220 NITELLA LESS. XXT 



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

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

 (rti) 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 orgamobium, 

 which reproduces by the conjugation of gametes (ovum and 

 sperm), and an asexual generation or agamobium, which 

 reproduces by budding. 



