Hydra 



TWO KINDS OF GENERATIONS 



The hydra and its relatives reproduce by budding and also by means of gametes, 

 which are discharged into the water. In the common salt-water jellyfish Obelia, there 

 is a sexual generation, called the "medusa", which is quite distinct from the vegeta- 

 tive generation 



leafy scales at the tips of some individuals flask-shaped structures develop. 

 These produce a single egg cell each, and are called archegonia. In other in- 

 dividuals the corresponding structures produce large numbers of swimming 

 sperm cells; these club-shaped organs are called antheridia. When the moss is 

 covered with water, usually in the spring, the antheridia burst open, and sperm 

 cells swim into the archegonia, where one fuses with each egg. The fertilized 

 egg immediately begins to develop into a new individual, but this new plant 

 is quite different in structure and appearance from either the male or the female 

 parent. It consists of a bare stalk which forms a capsule at the tip — the spore 

 capsule. Its base digs into the top of the mother-plant, from which it derives 

 most of its nourishment (see illustration, p. 412). 



When a moss spore alights on a suitable place, it absorbs water and puts 

 out a thin thread of protoplasm. This develops chlorophyl and looks like 

 one of the simpler algae. Later a clump of cells forms a sort of bud from which 

 the vertical leafy stem grows into either a male or a female moss plant. In 



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