472 PHYSIOLOGY 



maximum length, but from that time grows only in such a way as to 

 throw it into the spiral coils. One which does not beconu- attached grows 

 longer and longer, but finally shrivels, usually without coiUng. Sooner 

 or later, upon the cessation of this second phase of growth, the phase of 

 maturation is marked by the development of mechanical tissues, which 

 add strength to the elastic coils. The nature of the stimulus that brings 

 about the final coiling is uncertain. It may be the strain from the weight 

 of the plant after becoming fastened, or the spreading stimulation from 

 the contact pressure (for the attachment coils compress the support), 

 or some unsuspected stimulus may be brought into action. There are 

 many other stimuli which will evoke reactions from the tendrils, but 

 none which in nature has any importance. 



Sensitive petioles. — There are other plant organs that behave in a. similar way 

 to the tendrils, though none of them is so sensitive. The petioles of Clematis and 

 of the climbing Tropaeolum, or " nasturtium," are familiar examples. While such 

 petioles do not wrap themselves around the support nor form spiral coils as well as 

 a tendril does, nevertheless they are efficient prehensile organs, enabling the plants 

 to climb high. 



Dodders. — Any account of twining and climbing plants would be 

 incomplete without mention of the dodders (Cuscuta), leafless yellowish 

 parasites that wind their stems around and clamber over erect herbaceous 

 plants, sending haustoria into their stems, whence they obtain food and 

 water. In the first stages of development, the species that have been 

 studied germinate in the soil, and the young seedling behaves as a twiner; 

 but shortly after it has found a suitable host and begun to twine around it, 

 the lower part of the stem dies away, while the upper part continues its 

 growth at the expense of the host. The further twining, however, in- 

 stead of being dependent upon gravity, is the result of a contact stimulus 

 like that which enables tendrils to secure a hold, so that the parasite 

 enwraps supports in all sorts of positions. In the possession of these 

 two modes of response at diflferent periods of development, the dodders 

 are unique (see further Part III, fig. 1081). 



(3) Tiaumatropism 

 The wounding of plants produces immediate reactions, mostly invisible, but root 

 tips may be so wounded as to lead to curvature. If an active tip be branded on 

 one side with a hot iron or glass rod, or if it be similarly cut or otherwise injured, the 

 tip will turn to one side. When the injury is severe, this may so seriously impair the 

 tissues on the injured side that their growth will cease, and the injured side will 

 become concave near the point of injury, because there the tissues shrivel and the 

 growth of the other side goes on. This is not a true reaction, since the result is 



