August 20. Thirteen flowers marked to see if autogamy takes place.
August 22. All flowers still on the plant.
August 24. All but two flowers had fallen off. One of these seemed to
be developing; the other looked wilted.
August 26. One pod was thriving; the other had wilted and fallen off.
September 13. One pod fully developed.
From the structure of the flowers it would seem that self-pollination
would be impossible. When the flower is open, the stigma has never
been observed to be in contact with the terminal portion of the large
stamen. The stamens do not dehisce until after the flower has opened,
nor does the stigma come in contact with the tip of the anther in the
bud; thus, clistogamy would be out of the question. It appears from
the results obtained from plant III that spontaneous self-pollination
is possible. Of course, however, the positive result in this one case
should by no means be taken as conclusive evidence of self-pollination.
At the present, the most logical explanation to be suggested seems
to be that, when the flowers partially close at night, the tips of
the pistil and the large stamen are brought into contact. This might
occasionally occur, but it is by no means always the case. At the time
of the writing of this paper, material for the determination of this
point is not available, but two or three flowers examined at night
during the summer, before the results of the above experiments had
suggested the importance of a careful examination of a large series of
material, did not show the stamen and pistil in contact. Of course,
note will be taken of the fact that in only one out of thirteen flowers
on the plant did spontaneous pollination take place. Another suggestion
might be that, approaching so near as they do to each other, a puff of
pollen might be thrown from the large stamen and fall upon the pistil
when the plant is shaken.
In plants I and II, it will be seen that, in the first case, three
fully developed seed pods were obtained from twelve flowers the stigmas
of which were supplied with pollen from the large stamen of the same
flower. In the second case, one fully developed seed pod was obtained
from four pollinated flowers—just twenty-five per cent. in each case.
In the cases in which cross-pollination was effected between right-
and left-handed flowers opening simultaneously on the same raceme, we
find that, in the first, one pod of the two was only half developed at
the end of twenty days. Since the pods are normally fully developed
in somewhat less than this length of time, and this undeveloped pod
appears somewhat dried, its development seems doubtful. In the second
case, one pair of seed pods out of seven pairs of flowers crossed were
fully developed, and one seed pod from each of two other pairs were
fully and normally developed, making four out of fourteen flowers which
yielded seed pods—28.5 per cent.
Professor Todd observed only a small humblebee visiting the flowers
of this plant. Owing, probably, to more favorable opportunities for
observation, the writers have been able to secure other insects
collecting pollen.
The following is a list of the species:
_Agapostemon texanus_ Cress. Two specimens collected August 5, at
two P. M. The insects were collecting pollen from the small
stamens, to which they clung while they forced the pollen out by
pinching the anthers between their fore legs. Pollen was stored on the
hind legs. The insect was not seen to come in contact with the tip of
the large stamen or the stigma.
_Apis mellifica_ Linn. Taken at two P. M., August 5. They
sometimes came in contact with large stamen and pistil, but more often
did not touch them at all. Occasionally both stamen and pistil would
come in contact with the same side of the insect’s body. Short stamens
were sometimes approached from above, the large stamen and pistil
remaining untouched.
_Anglochora pura_ Say. Taken at 10:30 A. M., August 6.
Obtained pollen from the large stamen by alighting on it, crawling to
the tips, and collecting it from the terminal pores.
_Halictus_ sp. A smaller insect than the preceding one, but obtained
pollen in the same manner.[J]
No humblebees were taken around Lawrence, although many were noticed
working on the plants; consequently the names of the species noticed
cannot be given. In St. Joseph, Mo., there were taken at three P.
M., when bees were not generally seen working on the plant:
_Bombus virginicus._ One specimen.
_Bombus pennsylvanicus._ One specimen.
An examination of fifty flowers taken at this time showed from the
dented condition of the small stamens that they had all been visited.
Between eight and nine A. M., September 3, when bees were
numerous, there were taken:
_Bombus virginicus._ Two specimens.
_Bombus pennsylvanicus._ Twelve specimens.
_Bombus scutellaris._ One specimen.
The writers found that the humblebees were the principal agents
effecting cross-pollination. It was observed that the bee in visiting
the flower allowed itself to rest on the tips of the extending stamen
and pistil, which, being of the same length, came in contact with both
sides of the body just in front of the hind legs, these being left
perfectly free. The weight of the bee springs down both stamen and
pistil.
Professor Todd’s theory in regard to the pollination of this plant is
as follows: “The weight of the bee so springs down the flower, that
it is quite difficult, on account of the large, flexible corolla,
to see just what is done, but repeated observations led me, quite
satisfactorily, to this conclusion. The bee seeks the pollen—for the
flowers have neither nectar nor odor—and this she uniformly gets from
the four shorter stamens; never, so far as I could determine, from the
larger one. This she does by seizing each one, near its base, between
her mandibles, and with a sort of milking motion crowds the pollen out
of the terminal pores; meanwhile, by the movements of her feet, the
larger stamen is repeatedly sprung backwards, and as often throws a
cloud of pollen on one side of her body; this in a right-handed flower.
When she passes to a left-handed flower, which, as was explained
above, is very likely not to be on the same plant, the pollen is
carried directly to the pistil of that flower, and so on. We have here,
therefore, a novel apparatus for cross-fertilization, quite distinct
from those that have been most commonly noticed.”
A considerable quantity of pollen may be thrown from the terminal pores
of the large stamen upon tapping it. It thus seems quite possible that
some pollen is thrown upon the side of the insect, as described by
Professor Todd. All the meaning of Mr. Meehan’s[K] statement is not
clear to the writers, but he says, in speaking of Professor Todd’s
results: “In regard to the manner in which the pollen is extracted, he
found that ‘this she does by seizing each anther near its base between
her mandibles, and, with a sort of milking motion, crowds the pollen
out of the terminal pores.’ If this were the general way, there would
be no necessity for any pollen being ejected from the long stamens,
for the stigma would surely receive some during the ‘milking’ process;
and the pore at the apex in the long anther is beyond the line of the
stigma, so that on ejection from the pore the pollen would go still
farther beyond.”
It seems that this statement is of considerable importance for _S.
rostratum_ as well as for _C. marilandica_. Professor Todd very
evidently overlooked the fact that, in securing the pollen from the
small stamens and transferring it to the hind legs, the sides of the
insect are sure to be well dusted with pollen from these stamens. In
the case of _Apis mellifica_, as noted above, there is no certainty
that in visiting the flower the same side will be turned toward the
stamen or pistil. Even in the case of large insects, such as _Bombus_,
it would seem that the probability that the stigma will be supplied
with pollen from the large stamen exclusively is very small. It seems
improbable that _S. rostratum_ should depend exclusively upon such
an uncertain method of pollination as the projection, by the jarring
of a stamen, of a puff of pollen upon the side of an insect, and the
subsequent transfer of this pollen to the stigma of a flower of a
different type. Of course, it is not improbable that a part of the
pollen is furnished by the large stamen, as suggested by Professor
Todd, but that fertilization should be effected exclusively by this
means seems highly improbable.
The pollen from the large stamen has been shown to be fertile in a
certain number of cases, but unfortunately opportunity was not offered
for experiments on the fertility of pollen from the small stamens. A
rather hasty microscopic examination of fresh, unstained pollen from
the large and small stamens reveals no very striking difference in form.
In _C. marilandica_, Meehan[L] found that the large, strong stamens on
each side of the pistil served only as a platform upon which the insect
could rest while procuring the pollen from the small stamens. He found
that the lower stamens, while filled with pollen, did not dehisce of
their own account, nor were they opened by the insect.[M]
The lower stamens and the pistil of the _Solanum_ under consideration
serve the purpose of a platform when the flowers are visited by the
larger bees. It seems to the writers that this is not improbably the
function of the greatest importance of the observed arrangements of
the stamen and pistil in _S. rostratum_. In _C. marilandica_, the
pollen for fertilization, as well as for the attraction of the insect
visitor, is furnished by the small stamens, while the pollen produced
by the large stamens appears to have no function.[N] The condition is
not so specialized in the species of _Solanum_ under consideration.
Here the pollen produced by the small anthers serves for the attraction
of insects and, as it seems to the writers, for fertilization, while
the large stamen, in connection with the pistil, serves as a support
for the visiting insect, and possibly furnishes some pollen for
cross-fertilization.[O]
In reference to the relative amount of pollen produced by a large
and small stamen, Halstead has given a note, in his paper in the
Botanical Gazette.[P] The material in the hands of the writers at the
time of the writing of this paper is not suitable for a verification
of Mr. Halstead’s results; consequently they are simply quoted on his
authority. Even if the amount of pollen produced by the large anther is
no greater than that produced by one of the smaller, it is still very
considerable, as may be readily seen by tapping it out on a glass slip.
He says:
“The single large stamen of _Solanum rostratum_, with its beak-like
appearance, is a giant among its fellows, but does not exceed them
in the production of pollen, for, while three or four times larger
than the others, its thecæ are reduced to narrow, curved lines of
mother-cells. The ordinary stamens, on the other hand, possess
unusually large cavities in which the pollen is borne. The giant
stamen, in cross-section, is shown at _a_, in fig. 3, while a similar
section of an ordinary stamen is shown at _b_. The almost infertile
condition of the large stamen reminds one of the structure of the
stamens of the cultivated potatoes. In these, while large and plump,
there is almost no pollen-bearing layer, and usually no apical pore
opens for the discharge of pollen.”
In _C. marilandica_, as Meehan has shown, autogamy is impossible, while
in _S. rostratum_ autogamy may possibly sometimes take place.
The bee visits the flower for pollen; contrary, however, to the
statement of Professor Todd, that “the flowers have neither nectar nor
odor,” the writers observed that, especially in the early morning, the
odor was decidedly pronounced. It was observed that the bee collected
no pollen from the large stamen, but took it regularly from the four
smaller. This it did by grasping the anthers, one at a time, near
the base, and forcing the pollen out through the terminal pores, by
pinching it throughout the length between its mandibles. An exception
to this in the case of _Agapostemon texanus_ Cress, is already noted in
the list of species. It will be remarked that our observations on this
point correspond in general to those of Professor Todd.
Of course the statement of Professor Todd, that the next flower of
the opposite type which is visited by the bee is very apt to be on
another plant, loses entirely its significance, since it has been shown
that the flowers on a branch are not at all likely to be all right-
or left-handed. In visiting the flowers, the humblebees, as a general
rule, simply pass to the flower most conveniently at hand, and this
flower is very apt to be on the same plant, especially where the plants
are at all large. The humblebees especially work vigorously in the
early morning. In a patch of _S. rostratum_ examined between eight and
nine o’clock, in St. Joseph, Mo., nearly all the flowers had already
been visited. At this time fifteen specimens of humblebees were taken.
A great many flowers would be visited by the bee before it found one
which had not already been despoiled of its pollen. In visiting such
flowers, the bee would alight for a moment on the pistil and large
stamens, as described above, and then pass on to the next flower when
it had ascertained that there was no pollen present. In this way over
twenty flowers may be visited in a minute. It will be seen that, when
the bees are at all numerous and as well dusted with pollen as they
usually are, the pistil is almost certain to receive pollen, and
fertilization to be effected, especially if the pollen from the small
stamens is functional.
Among other insects found visiting the plant, the honey-bee was most
frequent.
As will be noticed from our list, some insects visit the plants
without effecting cross-pollination. Those insects which obtain pollen
in an illegitimate manner do not secure it from the small stamens
exclusively, but almost invariably visit the large stamen as well.
The adaptation of the plant to propagation by the production of seeds
is of considerable significance.[Q]
A normal plant will produce in the neighborhood of 7000 seeds. In
making observations on this point, it was found from five pods examined
there was an average of fifty-six seeds.
Pod 1 contained 66 seeds.
" 2 " 53 "
" 3 " 51 "
" 4 " 53 "
" 5 " 58 "
———
281 seeds; av., 56.
Pods 4 and 5 were from the same plant but separate racemes;
the others were from different plants. In determining the
average number of seeds produced by the plant, five plants
growing normally and in different localities were observed, with
the following results:
Plant 1 192 pods.
" 2 50 "
" 3 66 "
" 4 113 "
" 5 210 "
Taking the average of fifty-six seeds per pod obtained above, we see
that the plant producing 122.5 pods, the average from the preceding
table, would produce about 7000 seeds.
One plant was observed upon which occurred fifty-five to sixty racemes.
Allowing the low average of six pods to the raceme, the plant will
produce in the neighborhood of 20,000 seeds. Occasionally a very large
plant is observed which produces as many as 125 racemes. Allowing the
same low average of six pods to the raceme, it will be seen that on a
plant of this size there will be produced in the neighborhood of 40,000
seeds.
Only a very small proportion of the ovaries fail to develop. Out of the
forty-one racemes observed in five plants, taken at random in different
localities, results were obtained as follows:
Plant 1, 5 racemes, 53 pods, 4 failed.
" 2, 5 " 42 " 3 "
" 3, 9 " 55 " 5 "
" 4, 11 " 91 " 6 "
" 5, 11 " 121 " 5 "
—— ——— ——
41 racemes, 367 pods, 23 failed, or 6.2 per cent.
According to these figures, not more than 6.2 per cent. of the ovaries
failed to be fertilized.
While _Cassia chamæcrista_ is usually abundant in Douglas county,
owing, probably, largely to the severe drought, opportunities for study
were not nearly so favorable as for _Solanum_. The material studied was
found growing, for the most part, in somewhat shaded localities on the
banks of Lake View.
Professor Todd has given very well the points in the structure of the
flower of this species. He says: “The points that are of interest to
us are the sickle-shaped pistil, the stamens with long, rigid anthers
opening by terminal pores, and the most of them pointed toward the
incurved petal, which is always on the opposite side from the pistil.”
The flowers are arranged in small clusters a little above the axils of
the leaves. In some cases the axillary bud also develops into a flower
cluster. The axillary clusters have been considered separately in the
calculations made upon the conditions of the flowers.
Owing to the lack of material, Professor Todd was unable to determine
any definite law governing the arrangement of the flowers in _C.
chamæcrista_. This the writers have attempted to do. The determination
of any law governing the order of development of the flowers in a plant
like _C. chamæcrista_, where they are arranged in clusters developed
from buds produced on the main axis, and the development of which is
probably accelerated or retarded by various conditions, is much more
difficult than in _S. rostratum_, where they are produced on a definite
raceme, which is early differentiated from the terminal growing point,
and at first develops more rapidly than the bud which is to continue
the main axis of the branch.[R]