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Radical leaves ovate-lanceolate, light green. Flowers white and rather larger than the others, with a few whitish glandnlose hairs near the base of petals. Filaments and form of anthers similar to these of V. phamiceum, but beset with white instead of purple, glandular hairs. Pollen similarly copper-coloured in each.
Thus, judging from the characters of these three forms alone, there can be no doubt as to their being other than conspecific. In addition to this I may add, on the authority of Mr. Stirling of Edinburgh, that they have been raised from pure seed of the V. phoniceum, the rose-coloured variety frequently appearing amongst the seedlings of V. phceniceum, the white presenting itself more rarely.
In the first part of Tab. 1, the number of flowers fertilised, and the simple results are shown, and in the right hand, for the sake of comparison, the calculated produce of the number of seeds from 20 capsules of each is given.* If we compare the results, we see that reciprocal unions may be effected becween the V. phceniceum and varieties, with one exception, viz., V. phaniceum, roseum, by pollen of V. phceniceum, album, in which case 1 have found that though the pollen tubes are abundantly developed and freely penetrate the stigmatic tissues, the capsules nevertheless drop prematurely. The goodness, however, of both the male and female elements of the above varieties is nevertheless shown by their reciprocal unions with V. phamiceum. The individual potency of the respective sexual elements of these' varieties, in their reciprocal relations, is clearly shown; whereas by those experiments given in the three last lines of the table, in which the stigmas of each variety were covered by their own good pollen, no unions were effected, each proving utterly self-sterile!
This absolute, or conditional, sterility of the three varieties of V. phceniceum, when treated by their own good pollen, led me to examine
* From Mr. Darwin's suggestion in " The Origin of species" that the decreased fertility of mixed unions, as compared with that of the pare unions, might possibly be increased by the fact, that for perfectly satisfactory results, castration is necessary in the cross-unions; whereas in the latter, in pure unions, this not being necessary, we may have indiscriminate comparisons, of the two results theugh clearly castration may have a direct sterilising inflnonce. In view of this prudent suggestion, I took the precaution to castrato every flower both of the pure and mixed unions, from which I intended to draw results. The sole exception to this is that given in the first line of Table 2 of V. phceniceum as I was unable to get any of the plants under me to produce seed by their own pollen. Whatever be the effects of castration then on the fertility of the plants so treated, in the present cases, all having undergone it, the results will be mutual. into the apparent cause, as in certain cases we find it arising from the non-emission or non-penetration of the pollen tabes; the pollen through some mysterious cause being thus utterly impotent on its own stigma. The results of my present examination will, I trust, be found of sufficient interest to permit of my stating them here. They are as follows: first, I applied the pollen of each of the three varieties, reciprocally, to their etigmas; on dissecting these, I found them abundantly permeated by pollen tubes, many of which I distinctly traced into the ovary. Secondly, I fertilised several flowers in each variety, with its own pollen; on examining the stigmas of a few of these flowers, I found that many of the pollen grains had emitted tubes, but comparatively few had penetrated the stigmatic tissue, and of these still fewer permeated the conducting tissues of the styles. Several of the latter, however, I traced into the vascular bundles of the placenta, the pistillary cords, and in one or two instances, I believe that I detected them in the nucleus of the ovule. Nevertheless we have seen that, though these pollen tubes are developed, they most ineffectively perform their deputed function, inasmuch as not one of these matured even a single ovary I I have here to observe, however, that these pollen tubes do not seem utterly void of the fccundative influence, as many of the ovaries did undergo a certain degree of development; and on examination of these, as they dropped off, I found that the ovules also had undergone a partial and variable degree of development. In general, the fleshy albuminous envelope of the embryo was largely developed, whereas the embryo had undergone a very slight development, judging from a comparison of other good seeds of a similar stage, not at nil proportionate to the size attained by the albuminous parts. In nearly all the embryos which came under my observation, the development had ceased ere they exhibited any distinct separation of parts; a few only had reached that stage in which the axial and lateral projections were visible.
We thus sec, that whatever be the real cause of the inveterate selfsterility of the three varieties of the V. phcenicetim, it does not arise, as has been shown in other cases, from the non-emission of the pollen tubes. In these, as I have elsewhere noticed it, in certain individual plants of different species of Oncidin, Maxillaria, and Passiflor*, sterility apparently results from some slight differentiation of the mala element with respect to its own female element. I have also to remark, that the ultimate conditional sterility of these plants is not, relatively considered, an absolute but a graduated quantum; this is shown by the different degrees of development the embryos had undergone, thus illustrating a most interesting, though as yet imperfectly known fact, namely, that the male element, even though reaching the female element, may nevertheless fail to communicate that amount of vital stimulus necessary to the complete development of the embryo. Furthermore, I may in passing briefly refer to the perfect parallelism between these phenomena, and those occasionally observed in hybridisation, at least in the zoological kingdom, for unfortunately we are as yet nearly void of information on this point in the vegetable kingdom, hybridists having, in most instances, satisfied themselves by attending to the ultimate results, without troubling themselves to examine into the nature or degree of embryonic sterilisation. From the published papers of the Hon'ble and Rev. W. Herbert, we find, as might indeed be expected, that this point did not escape observation: thus in one case he remarks, " It has, I believe, not been duly considered, that the fecundation of the ovules is not a simple, but a complicated process. There seems to me to be three or four several processes: viz., the quickening of the capsule of the fruit, of the outer coats of the seed itself, of the internal parts or kernel, and lastly, the quickening of the
embryo." "It is further to be observed," he continues, "that there
is frequently an imperfect hybrid fertilisation, which can give life, but not sustain it well. I obtained much good seed from Hibiscus palustris by H. speciosus, and sowed a little each year till it was all gone, the plants always sprouted, but I saved only one to the third leaf, and it perished then."
To recur, however, to the above parallelism, of which we have here
additional and important illustrations: it has been stated by Mr. Darwin*
on the authority of Mr. Hewitt, that in the hybridisation of gallinaceous
birds a frequent cause of sterility in first crosses is the early death
of the embryo. Again Mr. Salter records similar results from
his experiments on the fertility inter se of several hybrid Galli,f thus
concluding, " the one striking point of these experiments (which I
believe has never been noticed before) is that a large proportion of
• loc. cit. p. 286.
t Nat. Hiat. Bev. 1863, p. 276.
these eggs from hybrid birds breeding inter se have failed to produce young, not from absolute sterility, but sterility in degree, from an amount of vitalization insufficient to carry out the whole result oi reproduction, in which the young individual has been completed, leaving it with vital resistance insufficient to maintain life and cop« with common and customary external influences." And thus in those curious cases of sterility of structurally hermaphrodite organisms, whose sexual elenfents have become differentiated with respect to their mutual fertile conjunctions, so in the phenomena of sterility from hybridism, we find, as Mr. Salter well remarks, with respect to the relations of hybridism and parthenogenesis, " that the Bterility is not absolute but in degree, and that the stimulus, whatever it may be, which starts the embryonic changes is feeble and imperfect rather than wholly wanting."
I have now shown that a regular more or less early embryonic abortion results from the self-fertilisation of certain individual plants of V. phaniceum and vars. roseum, and album; whereas by their reciprocal fertilisation, highly fertile unions may in general be effected. By again consulting Table 1, however, it will be seen that besides a reciprocal fertilisation, these three plants are also susceptible of fertilisation by pollen of other species. Thus in lines 7, 8, 9, of Table 1, the male element of V. nigrum is singularly enough effective in the fertilisation of each, while in a succeeding Table—4—the goodness of the male elements is also similarly shown by each effectively fertilising the female element of the V. lychnitis, lutea. Again, we have fuller illustrations of these curious sexual phenomena in Table 2, in which one of the above plants, V. phceniceum, yields a varying degree of fertility to four other distinct species; namely the V. ferrwjincum, Blattaria lutea and alba; Lychnitis lutea and ovalifolia. These are indeed remarkable physiological revelations. How strange that an individual plant could bo fertilised by the pollen of five distinct species, and yet not by its own good pollen: how singular also, as shown above, to see three hermaphrodite individuals incapable of self-fertilisation, yet having each sexual element reciprocally meeting and fertilising the opposite elements of other species. Thus, for oxample, the male element of V. phoeniceum and vars. roseum and album fertilise the female element of V. lychnitis, while the female elements of the three former arc also susceptible of fertilisation by the male element of*F. nigrum. The full explanation of these curious and complicated sexual relations, I leave for more sagacious and ingenious investigators, and simply confine myself to remarking on the apparent support that these and more especially those other cases which I have communicated to the Linnean Society,* on the fertilisation of certain species of Passiflorae, —in which I showed that individual plants perfectly self-sterile readily effected reciprocal unions with other similarly characterised individuals of the same species—give to that view which Mr. Darwin has propounded regarding the existence of a law in nature necessitating " an occasional cross with another individual, or, that no hermaphrodite fertilises itself for a perpetuity of generations," but " that some unknown great good is derived from the union of individuals which have been kept distinct for many generations. "f
In the following table, the results of the pure unions of V. phamiceam given on the first line are taken from capsules on a specimen in the Edinburgh University Herbarinm, as I have not yet been successful in getting good capsules from any of the plants which I have had an opportunity to experiment upon by their own pollen. The other plants of V. phceniceum and varieties mentioned in the table are the same as those from which I had the results given in Table 1. Indeed, in one or two instances, the same experiments are re-stated, with a view to show more clearly the relative degrees of sterility resulting from the crossing of undoubted varieties of a species on the one hand, with those from the hybridisation of distinct species on the other.
* "Journal Linn. Soe." Vol. 8. p. 197. t Orchid Fertilisation, pp. 1—360.