286 BOTANY PAKt i 



of the main or lateral axes of many plants. Thus the stems of many 

 spring plants, when the temperature is low, lie flat on the soil and 

 only assume the erect position as the result of geotropism when t lie 

 temperature becomes higher. Even more important are the modi- 

 fications arising from the action of light, by which the geotropir 

 irritability of rhizomes and foliage leaves may be so modified or 

 weakened as to permit of more advantageous heliotropic positions. 



While heliotropism was increased by the impurity of the atmo- 

 sphere (p. 278), geotropism is weakened by this influence. 



C. Hydrotropisin, TJii'i'iitii/r<>j>isin. n n<l nil,, / 7V( //*/.>;//.< 



While the illumination and the position with regard to the earth are the most 

 general and important relations of the plant to its environment, and heliotropism 

 and geotropism the most widely spread reactions of the plant, they are not tin- only 

 phenomena of the kind. 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. Roots in dry soil are 

 diverted to more favourable positions by the presence of greater quantities of 

 moisture. The force of this POSITIVE HYDROTROPISM may be so great as to over- 

 come 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 spermatozoids, roots, 

 fungal hyphse, and pollen tubes exhibit positive and negative CHEMOTIUHMC 

 CURVATURES. These vary according to the concentration of the solution, .so that 

 an attractive substance, at a higher concentration, may act repulsively. THERMO- 

 TROPISM or caloritropism (due to the stimulus of heat), RHEOTROPISM (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 stand in direct relations to certain vital 

 requirements of plants ( 89 ). 



In the case of EI.EI.TROTROPISM, which has also been demonstrated in plants, i,<> 

 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 th oil- 

 well-being. The fact of the existence of electro tropism 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. 



I). The Method of Slow Rotation The Klinostut 



All the curvatures of growth previously discussed have been induced 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 eqiiallv 

 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 uniformly rotated, with a velocity sufficient to preclude the 

 continuous operation of a stimulus on any one point long enough to occasion a 

 one-sided growth. As in that case, no one side will be exclusively acted upon, 



