LAND PLANTS AND WATEK-SUPPLY 257 



wall, is the solution of the water-supply problem, and 

 along with it of the means of obtaining other nutrient 

 substances. And such a solution sets the aerial portion 

 of the plant free to protect itself by various devices 

 against the ravages of heat and cold, of drought, of 

 bacteria and beast. 



I think it was Darwin who compared the root-tip to 

 the brain of an animal. The manner in which it in- 

 sinuates itself into rock crevices, negotiates stones, and 

 searches the soil for its richest supplies, is certainly sug- 

 gestive of sagacity. The root-hairs cling to particles of 

 soil enveloped with films of water containing dissolved 

 mineral salts. Sometimes they clasp these water-coated 

 particles so tightly that they can only with great diffi- 

 culty be removed. We can see this when we pull up 

 seedlings, which always bring soil along with them, and 

 can never be pulled " clean." 



But if the leaves of these normal land plants cannot 

 absorb water by their surface, and it is inadvisable that 

 they should, their arrangement and form is usually such 

 as to make them conduct and turn off from their sur- 

 face the rain-water which falls upon them, and to cause 

 it to fall on the ground in positions where it is most 

 acceptable to the roots ; or perhaps it would be more in 

 accord with the facts to say that the roots reach out to 

 the area of the soil which is kept moist by water dripping 

 from the leaves. Let us use the common cultivated 

 Beet, familiar to all, to illustrate our meaning. The 

 stout, fleshy, tapering root descends vertically into the 

 soil. It gives off short, thin branches, which never 

 extend far beyond the main root from which they 

 originate, and it is these branches which are furnished 



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