THE ROOT-TIP. 



39 



the root-tip is sensitive to mere contact, since a certain amount of 

 injury to the tissues was inflicted by the method employed ; and 

 this objection has not so far been fully met. Whatever may be 

 the true explanation, it is a fact that roots find their way into 

 worm-burrows, and otherwise follow in the earth lines of least 

 resistance, in a way that is strongly suggestive of a power to dis- 

 criminate between harder and softer regions of the soil. 



A. B. 



Fig. 7. A Seedling of Pea, with radicle extended horizontally in damp air, with a little 

 square of card affixed to the lower side of its tip, causing it to bend upward in opposition 

 to gravity. The deflection of the radicle after twenty-one hours is shown at A, and of 

 the same radicle after forty-five hours at B. (From Darwin's Power of Movement in 

 Plants.) 



An electric current passed through the tip induces curvature, 

 and in some cases roots have been found to bend away from the 

 light. Although it can hardly be supposed that sensitiveness to 

 these stimuli is of any special use to the plants, such behavior, 

 taken in connection with the highly useful modes of sensitive- 

 ness above described, surely indicates an almost animal-like irrita- 

 bility of the organ in question. 



From what has been said of the curvature of young roots, it is 

 obvious that, whenever the tip proper is stimulated, the effort 

 must be transmitted to the part above, since it is only this upper 

 portion which curves. A similar transmission of stimulus takes 

 place in the leaf of the sensitive-plant, and both suggest an anal- 

 ogy with the propagation of an impulse along the nerves in ani- 

 mals. Nevertheless, in the absence of all proof that anything 

 resembling nerves entered into the structure of plants, the anal- 

 ogy referred to was deemed rather fanciful, and certain mechani- 

 cal explanations of the phenomena were offered as more in keep- 

 ing with what was known. A few years ago, however, Gardiner's 

 demonstration of the continuity of protoplasm in plants * rendered 

 the mechanical theories superfluous, by showing that the living 

 matter of adjacent cells was connected by delicate protoplasmic 

 threads which might fairly be considered the analogues of nerves. 

 The essential similarity of many plant movements with those of 

 animals is thus seen to be even closer than was at first supposed, 



* Philosophical Transactions, 1883, p. 817. 



