DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



tosynthetic work when submerged to allow the plant to continue its 

 growth, and death speedily follows complete submergence. It might 

 be assumed that the dissection is a product of the environment instead 

 of an adaptation to it, and the apparent correlation of these three stages 

 of Slum with different environmental conditions would accord well with 

 the contention of Klebs (1903) and others that external factors are 

 directly determinative of the form and activities of plants. But the 

 plants which appear to demonstrate this proposition are quite excep- 

 tional. In Sium cicutaefolium this direct relation between the form 

 and the environment proves to be only apparent, and this species seems 

 to agree with the vast majority of plants in presenting a rather definite 

 cycle of development which it passes through whenever the environ- 

 ment supplies conditions which are favorable to growth. In such cases 

 the environment is only indirectly determinative of vegetative activities. 

 It supplies the energy, but the mechanism of the protoplasm determines 

 what shall be the product, just as the muscles of the hand and arm used 



in winding a clock supply 

 the power which is con- 

 verted by the peculiar 

 mechanism of the clock 

 into movement of the 

 hands, ticking of the es- 

 capement, and striking of 

 the hour. 



When a cycle of vege- 

 tative activity comes to a 

 close with the senescent 

 stages it can be repeated 

 only as a result of some 

 process of rejuvenescence. 

 Fertilization and seed pro- 

 duction are to be looked 

 upon as preparatory to or 

 part of the most common 

 process of rejuvenes- 

 cence, as was pointed out 

 long ago by Alexander 

 Braun (1851), but this is 

 not the only manner in 

 which it may be brought about. Whenever a bud grows immediately 

 without a resting period there is no sign of rejuvenescence and it con- 



c 



<0 



Gameto- 

 phyte 



Spores 



Gametes 



fertilization 



FIG. 4. Diagram of the life-cycle of a spermatophyte, 

 showing the relation of the processor rejuvenes- 

 cence as a "short-cut" across the cycle. Upper 

 "rejuvenescence" line may represent the rejuve- 

 nescence of buds at base of stem (fig. 7) ; the lower, 

 the proliferations of the flower-buds (figs. 8 and 9). 



