158 STRUCTURE AND LIFE HISTORIES 
cases are always only one cell thick, and always possess 
some form of annulus. 
146. Spores. As the sporangia mature the spore-case 
itself becomes differentiated into two distinct kinds of 
tissue, namely, sterile tissue on the outside, forming the 
wall, and fertile tissue within. In most species of ferns 
the fertile tissue of each sporangium becomes organized 
into 1 6 relatively large cells, rich in protoplasm. As soon 
as these cells are mature they divide once, and then, with- 
out resting, a second time, thus giving rise to four cells 
each. Each group of four cells is called a tetrad; each cell 
of a tetrad becomes a spore. There is always an even 
number of spores (usually 64) formed in each spore-case, 
and the cells from which they are formed by the two suc- 
cessive cell-divisions (tetrad-divisions) are spore-mother- 
cells. The spores finally become separated from each 
other by the dissolving of the. middle layer (middle lamella) 
of the cell-wall between adjacent spores. The solution 
of this middle layer is accomplished by an enzyme 
secreted by the cells, and which acts upon this par- 
ticular layer. The spores finally come to lie dry and 
perfectly free from each other within the spore-case. 
Their purpose is to reproduce the plant, and especially 
to multiply the number of individual plants. 
147. Number of Spores. The number of spores pro- 
duced by a vigorous fern is a great revelation to one who 
has never given such matters careful thought. Pro- 
fessor Bower, of Glasgow, has called attention to this fact 
in the following words: 
"A rough estimate may be made of the numerical output of spores from 
a large plant of the Shield fern, as follows: In each sporangium 48* 
1 Bower gives this number as the characteristic output for the species 
Aspidium Filix-mas. In other species the number may be 64. 
