176 



PLANT MORPHOLOGY 



Y chromosome. Consequently all the eggs carry an X chromosome, all 

 the sperms a Y chromosome, and the zygote is always XY. This devel- 

 ops into a sporophyte with eight pairs of chromosomes, seven ordinary 

 pairs and the XY pair. When meiosis occurs at sporogenesis, two of the 

 spores in each tetrad have an X chromosome and two have a Y chromo- 

 some. The spores with an X chromosome always develop into female 

 gametophytes and those with a Y chromosome into male plants. 



B ^ D E 



Fig. 143. Development of the archegonium of Sp/iaerorarpws iea;a«i/s, X500. .4, division 

 of initial into inner and outer cell; B, appearance of three vertical walls in the outer cell; C, 

 formation of cover cell and central cell ; D, formation of primary neck canal cell and primary 

 ventral cell; E, nearly mature archegonium with egg, ventral canal cell, and two neck 

 canal cells; F, cross section of neck. 



Summary. The Sphaerocarpales are an aberrant group of liverworts 

 showing a distinct relationship to the Marchantiales on the one hand and 

 to the Jungermanniales on the other. They resemble the Jungerman- 

 niales in the form and structure of the gametophyte, but are like the 

 Marchantiales in the structure and development of the sex organs and in 

 the structure of the sporophyte. As in the Marchantiales, the apical cell 

 is cuneate, the neck of the archegonium has six cells in cross section, and 

 the spore mother cells are not four-lobed. The development of the 

 antheridium shows a closer resemblance to the Marchantiales than to the 

 Jungermanniales. The sporophyte consists essentially of a foot and cap- 

 sule, the latter indehiscent and with a wall composed of a single layer of 

 cells. In addition to the spores, the capsule contains sterile nutritive cells 

 but no elaters. The most distinctive feature of the Sphaerocarpales is 

 the presence of a special involucre around each antheridium and each 



