266 INTRODUCTION TO BOTANY 



capsule in which the asexual spores are formed is a complex 

 structure. The stalk upon which it is borne is known as the 

 seta, which means " bristle" or " hair." In the capsule itself the 

 cap which covers the tip is known as the calyptra, which means 

 55 hood." The ealvptra is the old archegonium wall that was 

 carried upward by the developing stalk.. Beneath the calyp- 

 tra is the mouth, or peristome, of the capsule, and over the 

 mouth is an easily removed lid, the operculum. Beneath this 

 lid peculiar teeth (fig. 203, D and .") surround the mouth, and 

 through these teeth the spores are dropped or thrown as 

 changes in moisture cause the teeth to move in and out. 

 This elaborate arrangement is thought to secure thorough 

 distribution of the asexual spores of the mosses. 



251. Alternate stages in the life of the mosses. It is evident 

 that in the mosses sexual and asexual reproduction are limited 

 to distinct parts of the plant. A moss spore, when it ger- 

 minates, produces not the part of the plant from which the 

 spore grew, but the other part. Asexual spores germinate 

 and produce j)rotonema, from which the leafy shoot grows 

 by means of buds; the sex spore, or oospore, germinates and 

 produces the leafless stalk, upon which grows the capsule in 

 which asexual spores are formed. It is customary to speak 

 of that part of a plant which produces the asexual spores as 

 the xporopliijti' (spore plant), and of the part that produces 

 the sex spore as ^&~gametophyte (gaHaet_lant), or the part 

 of the plant which produces the sex cells. The sporophyte is 

 therefore the asexual generation of the moss, and the gameto- 

 phyte the sexual generation, and they alternate in completing 

 the life round of the whole plant. This relation of the two 

 phases is spoken of as the alternation of generations. The fact 

 that the protonema and the leafy shoot are distinct structures 

 does not make a third phase in the alternation, for the reason 

 that there is no spore intervening between them. Also, the 

 term altej^igMon, as used, refers only to sexual and asexual 

 generations, and not to cases such as that of the wheat rust, 

 where the asexual phase appears in several different forms. 



