52] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 223 



Similarly, the influences which ordinarily determine the direction 

 of growth of radial primary roots, are gravity and the distribution 

 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 negatively 

 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, diageotropic. The 

 oblique growth of lateral roots is simply due to their feeble 

 geotropic irritability. 



The conditions which determine the plagiotropic position of 

 dorsiventral members are these : they are both diageotropic and 

 diaheliotropic. But inasmuch as their heliotropic is higher than 

 their geotropic irritability, their ultimate position is that in which 

 the incident rays of appropriate intensity fall nearly or exactly at 

 right angles upon the upper surface. 



It will be observed that, as a rule, in growing members w r hich 

 are heliotropically irritable, this irritabilit}' is higher than any 

 other ; consequently the ultimate position assumed by the member 

 is mainly determined by the direction of the incident rays of light, 

 and it is termed, therefore, a light-position (see p. 215), although 

 other directive influences may have contributed to its assumption. 



The most remarkable case of combined effects is afforded by the 

 growth of twining stems. A twining stem, at its first development, 

 is straight, but after it has come to consist of two or three inter- 

 nodes its apex hangs over to one side, for the stem is not sufficiently 

 rigid to support its own weight. It then exhibits circumnutation 

 in a marked manner. If once it comes into contact with a more or 

 less vertical support of appropriate thickness, it twines round it. 



The commonly accepted explanation of twining is that it is due 

 to the negative geotropism of the stem, combined with its circum- 

 nutation modified by contact with the support ; but it is doubtful 

 if this explanation is adequate. It has been suggested, with some 

 show of reason, that twining stems may be irritable, like tendrils, 

 though in a less degree, to continuous contact with a support. 



52. Conditions of Movement. Inasmuch as the move- 

 ments under consideration are vital, they can only take place when 



