234 The Living Plant 



ingenious and highly probable theory has been advanced in 

 explanation, viz., that the epidermal cells, focussing the light in 

 a special manner, are light-sensitive organs, and that the leaf 

 keeps turning and shifting until all of these cells receive their 

 full quota of light at the most desirable angle. In some other 

 cases, however, the reception of the light stimulus is known to 

 take place in a specialized spot, as for example in the seedlings 

 of Grasses, which are light-sensitive only in the tip of the first 

 sheathing leaf. The same thing is true, for several stimuli, of 

 the growing-point of the root, and other cases are known. Evi- 

 dently some such structures advance pretty far in the direction 

 of the special sense organs of animals, such as eyes.* 



Thus much for the phototropism of stems, leaves, and roots: 

 what now of flowers and fruits? As to flowers, they turn their 



* The localized reception of stimuli by the growing points of the roots is strikingly 

 expressed by Darwin in the closing paragraph of his great book, The Power of Move- 

 ment in Plants; and this passage illustrates so well a number of other phases of 

 irritable responses that it is here reprinted in full. 



"We believe that there is no structure in plants more wonderful, as far as its 

 'functions are concerned, than the tip of the radicle. If the tip be lightly pressed 

 'or burnt or cut, it transmits an influence to the upper adjoining part, causing it 

 'to bend away from the affected side; and, what is more surprising, the tip can 

 'distinguish between a slightly harder and softer object, by which it is simultane- 

 'ously pressed on opposite sides. If, however, the radicle is pressed by a similar 

 'object a little above the tip, the pressed part does not transmit any influence to 

 'the more distant parts, but bends abruptly towards the object. If the tip per- 

 ' ceives the air to be moister on one side than on the other, it likewise transmits an 

 'influence to the upper adjoining part, which bends towards the source of moisture. 

 ' When the tip is excited by light (though in the case of radicles this was ascertained 

 'in only a single instance) the adjoining part bends from the light; but when excited 

 ' by gravitation the same part bends towards the center of gravity. In almost every 

 'case we can clearly perceive the final purpose or advantage of the several move- 

 ' ments. Two, or perhaps more, of the exciting causes often act simultaneously on the 

 ' tip, and one conquers the other, no doubt in accordance with its importance for the 

 'life of the plant. The course pursued by the radicle in penetrating the ground 

 'must be determined by the tip; hence it has acquired such diverse kinds of sensi- 

 'tiveness. It is hardly an exaggeration to say that the tip of the radicle thus en- 

 ' do wed, 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." 



