618 The Outline of Science 



avoided with care. If the root sustains injury, a stimulus 

 is immediately transmitted to the growing part, and the root 

 bends away from the quarter where the wound was inflicted. 

 When the exploring root-tip comes near a spot where water 

 occurs with food-salts in solution, it at once turns in that 

 direction, and when it reaches the place develops such absorp- 

 tive cells as are adapted to the circumstances. 1 



We have seen what part the roots play, in drawing nourish- 

 ment from the soil for the plant. The part of the growing root 

 most sensitive to stimuli is the tip, and the phenomena which are 

 exhibited in consequence of its great sensitiveness are so astound- 

 ing that Darwin compared the root-tip to the brain of one of the 

 lower animals. He wrote: 



It is hardly an exaggeration to say that the tip of the radicle 

 thus endowed, and having the power of directing the move- 

 ments 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. 



But modern botanists describe the same facts in less pictur- 

 esque language in terms of engrained "tropisms." 



The roots penetrate and explore and feel their way under- 

 ground, touching here, recoiling there, and insinuating themselves 

 in such a way that they search every part of the soil. The length 

 of roots is sometimes very great. The root-system of a large 

 cucumber plant was estimated by Mr. S. Clark in all its rami- 

 fications at 25,000 yards (fourteen miles), but this is open to 

 doubt. 



Clover roots are said to go down to depths of six or nine feet, 

 but many weeds go deeper still. Coltsfoot, for instance, may 

 be found, living at a depth of twenty spades. In Egypt and 

 other places the roots of acacias go down to twenty feet, or 



1 Kerner. 



