Page:Popular Science Monthly Volume 83.djvu/243

Rh but we have been unable to find that more than 2 atmospheres were involved. Professor Sachs, with the same data, estimated that the cell pressure developed was equivalent to a little more than one atmosphere.

Climbing and tendril-bearing plants, of which there are almost countless varieties, react to what is termed contact stimulation. Besides the many varieties which decorate our verandas and which are cultivated in our gardens for food, there are others with sensitive petioles (clematis and hook plant—Uncaria) which assist in anchoring the plant to supports. We have collected considerable data on the power displayed by tendrils and twining stems in clasping a support. Notwithstanding that the clasping results from the stimulation of the tendril, brought about by prolonged contact, the osmotic pressure does not ever appear to exceed the normal, only one to three atmospheres being found in these experiments. On the other hand, the effect of stimulation by contact in this case is to transmit the stimulus along the tendril, resulting in the formation of a spiral, and in most cases, if not all, the plant energy induced by the stimulus is directed towards the formation and modification of mechanical tissue, to render the union of the plant with the support more firm.

The formation of mechanical tissue in a tendril is well illustrated m the tendril of the common grapevine, and in various hook climbers. At first the tendrils of the grapevine are quite delicate and even edible, but later they become extremely hard and wiry. It would manifestly be a waste of energy from the economic point of view for tendrils to develop excessive clasping strength by means of an increased cell turgescence or osmotic pressure, since the clasping strength resulting from the normal turgidity or osmotic pressure of the cells is sufficient to answer all requirements. On the other hand, the increased production of mechanical tissue or a modification in the elasticity of the tendril is obviously of great advantage to it from the biological point of view. What is true for tendril plants appears to be true for climbing plants, such as the bean, as well as of plants with sensitive petioles, since there is no loss of energy displayed in the development of a superfluous osmotic presurepressure [sic] in the cells for the mere purpose of increasing its clasping powers.