26 DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



SUMMARY. 



The various forms of leaf which occur at different parts of a speci- 

 men of Sium cicutaefolium are described and their bearing on the phy- 

 logenetic history of the species is considered. The principal conclu- 

 sions are as follows : 



There is great variability in the early nepionic leaves, the first leaf 

 being the most variable and the variability being progressively lessened 

 in subsequent growth. 



The interpolation of an undivided leaf between the 3-lobed and the 

 pinnate leaf is illogical and might be interpreted as representing phylo- 

 genetic mutation. 



To show how wholly gratuitous is the attempt to draw conclusions 

 from juvenile stages regarding ancestral adult conditions, it is pointed 

 out that in the species of Onagra, where the ancestry is definitely 

 known, the juvenile leaves may resemble more closely the adult leaves 

 of the same species than they do either the seedling or the adult leaves 

 of the parent species. 



The senescent stages are totally different from those of the seedling. 

 The leaves show a regular reduction without sudden changes of type, 

 and there is increasing constancy in form as higher stages are reached. 

 Although the several stages of Slum present the appearance of being 

 closely correlated with the successive changes in its environment, the 

 plant passes through the same stages whenever the conditions are favor- 

 able for growth, regardless of the characteristics in the environment 

 which have been supposed to determine the several types of structure. 

 This cycle of development can be repeated only through rejuve- 

 nescence. This may result from fertilization and seed-production, 

 but can be brought about in other ways. Rejuvenations of axillary 

 buds and of flower-buds are figured and described. Rejuvenescence 

 in these buds was caused by submergence in water of stems in the 

 senescent state. The later the stage of senescence reached the earlier 

 are the juvenile stages produced on rejuvenescence. Proliferations of 

 flower-buds showed several leaf-forms which are simpler than any 

 found in the seedlings. 



Rejuvenescence may be due to increased available food-equivalent, 

 or to increased lability or fluidity of the protoplasmic substances, or a 

 combination of these two factors. The stage attained in any case of 

 rejuvenescence may be due to the relative value of the resultant of these 

 two complex factors, as compared with that necessary to the production 

 of the climax type of leaf of the adult plant. 



