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 



