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 Girikgo, 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 



