6 PART I. ORGANOGRAPHY. 



covered with a protecting sheath or cap of older and thicker- 

 walled cells, technically called the root-cap, which affords it a 

 decided mechanical advantage in penetrating the soil. 



The absorbing surface exposed by roots to the soil is much 

 greater than is usually supposed. In most plants it is probably 

 comparable in extent with that exposed by the leaves to the air. 

 This great superficial area which roots present to the soil is due 

 partly to their repeated ramifications into fine divisions, and 

 partly to the numerous delicate root-hairs that are found on the 

 ultimate branches just back of the growing-point. The finer 

 root-branches, together with their attached hairs, are the chief 

 agents by which the plant absorbs nutriment from the soil, 

 and on them, therefore, more than on the larger roots, is the life 

 of the plant dependent. The principal reason why transplanting 

 in midsummer is so dangerous to the life of the plant, is that in 

 the process of digging up and re-setting, numerous root-tips with 

 their absorbing hairs are broken off and destroyed, so that the 

 leaves evaporate water much faster than it can be taken up by 



Vlijk M 



Fig. 2. Fig. 3. Fig. 4. 



Fig. 2. — A root-tip considerably magnified, a, the growing point ; b, root-hair ; c, the 

 root-cap. 



Fig. 3. — The tap-root of the common Stock. 



Fig. 4. — The clustered and tuberous roots of the Dahlia. 



the remaining roots, and the plant, therefore, necessarily withers. 

 Fig. 2 represents a root-tip considerably magnified, showing its 

 growing-point a, its root-hairs b, and its root-cap c. 



Roots may be classified into primary and adventitious. A 

 primary root is the downward continuation of the embryonic 

 root of the seed. Commonly it is simple, or the branches it 



