FLORAL MECHANISM ETC . 
95 
Dioecism has the serious disadvantage of requiring a 
number of male plants whose sole function is to produce pol- 
len ; the chance of pollination, too, is less than in monoecism, 
and here again less than in cases of hermaphroditism. 
Another condition preventing autogamy is self-sterility , 
i.e. incapability of a flower to be fertilised by its own pollen. 
This condition is little understood ; the same flower varies 
in this respect in different countries. Corydalis sp., Abutilon 
sp., Passiflora sp., and others may serve as examples. It is 
very improbable that this character has been acquired as a 
preventive of autogamy. 
A phenomenon of the same kind is pollen-prepotency . 
If a stigma on a plant A be pollinated from the stamens of 
A and from those of another plant of the same species, B, 
in many cases the ovules will be fertilised by the tubes of B, 
not of A, even if A has had a start (provided of course that 
the tubes of A have not reached the ovules). 
Allied to these is the remarkable phenomenon of hetero- 
stylism\ seen in a typical condition in the primrose or cow- 
slip (Primula) and in Ly thrum (q.v.). In the former two 
kinds of flowers are found ( dimorphism ), in the latter three 
(trimorphism), each on a separate plant ; they are distin- 
guished by the different lengths of the stamens and styles. 
One flower of Primula has a long style and short stamens, 
the other a short style and long stamens. Complete fertility 
(i.e. a full yield of fertile seed) is only obtained when pollen 
is taken from long stamen to long style or from short to 
short, which of course involves cross-fertilisation. It is 
evident that this legitimate pollination will tend to be regu- 
larly effected by the visiting insects, which behave always 
in the same way upon the flowers. Illegitunate pollination 
(short and long) results in few seeds and these are more or 
less sterile. This is a remarkable fact, for it is exactly the 
phenomenon seen in hybridisation, i.e. in crossing of two 
distinct species. For further details see Lythrum ; cf. also 
Pontederia, Fagopyrum, Linum, Erythroxylon, Oxalis, Sta- 
tice, Hottonia, Menyanthes, Pulmonaria, Faramea, Bou- 
vardia, Mitchella, and many others. In Nolana, &c. the 
length of the stamens and styles is very variable. 
1 Darwin, Forms of Flowers. 
