196 MR. T. H. CORRY ON ASCLEPIAS CORNUTI. 
of the style *. This, however, soon ceases to progress further, and, indeed, never attains 
а very great length; the pedicel of the flower withers and becomes disarticulated from 
the peduncle, and then the flower itself falls off, both the pollinium and the stigma 
having remained fresh in appearance during this time; they seem, therefore, to have no 
deleterious effect upon one another. The emission of a skein of pollen-tubes by the polli- 
nium, in the usual manner, and the fact that this skein may ultimately enter the cavity 
of the ovary, appears to be a contingency insufficient in itself to ensure the production of 
an embryo. This was first clearly shown in 1867, by Delpino. Sterility must, then, be 
ascribed in these plants to some other cause than that of inefficient pollination, and can 
only consist in the want of adequate cross-fertilization. 
In warm weather hive- and humble Bees work very diligently, so that even in flowers 
of A. Cornuti which have been only a few hours expanded, no pollinia can be found 
remaining in their original situations, all being already removed. Notwithstanding this, 
І have been able to confirm Hildebrand’s interesting observation that in these flowers, 
while they are still young, it seldom happens (though it may do so, as will presently be 
shown) that pollinia are to be found inserted in the alar fissures of the same plant, and 
it is only when these flowers become somewhat older that pollinia are to be found 
inserted in their ајаг chambers, and producing their skeins of pollen-tubest. This 
observation clearly indicates that insects for the most part do not fertilize the flowers of 
this species with their own pollinia, but carry pollinia from younger flowers to older 
ones. Thus, if only the central flowers of the cymose umbel be expanded, the pollinia 
brought to them by insect visitors must, at least, be those of a different umbel, and 
very probably those of a different individual; while the pollinia extracted are carried 
away in like manner. If, however, the entire umbel is expanded, the more central 
flowers will already have had their pollinia extracted; and should the insect alight on 
one of the outer flowers first, it may possibly deposit in the alar chambers of the 
more central ones the pollinia it has extracted from those which are more external. 
In this case it will, however, sometimes have deposited in the alar chambers of the 
outer flowers the pollinia obtained from young and newly expanded flowers, either 
of the same individual or probably of a different one; though frequently it may come 
to these flowers destitute of pollinia of any kind. The insect, however, so far as my 
experience goes, very rarely pursues this order of visitation, but usually alights first 
on the central flowers of the umbel and pursues a reverse order. Having alighted, it 
deposits pollinia brought from another umbel or another individual, while it takes no 
pollinia in return, since these latter have already been withdrawn ; but it extracts pollinia 
from the outer and younger flowers by means of the ruptured appendages of the pollinia 
+ Т cannot agree with Mr. Weale's conclusion (Joc. cit. p. 57) that the flowers of Asclepiads probably require to be 
pollinated each with several pollinia in order to be productive, and that the absence of this leads to the production 
of such a very small quantity of mature fruit; for if a flower be pollinated with a single polliniwm from a perfectly 
distinet individual, ripe fruit and good seed will be produced. Such paucity of fruit is only due to insufficient nou- 
rishment if all the flowers are adequately eross-fertilized, and to inefficient fertilization if they are not so. 
+ Mr. E. Pott (loc. cit.) states just the reverse of this; but Hildebrand's conclusions are fully supported by my own 
researches. 
