CYTOLOGY OF REFRODLCTION IN PLANTS 153 



grows out directly into a gametophyte without the intervention of spores. 

 In such cases both generations have the same chromosome number; 

 this is the gametic number in some instances, the zygotic number in 

 others. When one of these aberrations occurs regularly in successive 

 cycles, compensation is made for it by another abnormality elsewhere 

 in the cycle. Apogamy, for example, may be followed by apospory 

 or by spore formation without haplosis. 



Mosses and Liverworts. — The bryophytes have a life cycle essentially 

 like that of the ferns, with the difference that the gametophytic phase 

 is in general more prominent than the sporophytic one. In mosses the 

 gametophyte typically passes through an early filamentous stage 

 (the protonema) and then the more familiar leafy stage. In the liverworts 

 it is either leafy and moss-like or develops as a broad and branching 

 ribbon. In both groups the sporophyte is usually a stalked spore 

 capsule standing on the gametophyte at the point where it began its 

 development from a fertilized egg in an archegonium. Because of their 

 fairly short life cycle, the prominence of their gametophytic phase, their 

 adaptability to greenhouse culture, their relatively low chromosome 

 numbers, and their response to experimental treatments, the bryophytes 

 hold a prominent place in cytology and have proved to be especially 

 valuable in the genetical study of gametophytic characters. 



Some bryophytes are homothallic, both archegonia and antheridia 

 being borne on the same gametophyte, while others are heterothallic, 

 with the two sexes in different gametophytes. The latter condition is 

 known to be due to the presence of two kinds of spore in a quartet, their 

 difference being determined at the time of meiosis (page 187). The 

 archegonium bears a single large egg containing relatively little stored 

 nutritive material. The antheridium develops characteristically a 

 very large number of small biciliate spermatozoids. 



In spite of the minuteness of the cells concerned, spermiogenesis 

 has been successfully studied in bryophytes, particularly in mosses (Fig. 

 112). The spermatid, like that of ferns, consists mainly of cytoplasm, 

 nucleus, plastids in some form, and a blepharoplast which has functioned 

 as a centrosome during the last mitosis. The behavior of these cell 

 components is of special interest when it is compared with that observed 

 in animal spermiogenesis (page 121). The nucleus draws out into the 

 form of a curved body within the cell. Meanwhile the blepharoplast 

 also elongates, unites with what is to be the anterior end of the nucleus, 

 and develops two long cilia. The limosphere, a large body formed by 

 the plastid material, extrudes a small globule which moves to the anterior 

 end of the nucleus near the blepharoplast and develops into the pointed 

 apical body. The remnant of the limosphere takes up a posterior position 

 and may eventually- disappear along with a portion of the cytoplasm. 



