56 BIOLOGY 



sedentary, fixed, or hydroid parent gives rise by budding 

 to a form which is free-swimming or pelagic. This 

 latter form gives rise either to male or to female repro- 

 ductive elements, which after fertilisation develop into 

 larval forms which swim to the bottom, and becoming 

 fixed, develop into the hydroid form from which we 

 started. Thus in the life history of this organism we 

 find an alternation of the asexual and sexual methods 

 of reproduction, an alternation of two distinct types ; 

 this is an alternation of generations which may be defined 

 as the alternate occurrence in one life-cycle of two or 

 more different forms differently produced. In the fern 

 we find a somewhat similar alternation of generations, 

 but in this case the male and female reproductive 

 elements are derived from different sources. 



Among the higher plants there are many interesting 

 devices to escape self -fertilisation, and to profit by cross- 

 fertilisation. Among the phanerogams we find many 

 plants bearing flowers with male and female organs 

 which are mature at the same time, and so many are 

 no doubt self-f ertilised. But this self -fertilisation is by 

 no means the rule, for we find that the anthers and the 

 stigma are not usually ripe at the same time, or it may 

 be that the position of the two sets of organs prevents 

 this self -fertilisation. Another method adopted is that 

 the plant bears two distinct kinds of flowers, and these 

 are not mature at one and the same time ; and still one 

 other method exists which completely prevents self- 

 fertilisation, namely that in which one kind of flower, 

 the male or the female, is found. Moreover the 

 numerous ingenious devices among plants which pre- 



