220 NITELLA LESS, xxi 



at its proximal end. Soon two nodes (nd) are formed on 

 the filament, or pro-embryo, from the lower of which rhizoids 

 (rJi) proceed, while the upper gives rise to a few leaves (/), 

 not arranged in a whorl, and to a small process which is at 

 first unicellular, but, behaving like an apical cell of Nitella, 

 soon becomes a terminal bud (term, bud] and grows into the 

 ordinary leafy plant. 



This is an instance of what is known as alternation of 

 generations. The Chara and presumably the Nitella 

 plant gives rise by a sexual process to a pro-embryo which in 

 turn produces, by an asexual process of budding, the Chara 

 (or Nitella) plant. No case is known of the pro-embryo 

 directly producing a pro-embryo or the leafy-plant a leafy- 

 plant. In order to complete the cycle of existence or life- 

 history of the species two generations which alternate with 

 one another are required : a sexual generation Q? gamobium, 

 which reproduces by the conjugation of gametes (ovum and 

 sperm), and an asexual generation or agamobium, which 

 reproduces by budding. 



