]\rOSSES AXD LIVERWORTS (BRYOPHYTES; 263 



mosses use this number as the basis of distinguishing one 

 species from another. On account of the regular thickenings 

 upon the teeth they are readily affected by moisture changes ; 

 that is, they are hygroscopic. When they extend within the 

 capsule the spores adhere to them. As they straighten and 

 extend outward they move ^\ith a jerking motion whicli serves 

 to tliTdw the spores about. A moss may be made to repeat the 

 characteristic tooth mo^'ements under a hand lens or low power 

 of a microscope, by being moistened and then fanned until dry. 



The spores developed ^^'ithin the 

 capsules are made entirely by cell 

 division and are therefore asexual 

 spores. As 



t 



seen m 

 Section 242, they 

 may germinate and 

 produce protonema. 

 Because of the large 

 number and wide 

 distribution of asex- 

 ual spores, abun- 

 dant production of 

 protonema occurs 

 when the favorable 

 conditions of mois- 

 ture, light, and tem- 

 perature exist. 



246. The moss plant : alternate stages in the life cycle. It 

 is evident that in the mosses sexual and asexual reproduction 

 are limited, each to a distinct part of the life cycle. It is also 

 evident that each of these parts of the life cycle forms a kind 

 of spore which, upon germination, produces not the same part 

 of the life cycle, but the other part. The asexual spore that is 

 formed in the capsule germinates and produces protonema, 

 which, by means of buds, produces the leafy plant; the oospore, 

 which is produced by union of gametes, — the egg and sperm, 

 — germinates and produces the foot, seta, and capsule. The 



Fig. 220. Tips of moss capsules 



.1, a side view of a moss capsule showing the teeth (i), 

 and the mouth or peristome (p), to which the teeth 

 are attached. (Considerably enlarged.) B, an end 

 view of a moss capsule. Xote the peculiar spiral 

 arrangement of the teeth and the transverse thicken- 

 ings upon them. (Greatly enlarged) 



