PTEEIDOPHYTA— HYDEOPTEEIDEtE 



149 



The lower cell takes no part in the formation of the 

 prothallium, but remains almost unchanged, becoming filled with 

 various reserve materials for 



Fig. 907. 



the nutrition of the young 

 embryo developed later on the 

 prothallium. In Azolla its 

 nucleus divides repeatedly, 

 but no cell-walls are formed 

 in it. 



The archegonia are de- 

 veloped at the apex of the 

 prothallium, three being 

 usually formed in Salvinia, 

 but only one in the Mar- 

 sileacese and AzoUa. If none 

 of the first formed ones be- 

 come fertilised, more are de- 

 veloped later. The structure 

 is similar to that of the ferns. 



In the germination of the 

 zygote or oospore Salvinia 

 shows some peculiar features. 

 It segments into octants as 

 in the Ferns, but all the 

 hypobasal cells go to form 

 the foot. No root is developed. 

 The first leaf or cotyledon, 

 developed from two of the 

 epibasal cells, is of curious 



shape. It is known as tEe scutiform leaf. In the other three 

 genera there are two cotyledons, stem, and root, as well as foot. 



Fig. 907. Gametophyte produced by the 

 macrospore of Salvinia. pro. Protlaallium 

 beariog a, young sporopbyte. After 

 Pringsheim. 



