Various Physical Factors 349 



its root in plaster and pulled gently. The soft central tissue disappeared 

 and a solid vascular core resulted. Flaskamper, who repeated and con- 

 firmed this, found that roots which had been subjected to traction had 

 somewhat less tensile strength than the controls. Newcombe (1895) 

 stretched roots of sunflower and squash and observed that they grew 

 somewhat stouter and were definitely stronger than the controls. Jaccard 

 (1914) studied some experiments of nature in this field, notably cases 

 where a small root crosses a larger one and is stretched by the growth in 

 diameter of the latter. In the root under tension the cell walls of the 

 wood were thicker than in the control (Fig. 16-2). The amount of me- 

 chanical tissue was less, however. 



Fig. 16-3. Effect of tension on a tendril. 

 Device for subjecting part of a tendril 

 to tension by weight over pulley (A) 

 and for relieving the other part by hav- 

 ing a cord bear all the tension ( B ) . 

 Stimulus of contact is the same in both. 

 ( From Brush. ) 



Among the organs of a plant most commonly subject to tension in na- 

 ture are tendrils. Many experiments have been undertaken with them 

 to determine whether traction (pulling) affects their structure. Much 

 difficulty was found by early workers in separating the effects of contact 

 (to which stimulus tendrils are particularly susceptible) from traction. 

 Brush (1912) placed a tendril in lengthwise contact with a thread to 

 the free end of which, thrown over a pulley, weights were attached. In 

 the control, this thread was continuous and sustained all the pull. In the ex- 

 perimental one the thread was interrupted in the middle so that the 

 tendril itself bore all the pull. Each tendril was thus in contact with the 

 thread through all, or almost all, its length, but one was under tension 

 and the other was not (Fig. 16-3). In both, there was more xylem than 



