86 HISTORY OF BOTANY 



the shoot should elongate and not the vessels and fibres 

 of the root also. 



During his studies Knight observed that roots were 

 attracted by moisture, and, by a series of experiments 

 devised for the purpose, he showed that water also could 

 counterbalance the action of gravity or, as we would say, 

 hydrotropism could overcome geotropism. Once more 

 Knight misses the idea of the sensitivity of the living 

 organism to an external stimulus. These experiments 

 were detailed in a paper published in 1810. 



A third series of experiments were carried out during 

 the next two years on the apheliotropic movement of 

 the tendrils of the vine and Virginian creeper and the 

 aerial roots of ivy. The explanation he offered, however, 

 still exhibits his persistent adherence to a mechanical 

 explanation of these responses to stimulus. " The 

 external pressure," he writes, " of any body upon one 

 side of a tendril will probably drive the fluid organisable 

 matter from one side of the tendril, which will conse- 

 quently contract, to the other side, which will expand, and 

 the tendril will thence be compelled to bend round a 

 slender bar of wood." The curvature, you see, is, accord- 

 ing to Knight's idea, a passive result of the transference 

 of fluid from the pressed to the free side, not an active 

 curving in response to the pressure. 



Knight was also struck by the way in which leaves 

 had a tendency to arrange themselves so that the incident 

 light always fell on the upper surface, or, as we would 

 put it, that leaves were diaphototropic. " I will request 

 your attention," he writes, " to the power of moving in 

 the Vine leaf, on which I have made many experiments. 

 It is well known that this organ always places itself 

 so that the light falls upon its upper surface, and that if 

 moved from that position it will immediately endeavour 

 to regain it ; but the extent of the efforts it will make, 

 I have not anywhere seen noticed. I have very frequently 

 placed the leaf of a vine in such a position, that the sun 



