DEVELOPMENT OF SIUM CICUTAEFOLIUM. 21 



tion may well be the relative degree of one or other of these factors or 

 of their resultant, as compared with that necessary for the produc- 

 tion of the climax type of leaf of the adult plant. This is essen- 

 tially in accord with Burns's (1904, p. 586) suggestion of poor vegeta- 

 tive conditions, and also with the earlier observations of Cushman 

 (1902, p. 885) that weak individuals and those growing on poor soil or 

 with insufficient moisture present earlier stages than do more vigorous 

 specimens or those growing under more favorable conditions with re- 

 spect to soil and water-supply. As applied to Proserpinaca this would 

 mean that the juvenile type of leaf in the submerged plant is primarily 

 the result of protoplasmic dilution, but that the failure to attain the 

 adult condition is due to less favorable relations as to food-supply. Or- 

 dinarily in submerged plants of Proserpinaca these two factors may be 

 conceived to balance each other in such a way as to produce the dis- 

 sected leaf continuously, but an unusually vigorous plant may be able 

 to provide a food-supply sufficient to overbalance the effect of dilution, 

 and so give rise to the adult type of leaf which is occasionally seen 

 even in submerged plants. When the dissected leaf is produced in aerial 

 parts it is primarily due to a condition as to food supply better than 

 that available in the resting condition, but not equal to that necessary 

 for the production of the adult leaf-type. With the return of spring, 

 conditions for food-manufacture are improved and the adult leaf ap- 

 pears at once. 



An explanation of senescence in the same terms would ascribe it to 

 decreasing lability or fluidity unaccompanied by a compensatory in- 

 crease in the available food-equivalent — a condition coming just as 

 fully under the head of poor vegetative conditions as does that pro- 

 ducing the juvenile stages. 



Not only are different regions of the axis supposed to bear leaves 

 characteristic of dift'erent periods in the history of the species, but it 

 is maintained by both Jackson (1899) and Cushman (1902, 1903) that 

 certain parts of the leaf, especially the apical region, likewise show 

 atavistic conditions. Leaving wholly aside for the present the relation 

 between the parts of the present adult leaf and the form of the ancestral 

 leaf, the question involved in this proposition is the localization of the 

 differentiative activities in the leaf. The simpler character of the apical 

 region in many leaves is undeniable, and it may correspond in certain 

 cases, therefore, with ancestral conditions, though this has not been 

 proved in any specific case. But the simplicity of the apical region has 

 led both of these writers into error when they conclude that new char- 

 acters arise at the base of the leaf. After examining: manv leaves and 



