96 



PLANT STRUCTURES 



.rh 



FIG. 82. A common moss 

 (Polytrichum commune), 

 showing the leafy gameto- 

 phore with rhizoids (rh), 

 and two sporophytes (sporo- 

 gonia), with seta (a), calyp- 

 tra (c), and operculum (d), 

 the calyptra having been re- 

 moved. After SCHENCK. 



plant from which it came. This 

 new leafy body consists of a slender 

 stalk bearing at its summit an urn- 

 like case in which are developed nu- 

 merous asexual spores (Figs. 82, 107). 

 This whole structure is often called 

 the " spore fruit/ 7 and its stalk is 

 imbedded at base in the summit of 

 the leafy branch, thus obtaining firm 

 anchorage and absorbing what nour- 

 ishment it needs, but no more a part 

 of the leafy branch than is a para- 

 site a part of the host. 



When the asexual spores, pro- 

 duced by the " spore fruit," germi- 

 nate, they reproduce the alga-like 

 body with which we began, and the 

 life cycle is completed. 



In examining this life history, it 

 is apparent that each spore produces 

 a different structure. The asexual 

 spore produces the alga-like body 

 with its erect leafy branch, while 

 the oospore produces the " spore 

 fruit" with its leafless stalk and 

 spore case. These two structures, 

 one produced by the asexual spore, 

 the other by the oospore, appear in 

 alternating succession, and this is 

 what is meant by alternation of gen- 

 erations. 



These two "generations" differ 

 strikingly from one another in the 

 spores which they produce. The 

 generation composed of alga -like 

 body and erect leafy branch pro- 



