290 



Science of Plant Life 



ferns produce asexual spores from which small, flat prothalli 

 develop. The prothalli in turn produce gametes and sexual 



spores, which then germinate 

 to form the fern plants proper. 

 This alternation of two unlike 

 generations in the life history 

 of the fern is most interesting. 

 The alternation of an asexual 

 generation with a small sexual 

 generation occurs in mosses 

 (page 279) as well as in ferns, 

 and it is found also in most 

 of the other great plant 

 groups. However, the alter- 

 nation of generations is not 

 so easy to study in groups, 

 other than the ferns, in which 

 the two generations do not 

 grow independently of each other. In some of the ferns and 

 also in some of the club mosses, the sporangia and the sexual 

 generations show stages in the development of these struc- 

 tures into the seeds of the higher plants ; but the origin of 

 seeds is too complex a question to be discussed here. 



The significance of the conductive system in plants. The 

 algae, being immersed in water, have a constant water supply. 

 The fungi have an adequate water supply because they live 

 inside other plants that have water-conducting tissue, or 

 because they grow in moist soil or within stumps and logs. 

 A water balance is maintained among the mosses and liver- 

 worts by their growing in spongelike masses and in close 

 contact with the soil water ; but within these plants water 



Fig. 172. Under side of fern prothallus, 

 showing egg-producing organs (archegonia) 

 (^1), the sperm-producing organs (anther- 

 idia) (B), and the rhizoids (C). 



