776 REMARKABLE PROPERTIES OF ROOTS. 



divided up into longitudinal strips, all of which were supposed not to grow at 

 the same time or to an equal extent, but rather that the wave of stronger growth 

 continually passes from one strip to the next one. This movement, however, like 

 that of twining stems is probably an alternate bending towards the different radii 

 of a circle drawn round the root, and since it is combined with an elongation of the 

 part of the root in question, the growing root-end describes a spiral line. 



The bend caused by the diversion of the root is either produced by a one-sided 

 contraction, or by a one-sided elongation. Since the bend oecui's in the growing 

 portion of the root, a more vigorous growth on one side may be regarded as 

 the immediate cause of the bend, and every impetus which would promote such 

 unilateral growth would also cause a bending. The bending of roots which 

 shun dry places may perhaps be referred to a withdrawal of water from one 

 side of the root-tip. Thus if the root lies imbedded between a damp and a dry 

 layer, that side which abuts on the dry stratum will transpire more actively than 

 the other, and it has been suggested that this active transpiration in some way 

 promotes an increased growth in length in that half, and in consequence of this 

 unilateral elongation on one side, the other half, adjoining the damp layer, will 

 become concave. 



The idea that the curvature is not produced directly in the place where the 

 external stimulus operates, but in the growing region lying behind the stimulated 

 root-tip, is much more interesting than these purely mechanical explanations. 

 According to this view the stimulus is transmitted as in the leaves of the Sundew, 

 Fly-trap, Aldrovanda, sensitive plants, and many other cases. The active stimuli 

 may be afforded by pressure, cold, dryness, and probably chemical conditions also. 

 Gravity, too, may be looked upon as a stimulus, indeed as one which influences 

 the direction of growth. It is believed that gravity acts on the root-tip as a 

 stimulus to growth and that this stimulus is conveyed to the growing region 

 behind, and that in consequence the primary roots grow down towards the centre 

 of the earth. But as primary roots are able to penetrate into mercury, and to bore 

 through paper, in their downward growth, something more than mere weight oper- 

 ates, since this would not be the case if the roots were influenced by gravity alone. 



The part of the growing root most sensitive to stimuli is — so far as experi- 

 mental evidence points — the tip, and the phenomena which are exhibited in 

 consequence of its great sensitiveness are so astounding that Darwin compared the 

 root-tip to the brain of lower animals. He writes, " it is hardly an exaggeration to 

 say that the tip of the radicle thus endowed, and having the power of directing 

 the movements of the adjoining parts, acts like the brain of one of the lower 

 animals; the brain being seated within the anterior end of the body, receiving 

 impressions from the sense-organs, and directing the several movements ". 



Remarkable and interesting as are these vital phenomena observed in roots, 

 there is still much to be wished for in the matter of their explanation and clear 

 comprehension. Here, as in so many similar cases, a phrase, a technical term, 

 a word, is introduced to designate the process observed, and not infrequently 



