The most striking case of geotropic response is seen in plants that climb up the stems of other plants. The twining around the support is the result of a geotropic response of the sides of the stem. The young seedling plant stands at first erect. As its end grows it begins to curve to one side in an oblique position, and this is due to an increase in growth on one side of the apex of the shoot. As a result the stem bends toward the other side. Not only does the end “sweep round in a circle like the hands of a watch,” but it rotates on its long axis as it revolves. As a result of this rotation “the part of the stem subjected to the action of the lateral geotropism is constantly changing; and the revolving movement once begun, must continue, as no position of equilibrium can be attained.” This movement will carry the end around any support, not too thick, that the stem touches. Most climbers turn to the left, _i.e._ against the hands of a watch, others are dextral, and a few climb either way.[32] Strasburger states that whenever any external force, or substance, is important to the vital activity of the plant or any of its organs, there will also be found to be developed a corresponding 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 suggest, 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 distribution of the plant or the places in which it is found. Footnote 32: These cases recall the spiral growth of the shell of the snail, but the spiral in the latter is due to some other factor. A number of plants climb in a different way, and show another sort of tropism. Those that climb by means of tendrils 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.