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 spore's the wall develops two layers, 
344 
FIGS. 343, 344. Gametophytes of Equisetum: 343, male gametophyte, showing an- 
thcridia at some of the branch tips; 344, female gametophyte. After GOEBEL. 
the inner called inline, the outer exine ; but in Equisetum another layer 
is laid down on the exine, called the perinium, 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 ; 
