36 



VARIATION AND DIFFERENTIATION IN CERATOPHYLLUM. 



plant which we have called "secondary" and "tertiary" main stems are 

 sensibly differentiated from whorls borne on any branches, of whatever 

 order. These whorls have a higher mean number of leaves and are 

 markedly less variable (absolutely and relatively) than are whorls on 

 branches. That is, they have those characters which we have just seen 

 hold for main-stem whorls generally. There can be, I believe, but one 

 conclusion, namely, that these portions of the plant are parts of the main 

 stem rather than primary branches. 



The results of this section of the paper may be summarized as follows: 

 It has been shown that (1) whorls borne on the main stem of the plant 

 have a significantly higher mean and modal number of leaves than do 

 whorls borne on branches; (2) main-stem whorls are markedly less vari- 

 able, both absolutely and relatively, than are whorls borne on branches; 

 (3) the distribution of variation in leaf -number is distinctly less skew 



Table 13. — Constants for variation in leaf-number in whorls on secondary and 



tertiary main stems. 



in the case of the main-stem whorls than in whorls borne on branches; 

 (4) the divisions of the main stem have the same characteristics in 

 respect to variation in leaf-number that the main stem proper does. 



From these facts we conclude that ivhorls borne on the main stem of 

 the plant are, as a class, clearly differentiated from ivhorls borne on 

 branches. 



VARIATION IN WHORLS ON PRIMARY BRANCHES. 



Anyone familiar with Ceratophyllum will recognize that it is the 

 primary branches which make up the greater portion of the plant. 

 They usually contribute a considerably larger number of whorls to the 

 total than does any other single division. This fact will be apparent by 

 comparing the data of table 14, which gives the frequency distributions 

 for primary branches, with those in the tables for other divisions of 

 the plant. 



