170 



REPRODUCTION 



which normally fuse before they can reproduce the filament. 

 It is known that the proportion of asexual to sexual repro- 

 ductive bodies changes with the time of the year. In the 

 spring the majority of the plants 2:)roduce gametes, in the 

 autumn they form zoospores. We can imagine that in the 

 course of evolution plants appeared in which this seasonal 

 differentiation was firmly impressed in the life of the individual. 



?^ 



o?i 



:'^v 



\ \ ■: 



/ 



Fig. 59. Vertical section through a fertile leaf of a Fern, Nephrodium. Upon 

 it are seated the numerous stalked sporangia, each containing many dark- 

 walled spores. The whole is covered over by the umbrella-like indusium. 

 After Krug. 



In many of the higher algae of this group there is a very fixed 

 alternation of plants forming zoospores which on germination 

 produce plants which form gametes only; in some the two 

 kinds of plants are alike to look at, in others they are quite 

 different; thus in the large kelp (Laminaria) the plant which is 

 so common produces spores, and these grow into minute 

 filaments about a tenth of a millimetre in length. On these 

 small threads the sexual cells are produced ; after their fusion 

 the product grows into the Laminaria plant, which may reach 



