CHAPTER III. — SPECIAL PHYSIOLOGY OF MOVEMENT. 759 



dorsiventi^l members are these : they are both diageotropio and 

 diaheliotropio. But inasmnch 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 which 

 are heliotropically irritable, this irritability is higher than any 

 other ; consequently the ultimate position assumed by the mem- 

 ber is mainly determined by the direction of the incident rays of 

 light, and it is termed, therefore, a light-position (see p. 748), 

 although other directive influences ma}' 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 develop- 

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

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

 sufficiently rigid to support its own weight. It then exhibits cir- 

 cumnutation 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 analysis of the phenomenon of twining is still a matter of 

 'discussion. The main facts are the following. Twining stems are 

 strongly negatively geotropic ; this is proved by the fact that they 

 will not twine round very oblique or horizontal supports. Again, 

 contact with a support interferes mechanically with the circum nu- 

 tation of the stem, and causes it to take place in an orbit of which 

 the support is the axis. Finally, a twining stem usually undergoes 

 torsion, that is, a twisting about its own axis, and this has been 

 suggested as an essential factor in the process of twining ; but there 

 is not evidence to support this suggestion. 



The commonly accepted explanation of twining is that it is 

 due to the negative geotropism of the stem, combined with its cir- 

 cumnutation 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 are irritable, like 

 tendrils, though in a less degree, to continuous contact with a sup- 

 port. It is admitted that the stem of the Dodder (see pp. 685, 

 743) is thus irritable. If this be so, then the irritability of twining 

 stems is the most important factor in the process. 



In order to investigate independently the degree and the kind of geotropic 

 and heliotropic irritability of a member, it is uecessary to be able to exclude 



