758 PART IV. — THE PHYSIOLOGY OF PLANTS. 



under the influence of various external directive influences, plant- 

 members may be conveniently classified into those which have 

 their long axis vertical, and those which have their long axis 

 oblique or horizontal, the former are distinguished as orthotropic, 

 the latter as plaqlo tropic. Most radial and isobilateral members 

 are orthotropic; all dorsi ventral, and some radial members are 

 plagiotropic. For instance, radial primary shoots and roots are 

 orthotropic; all dorsiventral leaves, etc., are plagiotropic ; lateral 

 branches of shoots and roots, even though radial, are plagiotropic. 



The directive influences which mainly determine the direction 

 of growth of radial primary shoots are gravity and the direction 

 of the incident rays of light, and the shoots themselves are nega- 

 tively geotropic and positively heliotropic. If only the conditions 

 are such that each side of the shoot receives an equal amount of 

 light, as when the plant grows quite in the open, no heliotropic 

 curvature takes place, and the shoot grows erect. But when one 

 side of the plant is shaded, as when it grows by the side of a hedge, 

 the shoot in most cases curves heliotropically out of the vertical. 

 This curvature is the resultant effect of the negative geotropism 

 of the shoot which tends to keep it straight, and its positive helio- 

 tropism which tends to make it curve more than it actually does. 

 Unilateral illumination usually causes some degree of curvature 

 in shoots, because, as a rule, their heliotropic irritability is higher 

 than their geotropic irritability. Exceptions to this rule have 

 been found in the inflorescences of Verbascum and Dipsacus, 

 which remain erect even when one side is shaded. 



Similarly, the influences which ordinarily determine the direc- 

 tion of growth of radial primary roots, are gravity and the distri- 

 bution of moisture in the soil. If the soil is uniformly moist, the 

 root grows vertically downwards under the influence of gravity; 

 but if the soil is not uniformly moist, the root will curve out of 

 the vertical towards the moister area, because its hydrotropic is 

 greater than its geotropic irritability. 



The conditions which determine the plagiotropic position of 

 most radial lateral branches of shoots are these : they are nega- 

 tively geotropic, and they are diaheliotropic, at least in intense 

 light. In darkness they grow erect, in virtue of their negative 

 geotropism. Some radial subterranean rhizomes are, however, dia- 

 geotropic (see p. 754). The oblique growth of lateral roots is 

 simply due to their feeble geotropic irritability. 



The conditions which determine the plagiotropic position of 



