FUNCTION AND STRUCTURE OF ROOTS 125 



its comparative imperviousness to prevent evaporation, or 

 reabsorption by the soil, of the sap as it flows from the root 

 hairs up to the stem and leaves. 



177. The Branching of Roots. Peel off 

 a portion of the cortex from any branch- 

 ing taproot and notice the hard, woody 

 axis that runs through the interior. Pull 

 off a branch from the stem and one from 

 the root ; which comes off most easily ? 

 Examine the points of attachment of the 

 two and see why this is so. This mode 

 of branching from the central axis in- 

 stead of from the external layers, as in 

 the stem, is one of the most marked 

 distinctions between the structure of the two organs. 



178. Distinctions between Root and Stem. In stems 

 the branches always occur, as we saw in our study of 

 leaves, at regular intervals called nodes (Sec. 50), while 

 in the root they occur quite irregularly. The root grows 

 only from just behind the tip; stems increase by the 

 development of successive internodes, each of which may 

 continue to grow for some time after the development of its 

 successor (Sees. 1 53, 1 57). The stem is normally an ascend- 

 ing, the root a descending, axis ; the one bears leaves and 

 buds at regular intervals, the other bears no leaves and 

 only occasional buds of the kind called adventitious ; that 

 is, buds which appear by chance, as it were, at irregular 

 intervals. There are other distinctions recognized by 

 botanists, but they are too technical to be considered here. 



PRACTICAL QUESTIONS 



1 . Why will most plants grow so much better in an earthen pot or 

 a wooden box than in a vessel of glass or tin? (168.) 



2. Which absorb most from the soil, plants with light roots and 

 abundant foliage or those with heavy roots and scant foliage? 



3. Which will require the deeper tillage, a bed of carrots or one of 

 strawberries? (173.) 



