222 PART III. PHYSIOLOGY. [ 51 



whilst the former means of transmission suffices for a short distance, 

 the latter is necessary when the distance to be traversed is 

 considerable. In Mimosa pudica there appears to be a special 

 tissue along which the stimulus is conducted : it belongs to the 

 bast, and consists of large elongated cells with pitted cellulose 

 walls. 



51. Combined Effects of different Stimuli. Inasmuch 

 as it is commonly the case that the motile members, whether 

 growing or mature, are irritable to stimuli of various kinds, it is 

 clear that the assumption by them of any particular position is the 

 resultant effect of the stimuli which may be acting simultaneously. 

 The phenomena in question are strikingly manifested by growing 

 members, and it is to these that the following account especially 

 refers. 



According to the position assumed in the course of their growth 

 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 plagiotropic. Most radial and isobilateral members are ortho- 

 tropic ; all dorsiventral, 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 negatively 

 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 heliotropism 

 which tends to make it curve more than it actually does. Uni- 

 lateral 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. 



