152 



PHYSICAL BASIS OF HEEEDITY 



the number to IN. It is at this time, too, in mosses with 

 separate sexes, that sex differentiation takes place, for as 

 the Marchals have shown, each spore gives rise to a male 



x(m 



%(n) 



y^'® ^ ® ® 



© ® XiU) 



Xy(2n) Sporophyte 



XiTl) Gamehphyie 



® ym) 



©xm) 



FiQ. 59. — Life cycle of moss. The mycelial thread and the moss plant constitute 

 the /n, or gametophyte generation; and the stalk and capsuie (with its contained spores), 

 arising after fertilization out of the moss plant, constitutes the 2n or sporophyte generation. 



or to a female thread that produces archegonia or else 

 antheridia regardless of the condition under which the 

 young plants are reared. Allen has recently shown in 

 related plants — the liverworts — that during the reduction 

 division (that gives rise to the spores) an unpaired sex- 



