308 ALLEN— SEX INHERITANCE IN SPH^ROCARPOS. 



The visible differences between male and female plants of Sphcs- 

 rocarpos fall naturally into two categories : 



(o) Differences in rate of growth and in the size at maturity of 

 homologous parts (compare, for example, Figs, i and 2, or 3 and 

 4), and, probably intimately connected with these, a dift'erence in 

 power of resistance to unfavorable conditions ; in each of these re- 

 spects the advantage is with the female. 



{h) Differences in form and structure of the sex organs (an- 

 theridia and archegonia), and of course of the gametes themselves, 

 as well as in the form and structure of the involucres surrounding 

 the sex organs. These differences in structure are associated with 

 size differences — for example, the archegonial involucre is much 

 larger than the antheridial involucre, as well as different in form — 

 but it does not follow that characters of size and those of form, 

 though constantly associated and even causally related, have been 

 brought to expression by the same chain of causative factors. 



Of none of these distinctive characters, it may be noted, is there 

 any apparent reason for suspecting that it has anything in common 

 with the " secondary sexual characters " of the higher animals. 



That the possession by each sex of its own complex of distinctive 

 characters — hereinafter referred to for the sake of brevity as " sex 

 characters " — constituting a constant phenotypic difference between 

 the sexes, is the expression of a genotypic difference, can hardly, I 

 think, be doubted ; for all the available evidence indicates that 

 SphcBvocarpos is strictly and under all circumstances dioecious, and 

 that an individual gametophyte possesses one and only one set of 

 potentialities so far as sex characters are concerned. This being 

 the assumption upon which discussion must for the present rest, it 

 follows that the respective groups of sex characters (or their phys- 

 ical bases) are separately inherited by the sexual from the asexual 

 generation through the spores, which latter are the only possible 

 vehicle of transmission from sporophyte to gametophyte. Since 

 two spores of each tetrad develop into plants of either sex, the con- 

 clusion already drawn by previous writers seems inevitable, namely, 

 that the physical bases for the sex potentialities were united in the 

 sporophyte down to the time of the formation of the spore mother 

 cells, and that the separation of these physical bases occurred in the 



