252 CAUSES OF DEGENERATIVE EVOLUTION 



the leaves of an etiolated plant 1 or of the muscles 

 of a limb which has been immobile for long, or in 

 the case of muscles which have become inactive 

 from disease of the central nervous system. The 

 results are similar in cases of normal atrophy. 

 In frogs, toads, and other Batrachia Anura, 2 the 

 disappearance of the tail before the adult state is 

 reached is the result of a struggle amongst the 

 cells. The active protoplasm of the muscular 

 fibres develops specially, and gives rise to many 

 cells, which enter the contractile material and 

 separate its elements. Gradually all the contrac- 

 tile material is absorbed by these isolated cells. 



Many plants, especially Sempervivum (see fig. 

 73, A, p. 236) possess a reduced stem with the 

 leaves closely massed upon it. This reduction 

 of the stem, which is nearly constant in the 



1 When a cutting from a potato or a seed (fig. 75) is allowed to 

 sprout in the dark, the young stems assume characters different 

 from those of plants grown in light. The absence of chlorophyll 

 produces important modifications of growth. In light the stem 

 is short, and the leaves are large and expanded ; in darkness the 

 stem is very long, and the leaves are much reduced. This atrophy 

 of the leaves is the result of the struggle for existence amongst the 

 organs of the plant. Light increases the rate of transpiration, 

 which is chiefly due to the presence of chlorophyll. As chloro- 

 phyll is most abundant in the leaves, the transpiratory current 

 sets strongly towards them, carrying in it the nutritive materials 

 for the formation of new cells. On the other hand, in etiolated 

 plants, transpiration is slower, and the nutritive materials delayed 

 in the stem give that the opportunity for specially active growth, 

 which takes place at the expense of the leaves. 



2 Metchnikoff, Annales Inst. Pasteur, January 1892. 



