20 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



The rejuvenescence of all these buds as the result of a change from 

 an aerial to an aquatic habitat is in perfect agreement with Burns's 

 ( 1904) interpretation of the changes induced in a similar way in Proser- 

 pinaca pahistris and very greatly strengthens his position. The fact 

 that the juvenile leaves of that species are finely dissected in the manner 

 so frequently found in the climactic or ephebolic stages of typical sub- 

 merged plants is either purely a coincidence or may be related in some 

 unknown way to the past environmental relations of the species, as sug- 

 gested by Goebel (1899-1901, p. 546). In Stum, where the juvenile leaf 

 is not at all of the hydrophytic type, submergence in water does not 

 produce a dissected, hydrophytic leaf, but the mesophytic form of leaf 

 characteristic of the Sium seedling. 



This change of view regarding the effect of water in producing the 

 modified leaf-form of these plants does not detract in the least from the 

 value of the negative results of McCallum's (1902) investigation into 

 the nature of the stimulus involved, but gives those results a bearing on 

 the phenomenon of rejuvenescence instead of the change from a terres- 

 trial to an aquatic type of leaf. To ascribe a phenomenon to good or 

 poor vegetative conditions (Burns, 1904, p. 586) does not yet trace it 

 very near to its ultimate cause or causes, but perhaps is as definite as 

 the present state of our knowledge regarding rejuvenescence would 

 warrant. The great vigor of the rejuvenated buds in Sium is scarcely 

 consistent with the view that they are due to starvation or any other 

 poor vegetative conditions, and, indeed, when we find flower-buds, rep- 

 resenting as they do the low ebb of vegetative vigor, suddenly awakened 

 into a new cycle of vegetative development we should assume that they 

 have found good rather than poor vegetative conditions. 



It is evident that a distinction must be made here between the cause 

 of rejuvenescence or the reawakening into the ascending side of the 

 vegetative cycle and the cause which determines just what point in the 

 cycle shall be attained in any specific case. I would offer the tentative 

 suggestions, (a) that a process of senescence resulting in a checking 

 or a cessation of growth is a necessary condition antecedent to reju- 

 venescence, and (b) that the cause of the reawakening may be due to 

 either or both of two complex factors, namely, an increase in available 

 food-equivalent and an increased lability or mobility (perhaps largely 

 fluidity) of the protoplasmic substances. The former may be predomi- 

 nantly o];)crativc when lateral buds are forced into development by 

 checking the growth of a terminal bud, and the latter may be the domi- 

 nant factor in cases of submergence, (c) The causes which determine 

 the point in the cycle which shall be attained under any specific condi- 



