384 



INTRODUCTION TO CYTOLOGY 



spores with 1 -\- X develop into female gametophytes, while those with 

 7 + F develop into males. The total chromosome volume in a female 

 gametophyte nucleus is about 1.7 times that in a male, and the volumes 

 of the cells differ by about the same amount (Lorbeer). 



Meiotic 



Spore 

 quartet. 



5PHAER0CAHP0S 



Female gametophyte 



Fig. 218. — Chromosome cycle in Sphcerocarpos. (Based on data of C. E. Allen, 1917, 1919.) 



The genus Pellia also shows well-differentiated sex-chromosomes-" 

 (Figs. 219, 220). In P. Neesiana the male gametophyte has nine chromo- 

 somes, including a Y with unequal arms. In the female there is an 

 equal-armed X in place of the F. Considerable portions of the X and F 



Fig. 219. — Sex-chromosomes in liverworts. A, B, Sphcerocarpos Donnellii: A, from 

 apical region of female gametophyte; B, from spermatogenous tissue of male. (After 

 Lorbeer, 1930.) C, D, Pellia Neesiana: sets from male and female gametophytes. Which 

 of the nine chromosomes in the male is the Y has not been determined. (After Showalter, 

 1928.) 



remain condensed (heteropyknotic) through the telophase and interphase. 

 A comparison of the chromosome complement with that of P. epiphylla 

 brings out the interesting fact that in homothallic species also a specialized 

 heteropyknotic "M-chromosome" may be present and suggests that the 



2" Heitz (1927hc 1928a6), Showalter (1927c, 1928). 



