DARWIN ON THE MOVEMENTS OF PLANTS. 511 



chanical resistance, because it occurred when there was not pressure 

 enough to produce it ; and, besides, Sachs has shown that the growing 

 part is more rigid than the part just above it, which should have 

 yielded first to resistance. Moreover, objects that yield with the 

 greatest ease will deflect a radicle. After various attempts to explain 

 the phenomenon, Darwin was led to suspect that the tip was sensitive 

 to contact, and that it transmitted an effect to the upper part of the 

 radicle, so exciting it to bend away. Such a thing had never been sus- 

 pected, although Sachs discovered that the radicle is sensitive a little 

 above the apex, and bends, like a tendril, toward the touching object. 

 Full details are given of the experiments by which this suspicion 

 was verified. We can only say, briefly, that " germinating beans were 

 pinned hilum downward inside the well-moistened cork lids of glass 

 vessels which were half filled with water, and the light excluded. 

 When the protruding radicles were the tenth of an inch or more long, 

 bits of card about one twentieth of an inch square, or bits of sand- 

 paper, were afiixed to the sloping sides of their tips by means of 

 thick gum-water, which by itself had no effect. To avoid confusion 

 from the bending known as Sachs's curvature, the bits were never put 

 in front." That the reader may have a clear idea of the kind of 

 movement excited by the bits of card, we give, Fig. 14, sketches of 

 three beans thus treated, which show the gradations in the degree 

 of curvature. Out of fifty-five beans experimented upon, fifty-two 

 were considerably bent away from the object attached, and the re- 

 maining three seemed to become sickly. As the radicle of the pea 



A. B. 



Fig. 15. Pisttm SATrvrrM: Deflection produced within twenty-four hours in the erowth of verti- 

 cally dependent radicles, by little squares of card aflSsed with shellac to one side of the apex: 

 A, bent at right angles ; B, hooked. 



was found to be rather more sensitive at a point above the apex than 

 that of the bean, he experimented with twenty-eight peas which had 

 been soaked for twenty-four hours, and then left to germinate in damp 

 sand. He tried them first with bits of card above the apex for Sachs's 

 curvature, and thirteen of them bent toward the card, the greatest 

 curvature being 62. Bits of card were then fastened to one side of 



