10 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



analysing morphogenesis and growth in particular, since 

 their most prominent feature is not growth but typical 

 motion. 



All of you know that the stem of a tree turns away 

 from the ground, whilst the root enters it. We speak of 

 negative and of positive geotropism in this case, for it has been 

 proved that it is gravity which determines the direction of 

 stem and of root here, in a manner that has been very 

 much elucidated by modern authors. 1 And in the same 

 style we call it positive and negative heliotropism, if a stem 

 of a plant turns toward the sun or any other source of 

 light, and if a root turns away from such sources. Thermo- 

 tropism, rheotropism, and chemotropism are similar pheno- 

 mena ; their names show most decidedly in what they 

 consist. There are a few similar phenomena in the so-called 

 stolons of hydroids. As we have said, it is only on growing 

 parts of fixed organisms that tropisms of all sorts are to 

 be observed. A marked correspondence of the directions 

 of the cause and of its immediate effect is exhibited in 

 all of them. 



Let us first state in a few words in what cases we may 

 speak of a real " direction " peculiar to an agent of the 

 medium. That a specific direction is given in the effect 

 of gravity and the rays of light going out from a radiant 

 body is clear without much explanation ; but there may be 

 direction in natural agents even when they cannot properly 



1 I refer to the work done by Noll, Nemec, Haberlandt, and many others 

 during the last ten years. Of more than usual importance seems to be the 

 discovery of Fitting (Jahrb. wiss. Bot. 44, 1907) that phototropic stimula- 

 tion may be transferred along broken (zigzag) lines, and that this stimula- 

 tion itself probably consists in a real induction of polarity in each cell 

 established from without. There is no machine-like apparatus simply set 

 going. 



