MR. T. H. CORRY ON ASCLEPIAS CORNUTI. 195 
The former, however, though possible, probably very seldom occurs, since the insect 
is unlikely to get the identical foot which extracted the pollinia again caught in the 
fissures of the same flower; and I have, moreover, frequently observed that the cir- 
cumstance of the insect having to exert force to extricate its leg from the apex of the 
alar chamber startles and annoys it a little, so that it immediately leaves that flower of 
the umbel and proceeds to the first adjacent flower. Some considerable time, moreover, 
must elapse after the pollinia are extracted before the corpuscular appendages are so far 
dried that both pollinia of the same corpusculum can be introduced through the fissure 
into the alar chamber, and in the meantime the insect has had time to reach another 
flower or plant. Yet, notwithstanding these possibilities, the great law first clearly 
enunciated by Andrew Knight * at the close of the last century, in the form that ** no 
hermaphrodite fertilizes itself for a perpetuity of generations; " and, after the lapse of 
years, again in 1862 by Charles Darwin, at the close of his * Fertilization of Orchids,' 
in the words, ** Nature tells us, in the most emphatie manner, that she abhors perpetual 
self-fertilization," will be found to hold good for the Asclepiads, as well as for the 
Orchidez and the rest of the Vegetable Kingdom. 
Each umbel is composed of from 20 to 50 closely approximated flowers; but of this 
extremely large number only one, or perhaps two, ripen seed, while all the others soon 
become disarticulated and fall off. 
The only ease which seems exactly to resemble it that occurs to me is that of the 
Horse-chestnut (сш из Hippocastanum), quoted by Darwin Т, where only one or two of 
the several flowers arranged in a thyrsus on the same peduncle produce a seed ; and this 
seed is moreover the product of one out of several ovules in the same ovary. The deter- 
mining cause in both eases which enables certain of the flowers to develop fruit to the 
exclusion of the rest, is to be found in the circumstance that these flowers have been 
pollinated with pollen or pollinia extracted from flowers belonging to another individual 
of the same species ; since, in this case, the resulting production of tubes appears to be 
attended with greater vigour and energy, while all the others have been pollinated simply 
with pollen or pollinia derived from the same individual to which they themselves 
belong ; in other words the fertilizing influence of pollen derived from another, is pre- 
potent over that derived from the same individual. We know from the experiments of 
Herbert and others detailed by Darwin T, that if one flower is fertilized with pollen which 
is more efficient than that applied to the other flowers on the same peduncle, the latter 
often drop off; and it is probable that this would occur with many of the flowers on 
the same plant which had been self-fertilized with pollen derived from the same indivi- 
dual, if other and adjoining flowers were cross-fertilized by pollen brought from another. 
So far as I have been able to determine by direct experiments, the flowers of Ascle- 
pias Cornuti are absolutely sterile when fertilized artificially with pollinia extracted 
either from the same flower or from one of a like age; though these pollinia when 
inserted produce each their skein of pollen-tubes which penetrates down into the interior 
* «Philosophical Transactions,’ 1799, p. 202. 
T ‘The Effects of Cross and Self-fertilization in the Vegetable Kingdom,’ p. 399. 
+ ‘The Variation of Animals and Plants under Domestication,’ 2nd edition, vol. ii. pp. 120-122. 
