DEVELOPMENT OF PLANTS 271 



conditions that would be fatal to the lower forms, tended to 

 make them independent of spore reproduction. ' But especially 

 has this change been brought about by the fact that the gameto- 

 spore in germinating develops an asexual plant or generation with 

 enormous possibilities in the way of spore production, so that the 

 increase and distribution of individuals is now fully provided for 

 by the asexual plant and consequently largely removed from the 

 sexual plant. This shifting of the responsibility for the increase 

 in number of individuals from the sexual to the asexual generation 

 is one of the most significant dei5artures in the evolution of plants. 

 We will be interested to note how this change came about and 

 how it steadily gains in importance in the remaining divisions. 



The sexual reproduction of the bryophytes reveals a series of 

 variations that are only remotely suggestive of the algae, but that 

 indicate very clearly relationship with higher plants. These 

 features will be considered in the discussion of the various groups. 

 There are two classes of Bryophyta: A, The Hepaticae or Liver- 

 worts; B, the Musci or Mosses. 



Class A. Hepaticae or Liverworts 



103. Classification of the Liverworts.^ — While these plants are 

 the simplest and least conspicuous of the Bryophyta, they are the 

 most interesting, because among them will be found reminders of 

 the algae and at the same time they present many features that 

 are suggestive of the mosses and ferns. They occur in very 

 moist places and in deep woods, on rotting logs and moist shady 

 banks. A few are aquatic, floating upon still ponds and streams, 

 and some have become adapted to dry conditions. There are 

 three orders: (a) Marchantiales or Thallose Liverworts; (b) 

 Jungermaniales or Leafy Liverworts; (c) Anthocerotales or 

 Horned Liverworts. 



104. Order a. Marchantiales or Thallose Hepatics.— The 

 liverworts of this order are characterized by flat, prostrate and 

 rather fleshy bodies which usually grow upon the earth, to which 

 they are attached by numerous rhizoids. Owing to the frequent 



^division of the apical cell of the thallus into two equal parts, there 

 result two growing points which thus produce the equal forking 



