DOMAIN OF EVOLUTIONARY HYPOTHESIS 123 



From the fertilized ovum the green-leaved moss plant 

 is not immediately produced, but it forms in the first 

 place another ' plantlet ' the sporogonium (Fig. 25). 

 When complete it consists of a stalk (seta) which 

 carries a capsule at its tip. The stalk is sunk within 

 the mother plant, without, however, an actual con- 

 nection by growth existing. In the capsule there are 

 finally numerous spores which are newly formed out of 

 an ' indifferent ' tissue. 



By the bursting of the capsule the spores are hurled 

 out, and only from these are there again developed 

 the green ' moss plants/ 



In regular succession there thus arise individuals 

 which produce the sexual cells proper viz. ova which 

 need fertilization, and seminal threads (antherozoids 

 in the plants concerned), and form others, 1 the spores 3 

 or germ cells, protected by a firm skin, which never 

 require to be fertilized, at least not with seminal threads, 

 in order to develop. This regular succession, one after 

 the other, of a sexual and asexual generation, is termed 

 ' alternation of generations/ 



1 Is the Sporogon an actual ' individual ' i.e. a new actual plantlet or 

 an organ of the mother plant for the formation of reproductive cells ? 

 It is very difficult to decide the question, but for our description it has 

 no importance, since we here consider only the actual differences which 

 can justify us in determining separated types. For the actual plant (not 

 organ) nature there speak the formation of various tissues with separate 

 vital functions, assimilation tissues, breathing apparatus (stomata), and 

 the circumstance that the germ cells (spores) are newly developed from 

 indifferent material. The spores are therefore not the immediate product 

 of the self -dividing fertilized ovum cell. 



2 It would go too far to enter exactly into the definition of the ' spore.' 

 The definition given above in any case suits the plant spores generally. 



