REPRODUCTION BY SEED iy 
flower is said to be protandrous (Gr. protos, first ; andvos, 
male); when the pistil, protogynous (Gr. gyne, female). 
The flowers, in these cases, may be regarded biologically 
as unisexual, being male when the stamens are ripe, and 
female when the pistils. The two periods, however, 
generally overlap, and during this time self-pollination 
is possible. In most flowers the stamens ripen first, but 
a few are protogynous—e.g., plantain, figwort, Christmas- 
rose, most grasses, and all fly-traps. The pellitory 
(Parietaria officinalis) exhibits very pronounced pro- 
togyny. The style protrudes from the flower before the 
bud opens, and by the time the stamens are ripe the 
stigmas have fallen off. 
3. The Mechanical Structure of the flower is such that 
the pollen cannot, naturally, get deposited upon the 
stigmas of the same flower—e.g., orchid, iris. These 
flowers, therefore, must be cross-pollinated, and if they 
are entomophilous, the insects touch and pollinate the 
stigmas on entering the flower, and on leaving it move 
the floral parts in such a way that the pollen they bear 
away is not deposited on the stigmas. 
4. Dimorphie and Trimorphie Flowers.—The primrose 
is dimorphic—i.e., it has two kinds of flowers (Gr. di, 
two; morphe, form). In one the anthers are situated 
upon the corolla-tube at a higher level than the stigma ; 
in the other the positions are reversed. Charles Darwin 
showed that effective fertilization is only brought about 
when the stigma of a long-styled or pin-eyed form is 
pollinated by pollen from the stamens of the short-styled 
or thrum-eyed flower, and vice versa—that is to say, 
pollination with the best results takes place between 
organs at the same level. In the purple loosestrife 
(Lythrum Salicaria), a marsh and river-side plant, three 
kinds of flowers occur, three levels being interchangeable 
among two whorls of stamens and the stigma. Here, 
again, pollination between organs at the same level is 
best, and these must be in different flowers. 
In all bisexual flowers, however efficient the devices 
may be to prevent self-pollination, it is always possible 
for it to occur. In some cases it regularly takes place 
if cross-pollination fails. The flowers of the Composite 
family, for example, always set a full head of seeds, and 
this could not happen if self-pollination were impossible. 
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