EEPEODUCTION 445 



the sporangium. In Ginkgo and in a species of Cycas the 

 male gametes are definite antherozoids, 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 ; thus 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 offspring ; 

 thus 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 parts 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 

 of the archegonium, forming the calyptra. The same 



