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 become attached grows 
longer and longer, but finally shrivels, usually without coiling. 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 different periods of development, the dodder? 
are unique (see further Part III, fig. 1081). 
(3) Traumatropism 
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 
