844 DIMORPHIC SPECIES.—HYBRIDISATION. 
stigma of the same flower is mature; while those plants in which 
the stigma is mature before the pollen are said to be protogynous ; 
either protandrous or protogynous plants being spoken of as 
dichogamous. It appears that entomophilous plants, by being 
protandrous or protogynous, or by the peculiar mechanism of 
their flowers, are generally incapable of self-fertilisation, though 
some may become self-fertilised in the absence of insect visitors, 
and these are not infrequently protogynous, so as to facilitate 
cross-fertilisation if an opportunity should occur. Apparently 
the form, colour, markings, odour, and nectaries of flowers exist 
in special positions to determine not only the visits of insects, 
but also the direction of their entrance so as to ensure cross- 
fertilisation, just as form, the presence of hairs, or disagreeable 
odours, or other secretions, impede or wholly prevent the visits 
of injurious insects. The form and characters of the pollen- 
cells themselves seem to have a direct connexion with the 
method of fertilisation, those of anemophilous flowers being 
generally smooth and dust-like, whereas those of entomophilous 
flowers are often very irregular and sticky. Anemophilous 
plants too have not infrequently a much larger quantity of pollen 
than those which are entomophilous. 
Dimorphic or heterostyled species are those which possess two 
forms of both sorts of sexual organs, as species of Primula, 
Oxalis, and Pulmonaria, which have both long and short stamens, 
and long and short styles. The long stamens are associated with 
the short styles, and vice versa, in the flowers; and Darwin has 
proved, by experiment, that, for the complete fertilisation of 
either kind of pistil, it is necessary that pollen from the stamens 
of corresponding length, and therefore from a different flower, 
be employed. Lythrwm Salicaria is trimorphic—1.e. has styles 
and stamens of three different lengths—and similar laws have 
been observed to prevail in its fertilisation. Legitimate fertili- 
sation is the impregnation of the style of one flower by the 
pollen from a stamen of equal length with itself, but belonging 
to another flower; while the fertilisation of a pistil by pollen 
from a stamen of different length is termed illegitimate. Some 
plants have inconspicuous self-fertilising flowers, which are said 
to be cleistogamic. Such flowers occur in large numbers on the 
common Violets (Viola odorata and V. canina), in addition to 
the more showy ones which are entomophilous. Similar flowers 
are found also on Lamiwm amplexicaule, Owxalis Acetosella, &e. 
Hybridisation, Hybridation, or the Production of Hybrids m 
Plants.—If the pollen of one species be applied to the stigma of 
another species of the same genus, should impregnation take 
place, the seeds thus produced will give rise to offspring inter- 
mediate in their characters between the two parents. Such 
plants are called hybrids or mules. The true hybrids, which are 
thus produced between species of the same genus, must not be 
confounded with simple cross-breeds, which result from the cross- 
