446 VEGETABLE PHYSIOLOGY 
the nucellus or body of the sporangium. In Ginkgo and in 
a species of Cycas the male gametes are definite anthero- 
zoids, furnished with cilia. In most of the Gymnosperms, 
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 advan- 
tageous 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 off- 
spring and in the possession of greater vigour by them. 
Plants not so closely related may, however, produce off- 
spring; 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 
increased 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 fertilisation is still more marked. In the Mosses its 
result is to cause not only the development of the sporophyte 
from the oosphere, but a considerable additional growth 
