BRYOPHYTA 



175 



Sphaerocarpus the first division of the fertilized egg is transverse, the 

 capsule arising from the outer segment and the foot and seta from the 

 inner segment, as in the Marchantiales. Each segment again divides 

 transversely at least once before vertical walls come in. The foot 

 becomes bulbous and the capsule spherical. In all genera some of the 

 sporogenous cells develop into spore mother cells and others into small 



Fig. 142. Early stages in the development of the antheridium of Sphaerocarpus texanus, 

 X500. A, division of initial into inner and outer cell; B, first di\'ision of outer cell; C, 

 formation of vertical walls; D, formation of perielinal walls; E, later stage, showing sterile 

 jacket surrounding spermatogenous cells, with stalk below. 



sterile cells that do not become elaters (Fig. 1445). Instead, they func- 

 tion as nutritive cells, being finally absorbed by the spores. As in the 

 Marchantiales, the spore mother cells are not lobed. The capsule has no 

 regular dehiscence. Its wall consists of a single layer of cells without 

 local thickenings. 



Sex Determination. The mechanism of sex determination in Sphaero- 

 carpus is of particular interest because here the occurrence of sex chromo- 

 somes in plants was first observed. In addition to seven ordinary 

 chromosomes, the cells of the female gametophyte have a very large 

 X chromosome, while those of the male gametophyte have a very small 



