HELIOTROPISM. 393 



the spectrum, has a retarding effect upon the growth of certain 

 parts, — for instance, upon that of shoots, — exhibiting itself 

 in the curvature of the part towards the side of greatest iUu- 

 mination. Such curvatures are said to be heliotropic. It is, 

 liowever, well known that the shoots and some other parts of 

 a few plants turn away from the light ; such are termed 

 aphelidtropic.^ 



1030. Little is known positivelj' as to the natui'e of the influ- 

 ence which light exerts upon growth. The studies of Vines 

 have shown that the influence is largel3- due to the modification 

 of the turgescence of growing cells. ' ' The conditions of growing 

 and of contractile cells are in some respects the same. Turgidit}' 

 is essential to the proper fulfilment of the functions of both, and 

 it has been shown that light has the power of inhibiting, more or 

 less completel}', the activity of both. The most general case of 

 the action of light upon growing cells has been shown to be a 

 diminution in the rapidity of their growth. The cell with dimin- 

 ished or arrested growth may be fairly- compared with one of the 

 cells of a rigid motile organ. In both, the micellse of the pi'o- 

 toplasm are in a state of stable equilibrium so that they do not 

 jield, in the former case to the force which tends to separate 

 tiiem, namely, the pressure of the cell contents, and in the latter 

 to the force which tends to bring them nearer together. The 

 theory that the action of light upon growing cells and upon those 

 of motile organs is due to such a modification of the relations 

 existing between the micellse of the protoplasm that the mobility 

 of the micelloe is diminished, thus gives a satisfactorj- explana- 

 tion of many phenomena which at first sight seem not to have 

 much in common."^ 



1031. Hydrotropism. It has been shown by several experi- 

 menters that rootlets when developing in moist air deviate 

 towards a moist surface. This phenomenon, whicli has been 

 examined in detail by Sachs, is termed Hydrotropism. The 



1 In order to examine the effects of the different parts of the spectrum upon 

 the growth and movements of plants, the student should cultivate in cases of 

 jjlass of different colors two or three seedlings, as many bulbous plants, and 

 some well-rooted cuttings of hardy house-plants, for instance Pelargonium. 

 Observe whether the growth is more or less rapid under blue glass, and note 

 whether all the seedlings circumnutate in the same manner in the different 

 cascjs. It should be borne in inind that the bulbous plant as it stai-ts has a 

 generous supply of available food, whereas the seedling has a more scanty stoit, 

 and the cutting very little. 



2 Arbeiteii 4es hot. Irjst. in ■Wiirzburg, 1878, p. 147- 



