THE POWER OF GROWTH IN PLANTS 



239 



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 culti- 

 vated 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 atmos- 

 pheres 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 FlG 10 showing growth 

 mechanical tissue, to render the union of the of tissue 0Ter street si s n 



,..,,., . „ placed on tree. The growth 



plant with the support more firm. is restl . icted on , y at ne 



The formation of mechanical tissue in a ten- point. The sign acts as a 



-, ■-, ■ n -li .in- ±i ± -1 -i j? j/i constant stimulus, inducing 



dnl is well illustrated m the tendril ot the com- the callus t0 grow over it 

 mon 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 turges- 

 cence 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 presure in the cells for the mere purpose of increasing its clasp- 

 ing powers. 



