312 ALLEN— SEX INHERITANCE IN SPH^ROCARPOS. 



influence on the rate of cell growth and of cell division. That this 

 is the case seems conceivable and indeed probable, because the over- 

 w^helming evidence of an influence of the chromosomes on inheri- 

 tance points to the exertion of this influence largely, at least, through 

 a determination of the rate and nature of constructive metabolism ; 

 and one effect of a modification of the rate of metabolism would of 

 course be an increase or decrease in the rate of cell growth and thus 

 also in the rate of division. 



But if such a simple quantitative explanation can be adduced to 

 account for the influence of the sex chromosomes on the one class 

 of sex characters, it cannot with equal ease be made to account for 

 the characters of the second category — those which concern the 

 form and structure of gametes, of sex organs, and of involucres. 



It is true that, in connection with the observed facts in animals, 

 several writers have suggested the possibility of a " quantitative 

 theory " of sex determination, which would make all primary sex 

 characters the expression of the amount or degree of activity of the 

 chromosome material present. Without entering into the discus- 

 sion of the validity of such a theory in the case of the metazoa, it 

 seems quite impossible of application to the class of characters under 

 consideration in Sphccrocarpos. For one thing, it is to be remem- 

 bered that in this plant, difl:'erently from the condition so common 

 in the higher animals as well as in the dioecious seed plants, there 

 are not two sets of reproductive structures, one functional, the other 

 rudimentary but conceivably capable (certainly capable in many 

 seed plants) of a normal development under particular conditions. 

 In such a case the stimulus leading to the functional development 

 of one set of structures or the other might conceivably result from 

 the presence of a greater or less quantity of particular nuclear sub- 

 stances. On the contrary, the male plant of Sphccrocarpos shows 

 no trace of archegonia or of archegonial involucres ; the female plant 

 shows no trace of antheridia or of antheridial involucres. Nor can 

 the differences between male and female structures be explained by 

 modifications of cell size operating differently in different cell axes. 

 Factors of some sort must be supposed to be at work which deter- 

 mine, very differently in the two sexes, the planes of successive cell 

 divisions; which modify, also in very different ways, the nucleo- 



