1 86 PRACTICAL BOTANY 



circular or, according to age, slightly quadrangular outline 

 of the section ; its constituent parts may be recognized as 

 follows : 



1. The wall of the capsule consisting of several layers of 

 chlorophyll-containing cells, of which the outermost has its 

 external wall thickened and cuticularized like a typical epider- 

 mis. 



2. Within this is an air space, traversed in a radial direction 

 by filaments of cells, containing chlorophyll : these are connected 

 internally with : 



3. The spore-sac, which is limited internally by 



4. An inner air-space : this also is traversed by filaments, 

 which connect it with : 



5. The central columella. 



Examine the spore-sac under a higher power, and, if the 

 sporogonium be of the right age, it will be seen to consist of 

 usually five layers of cells. 



a. Two external layers of thin-walled cells : 



b. A central layer of cells with dense protoplasmic contents : 



this is the archesporium, which is originally a single 

 layer, but may undergo divisions as it grows older : 

 ultimately by division of each of the resulting mother- 

 cells into four, and their separation from one another, 

 the spores are produced. 



c. The archesporium is sheathed again on the inner side by 



two internal layers of thin-walled cells. 



According to the age of the sporogonium various conditions 

 of the spore-sac may be observed, from the single archesporial 

 layer, to the condition of mature spores. 



XIV. Examine median longitudinal sections of a similar 

 young sporogonium, and recognize in it the several parts above 

 noted. 



The outline of a median section will show at the base the 

 narrow seta : passing upwards to the capsule it will be found to 

 consist of 



i. The apophysis, which appears as a basal swelling limited 

 by a definite epidermis with stomata : internally is a mass 

 of lax, chlorophyll-containing tissue. 



