192 MR. C. DARWIN ON THE SEXUAL RELATIONS OF 
general grounds of my belief that with all organic beings distinct in- 
dividuals at least occasionally cross together, and reciprocal dimor- 
phism is plainly one most efficient means for ensuring this result. 
and in the pollen-tubes which proceed from the grains within the anthers in 
V. canina, and from within the lower anthers of Oxalis acetosella, having the 
wonderful power of directing their course to the stigma. If these plants had 
produced the minute closed flowers alone, the proof would have been perfect 
that they could never have crossed with other individuals. I am aware that 
in some of these cases it has been stated that the perfect flowers never produce 
any seed; as far as Amphicarpea is concerned, I hear from Professor Asa Gray 
that the petaliferous flowers certainly sometimes yield seed. The completely 
enclosed flowers of that curious grass, the Leersia oryzoides, as described by M. 
Duval-Jouve (Bull. Soc. Bot. de France, tom. x. 1863, p. 194), apparently offer 
the best case of perpetual self-fertilization ; for when perfect flowers are pro- 
truded from the culms, they are, as far as is yet known, always sterile. Ina 
number of plants kept by me in pots in water, not one single perfect flower has 
protruded, but the enclosed flowers produced plenty of seed. Without wishing 
to throw any doubt on M. Duval-Jouve's excellent observations, I may add that 
with the enclosed flowers borne by my plants, the act of fertilization, that is, 
the penetration of the stigma by the pollen-tubes, took place in the air and not 
in fluid within the glumes. With the exception of the Leersia, as the case now 
stands, I cannot see how the production of the small, imperfect flowers invali- 
dates my doctrine that no species is perpetually self-fertilized, more than the 
multiplication of many plants by bulbs, stolons, Ze, As I observe that the pro- 
duction of seed by the perfect flowers of Viola is spoken of as something capri- 
cious and accidental, I may state that, although it varies much in different years, 
it depends exclusively on the visits of bees; I ascertained this by marking many 
flowers thus visited, and finding that they produced capsules, and by covering up 
many flowers which (excepting a few that I artificially fertilized) did not, when 
thus protected, produce a single capsule. After bees have visited these flowers, the 
pollen may be seen scattered on the papille and on the stigma itself, and they can 
hardly fail thus to cross distinct individuals. These remarks apply to V. canina, 
hirta, and odorata; with V. tricolor the case is somewhat different; but I 
must not enlarge any more on this subject. The production by so many plants 
of perfect and expanded, as well as of imperfect and closed flowers, seems to me 
to throw much light on many points; it shows how extraordinarily little 
pollen is necessary for full fertilization, for I ascertained with V. canina that the 
perfect and imperfect flowers (the latter producing so few pollen-grains) yielded 
the same average number of seeds ; it shows us that fertilization can be perfected 
in closed flowers; it shows us that large, highly coloured petals, perfume, and 
the secretion of nectar are by no means indispensable for this act, even in those 
species which properly possess these characters. It seems to me that the neces- 
sity of an occasional cross with a distinct individual of the same species explains 
the universal presence of at least some expanded flowers, at the expense of injury 
from rain and the loss of much pollen by innumerable pollen-robbing insects ; 
it explains the enormous superfluity of pollen from its liability to loss from these 
causes and during conveyance from flower to flower; it explains the use ofa 
gaily coloured corolla, perfume, and nectar, namely, to attract insects, except 1n 
those comparatively few cases in which wind is the agent, and in these the last- 
named attributes are deficient. 
