CHROMOSOMES AND SEX 



385 



heterothallic condition may have arisen through the loss of a portion of 

 this chromosome in some plants. This would give sex-chromosomes of 

 two kinds: an unmodified X and a shortened Y. It would also give two 



JPellia epiphylla. 



V 



// 



■0 





w, 



V 



4» 



\-:. 

 ^ 



■A 



■^ 



Pellia Neesiana, 



W 



^^ 



\<i 







^ V 



X-- 





c/' 



\\ 





if (\ 



if 



^^^ 



•if 



\\ 



» 





fi 



V 





Fig. 220. — Diagram of chromosome sets of homothallic and heterothallic species 

 oi Pellia. The fifth chromosome in each row is the sex-chromosome. At right: metabolic 

 nuclei, showing heteropyknotic chromosomal regions. Cf. Fig. 80. (After Heitz, 19286.) 



sexual manifestations, assuming that the M-chromosome carries factors 

 specially influencing sexual development. 



Sex-chromosomes essentially like those of Pellia Neesiana have been 

 described in the heterothallic moss Pogonatmn injiexum (Fig. 221). 





■fc. 



a 



Fig. 221. — Chromosomes of Pogonatum inflexum. a, from female gametophyte 

 (6 + x). b, from male gametophyte (6 + y). c, disjunction of Y and X in first mitosis 

 in sporocyte. d, e, f, metabolic nuclei from female gametophyte, male gametophyte, 

 and sporophyte, showing heteropyknotic sex-chromosomes (also nucleolus). (After 

 Shimotomai and Koyama, 1932.) 



In other heterothallic bryophytes, whose chromosomes are not well 

 known, there is evidence for the meiotic segregation of sex factors. In 

 certain mosses there are produced in the capsule spores of two kinds which 



