REPRODUCTION. 



645 



this further advantage that they facilitate the distribution of 

 individuals of the same species. They are light, readily 

 transportable by wind or water, and in some cases they are 

 actively motile. Hence that close aggregation of individuals 

 which would result from continued vegetative reproduction, 

 and which would be disadvantageous to the species, is ob- 

 viated by their formation. 



Admitting, then, the advantage accruing from the pro- 

 duction of spores, the further question arises why the asexual 

 production of spores should not suffice, why there should be 



mi 



fP' 



o 



Fig. 76 (after Strasburger) . Fertilisation in a Gymnosperm (Picea vulgaris] : 

 pt, pollen-tube ; o, oosphere ; mp, male pronucleus ; fp, female pronucleus. 

 A shews the first appearance of the male pronucleus in the oosphere ; B, its 

 movement towards the female pronucleus ; C, the fusion of the two pronuclei. 



any sexual production. In view of the often elaborate 

 arrangements by which the performance of the sexual process 

 is ensured, it may be inferred that it is an advantage to the 

 species that spores should be produced sexually ; that is, that 

 spores should be produced containing nuclear substance 

 derived from two more or less distinct sources. In illus- 

 tration of this, reference may be made to those plants in the 

 life history of which alternation of generations occurs : it is 

 clear that, normally, the sexual production of a spore is 



