Tropisms and Instincts as Adaptations 403 
Strasburger states that whenever any external force, or sub- 
stance, is important to the vital activity of the plant or any 
of its organs, there will also be found to be developed a cor- 
responding irritability to their influence. Roots in dry soil 
are diverted to more favorable positions by the presence of 
greater quantities of moisture. This may, I venture to sug- 
gest, be putting the cart before the horse. The plant may 
be only able to exist whose responses are suited to certain 
external conditions, and these determine the limits of distribu- 
tion of the plant or the places in which it is found. 
A number of plants climb in a different way, and show 
another sort of tropism. Those that climb by means of ten- 
drils twist their tendrils about any support that they happen 
to come in contact with, and thus the plant is able to lift its 
weak stem, step by step, into the air. The twining of the 
tendrils is due to contact, which causes a cessation of growth 
at the points of contact. The growth of the opposite side 
continues, and thus the tendril bends about its support. In 
the grape and in ampelopsis the tendril is a modified branch. 
The stalk of the leaves in a few plants, as in Lophospermum, 
act as tendrils. Other climbers are able to ascend vertical 
walls owing to the presence of disks, whose secretions hold 
the tendril firmly against the support, as in ampelopsis. 
It is interesting to find in practically all these cases that, 
whatever the stimulus may be, the results are reached in the 
same way, namely, by one part growing faster than another. 
The fact of importance in this connection is that the plant is 
so constructed that the response is often beneficial to the 
organism. 
Before leaving this subject there is one set of responses to 
be referred to that is not the result of growth. Certain move- 
ments are brought about by the change in the turgidity of 
certain organs. The small lateral leaflets of Desmodium 
gyrans make circling movements in one to three minutes. 
No apparent benefit results from their action. The terminal 
