GEOTROPISM. II 455 



that the perception of the stimulus of weight is a phenomenon of much wider 

 distribution than the occurrence of geotropic reactions would lead us to 

 suppose. It can scarcely be doubted that perception of the stimulus in very 

 many cases leads to no visible result because the capacity for response is 

 wanting in the organs concerned. The leaf articulations are organs of this 

 type which have retained their power of reaction long after growth has 

 ceased in them. 



We may now close our discussion of dorsiventral organs and of the external 

 agencies which often to a very marked degree affect their reaction, and conclude 

 this lecture by a reference to twining plants which are distinguished by very 

 special behaviour towards the geotropic stimulus. 



In the plants we have been studying hitherto the organs which exhibited 

 movements of orientation had sufficient strength to carry these out in opposition 

 very often to great resistance, evolving considerable energy in the process. 

 We have seen that not infrequently plants are capable of performing far more 

 work than is required for producing movement in the passive regions. A 

 certain rigidity is a fundamental condition of activities of this nature ; when 

 that is absent, the organ, such as a prostrate stem, can never completely raise 

 itself in spite of its possessing a negatively geotropic capacity. There are 

 many plants whose shoots would lie on the ground were they not capable of 

 making use of the rigidity of other plants for their own elevation. Plants 

 of this kind we term climbers. The simplest type of climber is represented 

 by such plants as Galium aparine, which, after reaching a certain height 

 by its own unaided efforts, sinks to the ground unless it manages to come in 

 contact with some other plant to which it adheres by means of the prickles 

 which it is provided with. Other climbers exhibit more complicated apparatus 

 designed for the same purpose, such, for example, as the hooks of Uncaria 

 or Strychnos (Treub, 1882-3 ; Schenck, 1892 ; Ewart, 1898). These organs, 

 however, attach themselves to the support accidentally only, so to speak, and 

 exhibit no active movements designed for the purpose of bringing about attach- 

 ment, such as are shown by the two great series of tendril-bearers and twiners. 

 From the biological point of view these two series have much in common, 

 since both save themselves the trouble of forming any special skeletal tissue 

 in their axes, and make use of some other rigid structures as a means of support- 

 ing the weight of their leaves. In nature these supports are always living 

 or dead plants and hence twiners and tendril-bearers are dependent, quite as 

 much as epiphytes, on other vegetation. Indeed, one might compare them in 

 a sense with parasites, since, if the supporting plants be alive, they shade the 

 light from them and thus inflict injury upon them though, it is true, only 

 indirectly. The movements, however, which the twiners exhibit in their efforts 

 to grasp their supports are so distinct in aphy Biological sense from those exhibited 

 by tendril-bearers, that a combined treatment of the two series is not advisable. 

 At present we will consider twiners only, since these plants carry out their move- 

 ments by means of a special form of geotropism (Noll, 1892-1902) and are 

 thereby brought into close relation to the subject we have just been discussing. 



Twiners grow round the stem of the supporting plant in a spiral manner, 

 and since those spirals lie close to and exert pressure on the support, and since, 

 further, the twining stems are frequently rough, the attachment is thus rendered 

 very secure and slipping away from the support is practically precluded. On 

 closer examination of a twiner, e. g. Calystegia, which begins to develop in spring, 

 we note that its shoots are at first strongly orthotropic and hold themselves 

 erect in virtue of a certain rigidity which they possess. After reaching a certain 

 height, however, the apex of the shoot bends over, and that too in consequence 

 of an active movement, not merely in consequence of the weight of the apex, 

 and takes up an almost horizontal or plagiotropic position. At the same moment 

 a special type of movement begins to make itself apparent, which may be termed 



