648 TEXTBOOK OF BOTANY 



lid, or operculum, and the whole structure, or at least a part of it, is 

 covered hx a hood or calyptra ( Fig. 297 ) . Extending partly across the 

 mouth of the urn is a peristome composed of from 4 to 64 variously 

 ornamented and radially disposed teeth. Large numbers of spores de- 

 velop in the sporangium; and when they are released, new protonemata 

 develop from them. 



Thus the life cvcle of a moss from spore to spore consists of two 

 alternating phases: the gamete-bearing phase (the gametophyte) and 

 the spore-bearing phase (the sporophyte). Reduction division takes 

 place at the time of the formation of the spores within the sporangium. 

 The spores and each of the cells of the gametophyte ordinarily have the 

 monoploid number of chromosomes, and those of the sporophyte the 

 diploid number. 



The leafless stalk with its sporangium is the sporophyte. Since it is 

 not a continuation of the stem of the gametophyte usually onl)' a slight 

 pull is necessarv to separate them. The sporophyte is green when young; 

 at most it is onlv a partial parasite on the green leafy gametophyte. 

 Owing to the location of the archegonium and the germination of the 

 oospore within it, the position of the sporophyte in some mosses is apical 

 on the main stem, and in others on minute lateral branches. 



The sporophytes of most species de^^elop at a definite season of the 

 year. This fact mav often be helpful in distinguishing one species from 

 another. In a few mosses, notably Tetraphis, the sporophyte persists for 

 a year or more. In others the sporangia mature in autumn and remain 

 until spring. In still other species the sporophytes mature in the spring 

 and early summer and soon disappear. There are a few species of mosses 

 on which sporophytes have ne\'er been found. 



The gametophyte phase of the moss begins with the formation of 

 spores in the sporangium of the sporophyte, and ends with the develop- 

 ment of gametes, and fertilization. The sporophyte phase l:)egins with 

 the fertilized egg, or zvgote, and ends with the formation of spores. 



Among flowering plants the plant that bears the flowers is the sporo- 

 phyte phase. The gametophvte phase consists of the embryo sac in the 

 ovule, and of the pollen grain and pollen tube: a female and a male 

 gametophvte. Among the mosses, the gametophyte (protonema and 

 leafy branches) ma)- be male or female, or it may be bisexual (monoe- 

 cious), bearing both antheridia and archegonia with their sperms and 

 eggs. Of all the plants in the world, the bryophytes have the largest 

 gametophytes, and the seed plants have the largest sporophytes. 



