LEAVES 597 



regarded as an amphibious plant tending once more toward meso- 

 phytism. However, there is no valid reason for supposing that juvenile 

 leaves, if indeed such terms as juvenile and adult represent the facts, 

 can furnish a trustworthy clue to ancestral adult forms.* Various fac- 

 tors may induce the replacement of the adult by the juvenile state, a 

 phenomenon known as rejuvenescence. This is not regarded as a reac- 

 tion to a new condition, but as an indication of a sudden shock, which 

 causes the plant to return to a youthful stage. External factors deter- 

 mine when, but not what, the change shall be. The development in 

 Sium of mesophytic juvenile leaves rather than of dissected leaves, 

 when an adult stem is placed in water, is cited as supporting the reju- 

 venescence hypothesis. 



The rejuvenescence theory, at best, is a statement of facts rather 

 than an explanation, and is likely to obscure the truth. To say that 

 a change in form is due to rejuvenescence, is merely to say that it is due 

 to unknown causes, the latter statement being less misleading than the 

 former. But in such a plant as Stigeocloniiim, or even Proserpinaca 

 at certain stages, it is scarcely correct to speak of rejuvenescence, inas- 

 much as definite external factors produce definite results, obliterating 

 any supposedly normal succession of stages. In the poison ivy the 

 juvenile stage may be eliminated even in the seedling, if the developing 

 plant is well nourished. Here and in Castalia and Sagittaria, where 

 water as a formative factor appears to be replaced by the complex of 

 factors known as nutrition, the assumption of such a complex is prefer- 

 able to the assumption of rejuvenescence, particularly because future 

 researches may analyze nutrition into its component factors. 



The advantages of leaf variation in amphibious plants. — The strik- 

 ing plasticity of amphibious plants has led to a search fco: marked 

 advantages in the different forms produced, but no such marked advan- 

 tages are known, nor is there any satisfactory proof that the reactions are 

 adaptive. The structural features of the two forms clearly are beneficial, 

 the water leaf often having capacious air spaces and epidermal chloro- 

 phyll, while the air leaf has small air spaces and abundant stomata. In 



' A more tenable theory than that of recapitulation is the repetition theory, which 

 represents that present juvenile stages repeat the juvenile stages of ancestral forms; even 

 here it is to be recognized that juvenile as well as other stages are subject to evolu- 

 tionary modification. If a present juvenile stage happens to resemble an ancestral adult 

 stage, it means merely that the ancestral form, as compared with the present form, 

 changed but little in passing from youth to maturity. In many cases, as in Proserpinaca, 

 it is thought that the juvenile leaves differ greatly from ancestral adult leaves. 



