548 



A TEXTBOOK OF THEORETICAL BOTANY 



Fig. 550. — Azolla fiUculoides. Floating frond with roots. 

 Twice natural size. 



^f- s*». 



Fig. 551. — AsoIIa fiUculoides. Superficial view of leaf 

 showing a stoma. Note that the pore is trans- 

 versely elongated with respect to the guard cells, 

 the wall between which is in process of breaking 

 down. 



and are obviously photosynthetic organs. The upper surface is velvety with 

 epidermal papillae and is unwettable, which is naturally advantageous in 

 preventing the blockage of the stomata with water. 



The lower lobes are somewhat broader than the upper and consist as a 

 rule of only one cell layer. They are very poor in chlorophyll, and it has 

 been shown that they absorb water freely, their surface being wettable. The 



