Neivcombe . — The Sensory Zone of Roots. 447 
continue to grow, and a lack of oxygen does not cause 
distortion so far as my observation goes. 
A final reference may be made to the significance and 
nature of rheotropism. Of biological significance, it is difficult 
to perceive any. The nature of the stimulus remains as 
obscure as ever. Of course there is pressure against the roots 
by the stream of water. But thigmotropism has not been and 
probably cannot be demonstrated inordinary roots by ordinary 
means. If rheotropism is not thigmotropism, we do not know 
what the nature of the stimulus is ; but until thigmotropism 
has been demonstrated in roots, no claim can be made for 
the identity of these two responses. 
