PTERIDOPHYTES 147 



Several layers of wall cells are formed, and the innermost wall cells 

 by their division add to the sporogenous tissue (fig. 342). The tapetum 

 investing the sporogenous tissue consists of two or three layers of cells. 

 At least one third of the mother cells do not produce spores, contribut- 

 ing to the nutrition of the remaining two thirds. 



Spores. — In the development of the spores a remarkable outermost 

 wall layer is formed. In all ordinary spores the wall develops two layers, 



Figs. 343, 344. — Gametophytes of Equisctum: 343, male gametophyte, showing an- 

 theridia at some of the branch tips; 344, female gametophyte. — After Goebel. 



the inner called inline, the outer exine ; but in Equisctum another layer 

 is laid down on the exine, called the pcriuium, which cracks into two 

 spirally wound bands that remain connected at one pole of the spore. 

 In shedding, these bands unwind (figs. 336-338), become entangled with 

 the bands of other spores, and thus the spores fall in clumps. This 

 seems to be advantageous since the gametophytes are mostly dioecious ; 



