332 



CAUSES OF VARIATION 



and gravity are opposed to each other the former is by far the 

 stronger force. 1 



ii. If a long piece of stem be suspended by both ends, roots 

 will start freely along the curved lower surf ace of the bent stem. 

 If now the bent stem be reversed, roots will not only form less 

 freely, but they will mostly be along the lower or under side of 

 the arch, the concave instead of the convex side of the curve, 

 and clustered closer to the basal end. 2 



SECTION VIII INTERNAL FACTORS IN REGENERATION 



Even among plants it is noticeable that internal influences 

 are strongly involved in the regenerative processes. Indeed, 

 one point of difference between plants and animals is that the 

 latter regenerate directly from the cut surface, while the former 

 regenerate by means of buds thrown out at the side and just 

 behind the point of severance. 



Again, it is to be observed that those organisms whose 

 regeneration is influenced by gravity are the attached species, 

 that maintain habitual and constant relations to gravity, but that 

 those species which, like most animals, move about freely, 

 regenerate according to internal influences, except in so far 

 as vital processes are involved through food, temperature, or 

 chemical conditions. In other words, the study of regeneration 

 is essentially a study of internal forces, and even among plants 

 subject to the influence of gravity the internal factors are yet 

 the dominant ones. They are well worth study, therefore, as 

 bearing upon the subject of this chapter and upon the inherent 

 forces of organized and living matter. 



Polarity and heteromorphosis. If a short piece be cut from 

 the anterior end of an earthworm the main piece regenerates 

 promptly, the small piece with difficulty or not at all'; and the 

 same is true as to the posterior end. Again, if the short piece 

 regenerate at all, it does not complete itself by growing a new 

 worm, but by growing a reversed head (or tail) like itself, so 

 that we may have an individual with two heads and no tail, or 

 vice versa. In other words, the polarity is reversed ; for if the 



1 Morgan, Regeneration, p. 78. 2 Ibid. pp. 79-80. 



