Part 4

The writers are not sure that the insect in flying to another flower strikes the tip of the pistil against the side, as stated by Robertson. Certainly, in many cases, the insect, while collecting the pollen, supports itself by placing one leg over the tip of the pistil. When the leg bears a large mass of pollen, which is being stored there, it seems hardly possible that the flowers could fail to be pollinated. It might be suggested that, since the stamens for the most part point in the direction of the incurved petal, the function of this petal is to prevent access to the stamens, except in the cases in which the insect supports itself by means of the pistil. While this seems to the writers, at the present time, the most logical of the two functions so far suggested, much more careful observation work must be done before this point is finally decided. The petal may to a certain extent, in connection with the pistil, serve as support for the insect. Todd and Robertson observed only humblebees visiting the flowers. The writers obtained:

_Apis mellifica_ Linn. Lake View, August 7. Seven specimens. _Agapostemon texanus_ Cress. Lake View, August 7. _Mellisoides bimaculata_ (St. Farg) Lepl. Lake View, August 7. _Megachile petulans_ Cress. Lake View, August 7. _Bombus separatus_ Cress. Lake View, August 7.

As in the case of _Solanum_, it will be seen that the collecting period extended over a very short period of time. More search would doubtless greatly increase the list.

Robertson reports the following species as collecting pollen: _Bombus virginicus_ Oliv., _B. separatus_ Cress., _B. americanorum_ F., and _B. scutellaris_ Cress.

August 28, when the blossoming season for _C. chamæcrista_ was almost over, an examination of material from the above-named region was made for the purpose of determining the number of seeds produced by a single plant. Fifteen pods were selected at random from different plants and the number of ovules counted. It was impossible to tell about the number in each pod which were fully and normally developed seeds or which would become such; consequently this factor is not taken into consideration. The percentage of ovules which fail to develop is, however, small. The number of seeds found to the pod is shown by the following:

TABLE F.

Pod I, 13 seeds. II, 14 " III, 11 " IV, 11 " V, 10 " VI, 12 " VII, 16 " VIII, 12 " IX, 8 " X, 14 " XI, 17 " XII, 18 " XIII, 17 " XIV, 15 " XV, 14 " ———— Total XV, 202 seeds, or 13.4 per pod.

From this it will be seen that the minimum number of seeds found was 8, the maximum 18, with an average of 13.4. Since the pods were simply gathered at random, there is no certainty of gaining the maximum or minimum number of seeds, but a fair average of the number produced may be expected. September 4 three plants were examined to determine something about the range of variation in the number of ovules produced in the pods of a single plant. The results are given as follows:

Plant 1 varies from 5 to 11.

Plant 2 varies from 8 to 18.

Plant 3 varies from 9 to 14.

Plant 1 had 35 pods, plant 2 had 64, and plant 3 had 27.

Plant 1 was selected on account of the small number of seeds produced per pod.

It will be seen from table D that an average of nearly 3.3 flower buds per cluster is produced. These were moderate-sized, healthy plants, producing on the whole probably more than the average number of clusters per plant. On the ten plants, there were produced 342 clusters, which bore 344 seed pods, instead of about 1120, the number of flowers which might be expected, thus giving less than thirty-three per cent. of the buds which produce mature seed pods.

It will be seen that, while in the observations made on _S. rostratum_ the flowers which failed to produce seed did not reach much over six per cent., in _C. chamæcrista_ it is over sixty per cent. In addition to this fact, it is rare to see a seed pod of _S. rostratum_ which has been destroyed by insects or other destructive agencies, while in 460 pods of _C. chamæcrista_ which were examined at Lake View, September 4, not one was found which did not have some of the ovules destroyed by the larvæ of some insect, and probably this would amount on the average to fifty per cent. of all the seeds produced, being in the case of some plants as high as seventy-five per cent.

A convenient method of approaching the question of the production of seeds might be to determine the number of seed pods produced on a given area of ground. A general idea may be obtained from the examination of the plants growing upon two square feet of ground. In the first case, the plants were much crowded; in the second, not nearly so much so; in fact, it may be said they were growing under “normal” conditions. It might be interesting to compare the results. The material for the two tables was taken September 4.

FIRST SQUARE FOOT. Plant 1, pods 0 " 2, " 10 " 3, " 4 " 4, " 13 " 5, " 0 " 6, " 12 " 7, " 3 " 8, " 1 " 9, " 4 " 10, " 3 " 11, " 0 " 12, " 25 " 13, " 2 " 14, " 0 " 15, " 0 " 16, " 10 " 17, " 2 " 18, " 0 " 19, " 0 " 20, " 0 " 21, " 0 " 22, " 6 " 23, " 1 " 24, " 5 " 25, " 2 " 26, " 3 " 27, " 0 " 28, " 3 —— ——— Total, 28, pods 109

SECOND SQUARE FOOT. Plant 1, pods 1 " 2, " 50 " 3, " 36 " 4, " 15 " 5, " 48 " 6, " 34 " 7, " 9 " 8, " 11 " 9, " 9 " 10, " 3 " 11, " 0 " 12, " 7 —— ——— Total, 12, pods 223

In the first square foot of ground, where the plants were much crowded, of the twenty-eight plants, ten produced no seed pods at all, and of the remaining eighteen only six produced over five each. On these plants an average of a little less than four pods per plant was produced. In the second lot, where, evidently, the plants were not nearly so crowded, only four produced fewer than five seed pods, and there was a general average of 18.7 pods per plant.

On the first foot of ground, then, there might be produced in the neighborhood of 1300 seeds; on the second, 2600. The large _Solanum_ upon which 40,000 seeds were estimated would probably cover an area of 12.5 square feet, giving 3200 seeds per square foot. Of course, these figures represent only certain isolated cases, which in a way are typical, but must not be taken to represent the average condition.

The largest plant noted September 4 had produced 100 pods, with an average of thirteen seeds per pod; this plant might show 1300 seeds.

Professor Todd discusses in his paper the occurrence of similar divergences from the typical form in other Solanaceæ and Leguminosæ, and tries to discover some hint as to their origin. Lack of material for observation precludes any present discussion of these points.

The results of these observations may be briefly summarized as follows:

Solanum rostratum.

1. As Professor Todd observed, the numbers of right-and left-handed flowers on a plant of any considerable size are about equal.

2. As a general rule, only one flower opens at a time on a raceme, but very commonly two will open on the raceme the same morning, giving a right-and left-handed flower opening simultaneously, and thus permitting in a considerable number of cases pollination between flowers on the same raceme, even if Professor Todd’s theory of the method of pollination be the correct one.

3. Even on the smaller branches of the plant, the flowers are almost always approximately divided into the two types.

4. The flower has a distinct odor.

5. Various species of insects visit the flowers for pollen. Many insects secure pollen without effecting pollination.

6. In a rather hasty microscopic examination, no very apparent difference was detected between the pollen from large and small stamens.

7. A very important function of the observed arrangement of stamen and pistil in _S. rostratum_ seems to the writers to be that of support for the visiting insect.

8. It might seem that the pollen from the small stamens is of much more importance in the process of fertilization than Professor Todd suspected, especially since it seems that there is much more certainty of the pollen from the small stamens reaching the pistil than there is of that from the large stamen. The fact that there is some question as to the fertility of the pollen from the large stamen in all cases, and that in the case of another plant stamens of somewhat similar arrangement seem to have lost entirely their direct reproductive function, would indicate the same.

9. In a limited number of cases the pollen from the large stamen of a flower seems to be fertile on its own stigma, as well as upon the stigma of a flower opening simultaneously on the opposite side of raceme.

10. Spontaneous self-pollination seems sometimes to occur.

11. The percentage of cases in which seeds develop in those flowers in which artificial pollination is effected in the same flower or in two flowers of the same raceme is much smaller than when cross-pollination is effected by insects, reaching, in the case of the somewhat limited experiments of the writers, only as high as 28.5 per cent. Whether this is partially due to the method of applying the pollen or not has not been determined; whether the seeds produced by these cases of pollination of the same flower or flowers on the same raceme are capable of germination or not has not yet been determined. It might be suggested that the low percentage of cases is due to a lack of fertility in the pollen of the large stamen.

12. Estimated from the number of seed pods which normally develop, the number of flowers in which pollination is not effected is very small, not reaching, in the observations of the writers, much over six per cent.

Cassia chamæcrista.

1. Right- and left-handed flowers are produced at the same time on the plant. When several plants are taken, the number of right-and left-handed flowers produced is practically the same.

2. So far as observed, two flowers were never seen open at the same time on a cluster, nor was a bud ready to open the following morning ever found on a cluster with an open flower. Thus, cross-pollination between flowers on the same cluster would not be possible, as it frequently is in _S. rostratum_.

3. So far as the writers have been able to ascertain, there is no law governing the producing of right-and left-handed flowers on the opposite sides of the main axis.

4. Various species of insects visit the flowers for pollen.

5. It seems that pollination is effected in many cases by the transfusal of pollen from the leg of the insect, where it is being carried, to the stigma of the stamen upon which it is supporting itself. The function suggested by Professor Todd for the incurved petal seems to the writers entirely improbable.

BOTANICAL LABORATORY, UNIVERSITY OF KANSAS, SEPTEMBER 28, 1901.

EXPLANATION OF PLATE I.

=Solanum rostratum= Dunal.

FIG. 1. Right-handed flower from the front and a little to one side, showing large and small stamens, pistil, and wings of the corolla, which enfold pistil and large stamen in the bud. × 2.

FIG. 2. Tip of a raceme from the front, showing one left- and two right-handed flowers; also the decurved end of the raceme, with the buds.

FIG. 3. Lateral view of decurved tip of raceme, showing the buds.

FIG. 4. Lateral view of bud ready to open the following morning, showing the two lower lobes of the corolla, enfolding pistil and large stamen.

FIG. 5. Ovary and pistil. × 5.

FIGS. 6 and 7. Lateral and dorsal view of large stamen. × 5.

FIGS. 8 and 9. Lateral and dorsal view of small stamen. × 5.

FIGS. 10 and 11. Hind leg of _Bombus_, with and without mass of pollen.

FOOTNOTES:

[D] Todd Prof. J. E.: On the Flowers of _Solanum rostratum_ and _Cassia chamæcrista_, Amer. Nat., vol. XVI, pp. 281-287, 1882. A brief review of Todd’s paper is given by Dr. P. Knuth, Handbuch der Blutenbiologie, Leipzig, 1898.

[E] According to Carruth—Carruth, J. H., Catalogue of Plants seen in Kansas, with additions by Prof. F. H. Snow and Prof. E. Hall—_S. rostratum_ first appeared in Kansas in 1864. This date probably refers to eastern Kansas. Dr. S. W. Williston informs the writers that it appeared around Manhattan in 1860 or 1861.

[F] One specimen observed growing in rich soil back of a feed-store in St. Joseph, Mo., in early September had a diameter of over seven feet and a height of three feet. The plant might be considered as normally developed, having produced apparently the normal number of seed pods, and so would not be classed with the rank vegetative development which plants sometimes show when grown in very rich soil.

[G] _S. rostratum_ appears to be better adapted to xerophytic conditions by its extensive root system than by any adaptation for the prevention of evaporation of water. When cut down on a warm day, the plants wilt in a very few minutes. Roots extend down sometimes for more than three feet, so that the plants generally appear perfectly fresh when others around are wilted and drying up.

[H] Mueller, Fritz: Two Kinds of Stamens with Different Functions in the same Flower, Nature, vol. XXVII, pp. 364, 365, 1883.

[I] Experiments for the determination of the fertility of close and cross-fertilization are always interesting, but are of especial interest in the case of a plant such as _S. rostratum_, in which, if the method of pollination described by Professor Todd is the one actually depended upon, cross-pollination is sometimes possible and sometimes impossible on the same raceme. Of course, if, as suggested in the latter portion of this paper, the method of pollination suggested by Professor Todd is not the only one, these experiments do not have the interest which they otherwise would.

[J] As will be remarked, the above insects were all taken August 5 and 6. Careful collecting extending over a considerable period of time would doubtless secure many other forms which visit the plant more or less frequently.

[K] Meehan, Thomas: On the Fertilization of _Cassia marilandica_, Proc. Acad. Nat. Sci. Phila., 1886, pp. 314-318.

[L] Meehan, Thomas, _loc. cit._

[M] It must be stated that in a later paper (Robertson, Charles: Flowers and Insects, V. Bot. Gaz., vol. XV, No. 8, pp. 199-204), Charles Robertson does not give the same results as those found by Thomas Meehan. Robertson says: “Two long stamens, one on each side of the style, furnish pollen for cross-fertilization. They have inflated anthers which probably have a bellows-like action, like the long stamen of _Solanun rostratum_ and _Rhexia virginica_.” Meehan states expressly in his paper that in the case of _C. marilandica_ he was sure no pollen was ejected, as Todd found for _S. rostratum_, since in the flowers, which were covered with a gauze bag, the membrane at the apex was never ruptured when the stamens were ready to fall. Robertson describes the method of extracting the pollen in _C. chamæcrista_ in a way which is essentially the same as Todd gives for _S. rostratum_. He then says, in speaking of _C. marilandica_: “Four small stamens furnish pollen for the visitors. Bumblebees milk the pollen out of these, using their jaws, as in the case of _chamæcrista_.” Meehan says: “Nor was there any draw-out of the pollen, as observed by Professor Todd. It is abstracted solely through the pores; and, although I could see no evidence that such was actually the case, I suspect that fertilization could only occur through some of this extracted pollen escaping from the insect to the stigma.” It must be noted here that the method which Meehan describes for the method of opening of the anthers, the pollen being “abstracted solely through the pores,” does not agree with the method described by other observers. Leclerc du Sablon, in a paper, “Recherches sur la Structure et la Dehiscence des Anthers,” in vol. I of the seventh series of Annales des Sciences Nouvelles, discusses the anatomical modifications of the anther walls, by which dehiscence is secured. His observations cover _Cassia cremophilia_ and _Solanum_. His observations do not cover a sufficient number of species to make them of the greatest value in deciding the present points. The author presents, in a condensed form, his results in: Note sur la Dehiscence des Anthers, La Belgique Horticole, vol. XXXIV, pp. 148-150, 1884. Robertson says, in speaking of the central of the three long stamens: “Bees, no doubt, force the pollen out of this as they do from the short stamens.” Meehan says: “I watched a mass of plants containing eighty-eight flower-stems on the 30th of July, and the same lot for an hour on the 6th of August, but saw no attempt to get pollen from the longer anthers or to use them in any way but as a platform. It would indeed be hardly possible for the bee to stand anywhere so as to get power to pierce the apical membranes of the longer stamens. When the flower matured and the anthers were ready to fall they were examined-the four short ones were empty sacs-the three lower ones proved that they had not served any purpose to the bees, for they were full of pollen.”

[N] This, of course, in case, as Meehan states, the large anthers do not dehisce. Of course the statement loses entirely its significance if, as Robertson states, the large stamens furnish pollen for cross-fertilization.

[O] While the experiments made upon artificial pollination were very limited, it will be seen that the pollen from the large stamen in no case fertilized over twenty-nine per cent. of the flowers pollinated from it. These flowers, however, were on the same raceme; so the low per cent. might be due to this, or to the mechanical manipulation. The suggestion that the pollen of the large stamen is less fertile than that of the smaller ones is at least interesting as a working hypothesis.

[P] Notes on Stamens of Solanaceæ, Bot. Gaz., vol. XV, pp. 103-106, 1890.

[Q] Observations on the number of seeds produced and the surety of fertilization may be of especial interest, when the wonderful distribution which this plant has attained in recent years is taken into consideration. The original habitat of _S. rostratum_ was the southwestern portion of the United States. It has since spread over a large part of the United States, in many places being recognized as a very noxious weed. It is also reported from several European localities. Reports on the destructiveness of the plant as a weed may be found in publications of the agricultural departments, as: Dewey, L. H., A Weed Bulletin, Farmers’ Series, No. 28, U. S. Dept. Agr.; Pammel, L. H., Two Noxious Weeds, Bull. Iowa Exp. Sta., 1895. L. H. Pammel,—Distribution of Some Weeds in the United States, especially _Iva xanthifolia_, _Lactuca scariola_, _Solanum carolineum_, and _Solanum rostratum_, Proc. Iowa Acad. Sci., 1895, vol. II, pp. 103-127—gives the eastward migration of this weed up to 1895.

[R] The racemes of _S. rostratum_ are produced by a scorpoid sympodial dichotomy of the branch, in which the racemes represent the alternate branches. At first the raceme develops much more rapidly than the bud which is to continue the main stem, and so the racemes, when flowering, are always well towards the outside of the plant.

[S] Robertson, Charles, _loc. cit._

Transcriber's Notes:

Underscores "_" before and after a word or phrase indicate _italics_ in the original text. Equal signs "=" before and after a word or phrase indicate =bold= in the original text. Small capitals have been converted to SOLID capitals. Typographical errors have been silently corrected but other variations in spelling and punctuation remain unaltered.