456 VEGETABLE PHYSIOLOGY 
detached from the parent plant before it has reached the 
embryo-sac, from which it is separated by a bulky portion of 
the nucellus or body of the sporangium. In Ginkgo, Zamia, 
and in a species of Cycas, the male gametes are definite 
antherozoids, furnished with cilia. In most of the Gymno- 
sperms, however, this degree of differentiation has not been 
observed. The character of the female gametophyte has 
been already described. 
Though cross-fertilisation is seen to be most advantageous 
throughout the vegetable kingdom, it is only possible within 
certain limits. For a new individual to be produced, the 
sexual cells taking part in the process must have a certain 
degree of relationship ; for instance, the antherozoid of a 
moss cannot fertilise the oosphere of a fern. The most 
favourable degree of relationship is that the two gametes 
shall be produced by different plants of the same species. 
Such a union results in greater numbers of offspring and 
in the possession of greater vigour by them. Plants not so 
closely related may, however, produce offspring ; we may 
have the union of gametes of plants standing to each other 
in the relation of varieties of the same species, or very 
frequently of distinct species belonging to the same genus, 
or even of species of different genera. Such fertilisation is 
known as hybridisation. 
Hybrids, the offspring of such fertilisation, generally 
exhibit peculiarities of form and structure intermediate 
between those of their parents; they are generally fertile 
with either of the parent species, but not usually so with 
another hybrid, or to a much smaller extent. When 
crossed with one of the parent forms the offspring tend to 
revert to that form. 
The immediate result both of pollination and of fertili- 
sation is generally to stimulate the part concerned to in- 
creased growth. In some Orchids the ovules are not formed 
in the ovary until the stigma is pollinated, and seem to arise 
in consequence of that process. The stimulus of fertilisa- 
tion is still more marked. In the Mosses its result is to 
