MR. T. Н. CORRY ON ASCLEPIAS CORNUTI. = 197 
which it previously bore, and carries them in turn to the central older flowers of another 
umbel. In this case cross pollination, between the umbels at least, invariably takes 
place, and is also frequent between different individuals. In the case of curious combi- 
nations of corpuscula-bearing pollinia, the possibility of the insertion of pollinia from 
one flower into a neighbouring one of the same umbel occurs. Yet even here I have 
frequently observed that the combination is formed by the successive insertion and 
extraction of corpuscula derived from young flowers (whether central as in the early 
condition, or circumferential as in the later) of one umbel into the central flowers of 
another which are of a similar age, so that even in this case cross pollination between 
separate umbels and often between distinct individuals is ensured. 
The extraordinary number of flowers which a hymenopterous insect is able to search 
within a very short space of time increases the chance of cross pollination, as does the fact 
that they are unable to perceive without practical experience for themselves whether the 
store of nectar in a flower has been already exhausted. 
If all or nearly all the flowers of an umbel are pollinated, those which are so by 
pollinia derived from another individual will, as we have already seen, fare best and out- 
strip those which are fertilized by pollinia from the same or other umbels of the same. 
individual; so that the former alone in this case will set seed. Further, should a flower 
be pollinated by means of pollinia derived from the same individual, and also even at a 
subsequent period by pollinia obtained from a different one, the pollen-tubes of the latter 
will outstrip those of the former in their action, and the ovary which they enter will 
alone ripen into fruit. The relative priority of pollination in the different flowers which 
have been efficiently cross-pollinated no doubt exercises an important influence in deter- 
nining which of these shall develop further in preference to the rest, since only a limited 
amount of fruit can be efficiently nourished. In another species, 4. Michaurii, Decaisne 
(A. angustifolia, Ell.), I have found that the flowers of one individual are fertilized more 
freely by pollen from a distinct individual than from its own, yet, if access to the pollinia 
of another individual be denied, those flowers which are pollinated by the pollinia of 
younger flowers of the same individual set seed easily and readily, although they are 
sterile to their own pollinia; they are, therefore, only partially self-sterile*. І am by no 
means, however, prepared to affirm that the same is true of 4. Соғпш as of this last- 
named species. On the contrary, what observations and experiments I have been able to 
make on the subject tend distinctly to the reverse conclusion, and I regard the flowers of 
A. Cornuti as utterly self-sterile. For while the stigmas of the flowers of every individual 
are perfectly normal, and their ovules are perfectly fertilized and produce a full comple- 
ment of seed with the pollinia of any other distinct individual of the same species, not 
one of these flowers produces a single seed when fertilized by pollinia derived from the 
same individual plant to which it belongs. These pollinia, notwithstanding, fertilize 
completely the flowers of any other distinct individual save their own. This self-impo- 
tency may be due іп this case, as in so many others, to the change of circumstances to 
which the plant i is exposed in the climates of Europe; but Prof. Gray tells me that no 
* T unfortunately did not observe the proportion of seeds produced by cross pollination to that produced by self- 
pollination in this species, nor the relative vigour of the seedlings produced from them. 
SECOND SERIES.—BOTANY, VOL. II. Sn 
