sect, n PHYSIOLOGY 263 



Of the changes in the geotropic conditions of plant organs due to 

 external causes, those are particularly noticeable which result from a 

 failure of a sufficient supply of oxygen, by which, for example, roots 

 and rhizomes are made negatively geotropic. And even more im- 

 portant are the modifications arising from the action of light, by 

 which the geotropic irritability of rhizomes and foliage leaves may 

 be so modified or weakened as to permit of more, advantageous 

 heliotropic positions. 



C. Hydrotropism, Caloritropism, Thermotropism, etc. 



Whenever any external force or substance is important to the 

 vital activity of a plant or any of its organs, there will also be found 

 to be developed a corresponding irritability to their influences. Eoots 

 in dry soil are diverted to more favourable positions by the presence 

 of greater quantities of moisture. The force of this positive hydro- 

 tropism may be so great as to overcome the geotropic equilibrium of the 

 roots, and thus give rise to hydrotropic curvatures. Conversely, the 

 sporophores of many mould Fungi avoid moisture. To this property is 

 due the fact, so advantageous for the distribution of the spores, that 

 their sporangiophores grow directly away from a moist substratum. 

 Corresponding to the chemotactic irritability of Bacteria and spermato- 

 zoids, roots, fungus hyphse, and pollen tubes exhibit positive and 

 negative CHEMOTROPIC CURVATURES. These vary according to the 

 concentration of the solution, so that an attractive substance, at a 

 higher concentration, may act repulsively. Thermotropism (due to 

 the stimulus of heat), Eheotropism (occasioned by the direction of 

 water currents), and Aerotropism, a form of chemotropism, are 

 additional phenomena, which have been distinguished as arising from 

 the special action of external stimuli, and which stand in direct rela- 

 tions to certain vital requirements of plants. 



In the case of electropism, which has also been demonstrated in plants, no 

 such essential relations have been discovered ; the disposition of plant organs in a 

 direction contrary to that of an electric current, seems in no way to affect then- 

 growth. The fact of the existence of electropisni in plants shows clearly that an 

 irritability may be present, from which no direct benefit is ordinarily derived, and 

 which accordingly could not have been attained by natural selection. 



D. The Method of Slow Rotation — The Klinostat 

 All the curvatures of growth previously discussed have been in- 

 duced by the one-sided action of stimuli, the source of which determined 

 the direction of the movements as well as the position of equilibrium. 

 An influence operating equally on all sides is unable to produce a 

 curvature in an organ of which the irritability is equally developed on 

 all sides. In like manner no curvatures can take place when the plant is 



