation to the expected ratio

*) Nilsson-Ehle, H., Kreuzungsuntersuchungen an Hafer und Weizen.
pp. 122. 1909. Lund: Hakan Ohlssons Buchdruckerei.

15 : 1, considering the number of individuals involved in the F2
cultures and the consistent results yielded by three different
pedigrees involving reciprocal crosses. After the pedigrees were
all arranged in the spring of 1910 for the testing of this hypoth-
esis, Baur J) suggested the same possible explanation in a
review of one of my papers.

The demonstration of two independent genes for the deter-
mination of the same external character is to be found in the
composition of the F3 families grown from self- fertilized F2 individ-
uals possessing the dominant character in question, — in this
instance the bursa-pastoris type of capsule. Letting the two sup-
posed genes for the triangular capsule be represented by the
symbols Cc and Dd, the gametic composition in F2 and the
expectation in the F3 is indicated in the following table:

Table II.

Number of r ® *™£L Expected

F2 plants in 16. C^he F2 Re8U^8 in F*

1 CDCD All bursa-pastoris.

2 CDCd All bursa-pastoris.
2 CDcD All bursa-pastoris.

4 CDcd Bursa-pastoris and Heegeri, 15 : 1.

1 CdCd All bursa-pastoris.

1 cDcD All bursa-pastoris.

2 Cdcd Bursa-pastoris and Heegeri, 3:1.
2 cDcd Bursa-pastoris and Heegeri, 3:1.
1 cdcd All Heegeri.

The facts shown in this table may be summarized in the
statement that among fifteen families raised from F2 plants
having the bursa-pastoris type of capsule, there will be on the
average seven (1 CDCD + 2 CDCd + 2 CDcD f 1 CdCd + 1 cDcD)
which will breed true to that type, four (CDcd) will produce
bursa-pastoris and Heegeri in the ratio 15 : 1 as in the F2, and
four (2 Cdcd + 2 cDcd) will give these two types of capsules in
the ratio 3:1. As in all other recessive types, the offspring of
F2 plants having Heegeri capsules should produce no plants with
bursa-pastoris capsules in the F3.

l) Zeitsehrift fur Induktive Abstaininuugj- und Vererbuugslehre
:\ : 341—342, Je 1910,

6

Eleven families were grown during the winter of 1910 — 11
from seeds of F2 plants having bursa-pastoris capsules, and live
families from plants having Heegeri capsules. The results are
brought together here in the form of a table.

Table III.

Pedigree
Number

Capsule of
parent

Rosette of
parent

Result iii F3

Ratio

capsules

Ratio
rosettes

09258

bursa-pastoris

heteris

1 H. heteris

0:1

1 :0

09281

bursa-pastoris

heteris

307 bp. heteris

1:0

1 :0

09284

bursa-pastoris

heteris

( 31 bp. heteris \
1 11 bp. tenuis 1
) 5 H. heteris f
1 4 H tenuis

4.67 : 1

2.47 : 1

09271

bursa-pastoris

rhomboidea

f 175 bp. rhomboidea }
\ 70 bp. simplex }

1 :0

2.50 : 1

09272

bursa-pastoris

rhomboidea

{ 96 bp. rhomboidea \
< 31 bp. simplex
[ 2 H. rhomboidea J

63.5 : 1

3.16 : 1

09273

bursa-pastoris

tenuis

375 bp. tenuis

1:0

1:0

09274

bursa-pastoris

tenuis

/224 bp. tenuis }
) 64 bp. simplex
\ 10 H. tenuis (
[ 2 H simplex

24.0 : 1

3.55 : 1

09283

bursa-pastoris

tenuis

(250 bp tenuis \
{ 16 H. tenuis J

15.6 : 1

1 :0

09275

bursa-pastoris

simplex

f443 bp. tenuis \
\213 bp. simplex j

1:0

2.08 : 1

09275 i

bursa-pastoris

simplex

1 86 bp. tenuis }
\ 35 bp. simplex j

1 :0

2.43 : 1

09276

bursa-pastoris

simplex

472 bp. simplex

1:0

0: 1

09282

bursa-pastoris

simplex

156 bp. simplex

1:0

0: 1

09278

Heegeri

heteris

{7 H. heteris ]
3 H. rhomboidea >
1 H. tenuis )

0: 1

7:3:1:0

09288

Heegeri

heteris 1 H. heteris

0: 1

1 :0

09289

Heegeri

heteris 1 H. heteris

0:1

1 :0

09277
09290

Heegeri
Heegeri

tenuis
tenuis

J167 H. tenuis \
\ 79 H. simplex j

10 H. tenuis

0: 1
0:1

2.11 : 1
1:0

Second sowing.

Several of these families consisted of but one individual each,
and these may be left out of account as having no significance. The
most interesting is 09258 in which a bursa-pastoris parent produced
a single Hezgeri offspring, thus showing that this parent was
heterozygous in respect to capsule-character. Six of the eleven
families derived from bursa-pastoris parents bred true to the
bursa-pastoris type of capsule, this being slightly in excess of
expectation on the assumption that this character is determined
independently by two genes, and considerably in excess of the
one- third which should have bred true if but one gene differentiated
the Heegeri capsules from the bursa-pastoris capsules of the
Pj. However the number of families is wholly inadequate to permit
the attachment of any special significance to this closer agreement
with the requirements of the two-gene hypothesis. All of the
families from parents having Heegeri capsules have bred true to
the parental character, as they should do to agree with Mendelian
interpretation. The four families which split into the two parental
types show the ratios, 4.67 : 1, 15.6 : 1, 24.0 : 1, and 63.5 : 1, all
of these ratios differing in the same direction but in quite various
degrees from the two available ratios 3 : 1 and 15 : 1. Two of
the families show a suggestive approximation to the expected
ratios, while the other two depart widely from the nearest
available ratio 15 : 1. All of these families as well as the three
families of the F2 have shown a smaller proportion of Heegeri
plants than required by the hypothesis that the bursa-pastoris
capsules are determined independently by two genes.

It appears to me that the explanation of these results is to
be sought in some modifying influence acting upon the normal
Mendelian processes. The ratio 4.67 : 1 may then represent a
modified ratio of 3 : 1, and the other three ratios may be referred
to the ratio 15 : 1. These three families taken together give
a ratio of 22.2 ; 1, essentially identical with the observed ratio
21.9 : 1 in the F2, and showing almost exactly the same pro-
portional departure from 15 : 1, that 4.67 : 1 shows from 3:1,
for 4.67 : 3 = 23.3 : 15.

The nature of the modifying cause or causes which may be
operating to produce these defective ratios need not be discussed
at length here, as the matter is capable of experimental treat-
ment and is being investigated; but it may be pointed out that
either „ selective fertilization" favoring the union of unlike gametes,

or „ selective elimination" of the Heegeri homozygotes, would
produce the observed results. Of these two sources of modified
ratios, „ selective elimination" seems to be the more promising,
because there appears to be at present no satisfactory evidence that
„ selective fertilization" occurs in any other organism, while „ selec-
tive elimination" has been clearly demonstrated in Antirrhinum1)
and in yellow mice.2) In both of these, the one class of homozy-
gotes is entirely eliminated, so that the normal ratio 3:1(1+2:1)
becomes 2:1. If „ selective elimination" is the source of the
discrepancies between the theoretical and observed ratios in
the capsule-characters of Bursa, its operaton must differ from that
in Antirrhinum and in yellow mice in two particulars, namely,
there must be only a partial elimination of one homozygous class
in Bursa, and this elimination must affect the negative, instead
of the positive, homozygotes.

The ratios for the rosette - characters in several of the
families also deviate considerably from the expected ratio 3:1,
and it is a pertinent question whether any of these deviations
is significant, or whether they may be accounted for by the
errors of random sampling, due to the small size of the families.
The fact that several of the ratios lie above 3 : 1 and others
below, suggests that at least a considerable part of the variation
in the ratios is due to purely chance causes of this kind. The
ratio 3:1 is the ideal, but can be absolutely expected only
when the number of offspring is infinite.

Too little attention is paid by students of genetics perhaps
to the probable errors of their results. If we follow Johannsen 3)
in computing the standard deviation in the case of alternative
characters by the formula <r = V % p(l . % pl5 in which % p0
represents the percentage of individuals in the one class and
%pj the percentage in the alternative class, it is easy to deter-
mine within what limits a single observed ratio might be referred
to 3 : 1 with a sufficient degree of probability. These limits for

!) Baur, E., Untersuchungen iiber die Erblicbkeitsverhaltnisse in
einer nur in Bastardform lebensfalrigen Sippe von Antirrhinum majus.
Ber. d. Deutseh. Bot. Gesell. 25 : 442, 1907.

a) Castle, W. E., and Little, C. C., On a modified Mendelian ratio
among yellow mice. Science N. S. 32:868—870, 16 D 1910.

3) Johannsen, W., Elemente der exakten Erblichkeitslehre. pp. VI |
516, 1909. Jeiia: Gustav Fischer. See p. 57.

9

families of various sizes from 100 to 1000 are given in the
following table, and are calculated to allow a departure from
the observed ratio, equal to three times the probable error. The
adoption of three times the probable error as a crtrerion of
significant differences is purely arbitrary, and about three families
in one-thousand having the given number of individuals could
be expected to transgress the limits indicated in the table, and
such departures would still be due only to the errors of random,
sampling. Some biometricians accept 2.5 times the probable error
as the limit within which results may not be confidently claimed

to be significant.

Table IV.

Number of Observed percentages Observed ratios

individuals theoretically referable to 75 % referable to ratio 3 : 1

100 60.34 % to 85.54 % 1.52 : 1 to 5.91

200 64.87% to 82.97% 1.85:1 to 4.87

300 66.84 % to 81.70 % 2.02 : 1 to 4.46

400 68.00 % to 80.90 % 2.13 : 1 to 4.23

500 68.78 % to 80.33 % 2.20 : 1 to 4.08

600 69.35 % to 79.91 % 2.26 : 1 to 3.97

TOO 69.80 % to 79.57 % 2.31 : 1 to 3.89

800 70.14 % to 79.29 % 2.35 : 1 to 3.82

900 70.43 % to 79.07 % 2.38 : 1 to 3.77

1000 70.68 % to 78.87 % 2.41 : 1 to 3.73

1
1
1
1
1
1
1
1
1
1

Only one of the ratios for the leaf-characters in the F3 cul-
tures transgresses the limits indicated in this table. The
family 09275 in which 656 individuals gave a ratio of 2.08 : 1,
clearly presents a defect not due to random sampling, and the
cause of the deficiency was easily discovered. The parent of this
family was classified as Bursa bursa-pastoris simplex, and was
expected to produce only the parental characters in its offspring ;
but the progeny consisted of 443 B. bp. tenuis and 213 B. bp. simplex,
thus demonstrating that the parent was a heterozygote in which
the normally dominant tenuis characters failed to appear. The
relative impotency of the tenuis character which allowed it to
remain undeveloped in the parent, seems to have affected the
offspring in a similar manner, so that without doubt many of
the heterozygotes were classified as B. bp. simplex.

During the early development of this family, it appeared
to consist of about three B. bp. simplex to one B. bp. tenuis, and

10

only much later did it become obvious that many of the supposed
simplex plants were producing a greater elongation of some of
the lobes than is to be found in pure B. bp. simplex. The ratio
2.08 : 1 here reported for this family, was derived by waiting
until the flower- stems were about 5 — 10 centimeters high, and
then calling everything tennis which produced at least one lobe
more elongated than those of pure-bred B. bp. simplex. Plate V
shows the most highly developed leaf-characters attained in each
of twenty-six individuals taken quite at random from plants in
this family, which had been finally classified as B. bp. tennis.
In normal, well-developed specimens of B. bp. tenuis there is a
long series of leaves in the middle („ climax") region of the
rosette, in which there is marked elongation of the lobes (see
plate III), but in family 09275 many of the plants which were
finally regarded as B. bp. tennis, had but one or two leaves in
which recognized tenuis characters appeared. Thus the wide gap
which ordinarily separates the dominant and recessive types in
these hybrid families of Bursa, was in this particular family not
only reduced to zero, but it appears certain that the heterozy-
gous and recessive categories overlapped to such an extent that
many individuals which belonged in the former were necessarily
classified in the latter : hence the defective ratio is to be explained
by the failure of dominance of the tenuis characters in the heterozy-
gotes. v '.

The cause of this failure of dominance is not apparent. The
environment has a very considerable influence in determining the
various features of Bursa plants; and especially in limiting the
development of such distinctive characters as ordinarily appear
only in the climax leaves. Crowding in the seed-pans, poor
illumination, and other unfavorable conditions, have caused many
plants in certain of my cultures, to develop flowers and ripen
seeds without having developed their leaves beyond the early
juvenile stages. While the rosettes in 09275 were not in any
sense juvenile, the simplex characters do represent a less highly
specialized type than tenuis, and therefore any influence which
tends to abbreviate the cycle of development, might conceivably
reduce tenuis plants to a form indistinguishable from simplex. This
large family of 656 individuals was germinated in a single square
seed-pan, 30 X 30 cm, and the young plants grew in this seed-pan
for seven weeks before they were potted. Perhaps this long

11

crowding might be expected to have some such effect as that
observed. So far as known, all other conditions under which these
plants grew, were conducive to vigorous development. At any
rate this culture was in all other regards on an equal footing with
the other cultures which were being grown during the same
period, and which gave normal development of the several
biotypes.

To test the suggestion that crowding might be respon-
sible for the failure of dominance, a second sowing was made
on February 25, 1911, each seed being sown separately and
spaced in such a manner that the seed-pan contained only 120
plants. On April 7 these were potted and their growth has been
continuously healthy and vigorous. They have not been subjected
at any time to the least injurious crowding, yet they show the
same tardy development of the tenuis characters as observed
before. On April 22 only three of the 120 plants could be distin-
guished from B. bp. sim/ lex, though nearly all were sufficiently
advanced that if it had been a normal family representing the same
hybrid combination, almost a complete separation of the alter-
native types might have been made. The final census of the plants
derived from this second sowing, made on the same basis and
with the same care as in the case of the first sowing, showed
85 B. bp. tenuis and 35 B. bp. simplex, - - a ratio of 2.43 : 1.
The conclusion is reached therefore that the low grade of the
tenuis characters in this family is inherent, and not a direct
effect of unfavorable conditions of the environment.

This result appears to furnish an illustration of the phenom-
enon known as „ variable potency" *), but whether the gene A
for the tenuis characters is really different in family 09275 from
that in the other families must remain for the present an open
question. It is conceivable that the relative inefficiency of A in
this family is due to the operation of some other factor which
acts as a partial inhibitor, so that although the actual character
of A remains unchanged, it must meet a greater resistance, and
therefore produces a less effect. To avoid a decision of the
question whether the gene has less power or whether it must
meet a greater resistance, I use the expression Mrelative

J) Davenport, C. B., Heredity and Mendel's law. Proc. Washington
Acad. Sci. 9 : 179-187, 31 Jy 1&07.

12

potency". The ^inhibiting factor", if such there be, need not
even be genotypic in nature, but may be the result of some
somatic quality of the parent (such for instance as its state of
health) projected to the offspring through influences surrounding
the latter during their embryonic development.

Summary.

Crosses between Bursa (Gapsella) bursa-pastoris simplex and
B. Heegeri heteris have demonstrated the existence of two genes,
A and B, which determine the differentiating characteristics of
the rosettes and which result in the production of four forms in
the F2 in the ratio 9:3:3:1.

They seem to indicate also the presence of two genes, C
and J), each of which is independently responsible for the bursa-
pastoris-type of capsule. The Heegeri-tyipQ appears only in the
absence of both G and D. On this basis the two forms should
appear in the F2 in the ratio 15 : 1. The observed ratio was
21.9 : 1. In the F3, some families should give ratios of 3 : 1, and
other families should again give ratios of 15 : 1. The observed
ratios in F3 were 4.67 : 1, and 22.2 : 1. These observed ratios
show a corresponding deviation from the theoretical ratios, and
are thought to indicate the action of some modifying influence,
such as „ selective elimination", distorting the results of an other-
wise normal Mendelian segregation.

A defective ratio in the rosettes of one family is shown to
be due to failure of dominance, and certain facts are presented,
which indicate that this failure of dominance is attributable to a
less „ relative potency" of the gene A which determines the
tenuis- character. This change in the relative potency of A may
be due to a decline in " the efficiency of the gene itself, or to
the operation of some other factor or condition which offers an
increased resistance to the development of the

Drnck von W. Burkart in Bj-uim. — Yerlag dcs Verfasseys.

Shull — Defective ratios in Bursa. Plate I.

Ontogenetic succession of leaf- forms in Bursa. . . .heteris.

Shull — Defective ratios in Bursa. Plate II.

w

Ontogenetic succession of leaf- forms in Bursa.. . .rhomboidea.

Shull - - Defective ratios in Bursa, Plate III.

Ontogciietic succession of leaf- forms in Eurm . . . .tenuis.

Shu 11 — Defective ratios in Bursa. Plate IV.

Ontogenetic succession of leaf- forms in Bursa simplex.

Shu 11 — Defective ratios in Bursa. Plate V.

Inflorescences of Bursa bursa-pastoris (at right) and of
B. Heegeri (left).

Shu 11 — Defective ratios in Bnrsa. Plate YT.

;••• :•:::•

Climax leaves of twenty- six individuals classified as B. bp. tenuis ^

in a family (09275) which showed an unusually low grade of

development of the tenuis characters.

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AN INITIAL FINE OF 25 CENTS

WILL BE ASSESSED FOR FAILURE TO RETURN
THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO 5O CENTS ON THE FOURTH
DAY AND TO $1.OO ON THE SEVENTH DAY
OVERDUE.

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