THE ROOT. 35 



may be mentioned, the napiform roots of most turnips, the 

 conical roots of carrot, salsify, etc., the moniliform roots of 

 some pelargoniums, and so on. 



Besides acting as organs of absorption and as storehouses 

 of reserve materials, roots fulfil an important function in 

 holding the plant firmly in its place. A study Mechanical 

 of the arrangement of their tissues shows a fuuotions. 

 manifest adaptation to this function, the mechanical ele- 

 ments being placed compactly at the center, a position in 

 which thej'^ are able to resist to the best advantage a 

 pulling force that tends to break the root or draw it out 

 of the ground. Such aerial roots as those of the poison 

 ivy serve to hold the stem securely to some external sup- 

 port, and the prop roots of Indian corn that arise a little 

 above the surface of the ground constitute an admirable 

 system of braces and guys, by which the stalk, with its 

 heavy load of ears, is enabled to maintain an erect posi- 

 tion. Considering the size and weight attained by a 

 single cornstalk with its fruit; and its exposure to heavy 

 winds and rain, it is difficult to conceive of a more 

 effective and, at the same time, more simple mechanical 

 arrangement. 



In their mode of growth roots exhibit a remarkable 

 adaptation to their environment. Growth in length takes 

 place just behind the tip, which is thus free to jj;,^^ „f 

 turn in any direction, curving aside as it meets growth, 

 obstacles, and directing its way towards moisture or food, 

 as occasion requires, without involving any disturbance of 

 the older parts that have already become fixed in the soil. 

 The growing point is covered by the root-cap, and thus 

 protected from injury. 



The primary root grows perpendicularly downwards, 

 but the secondary roots, reacting differently to the pull of 



