28 



BOTANY 



plant, and that of absorbing water and soluble food-compounds from 

 the earth. As the amount of the water absorbed varies with the 

 extent of leaf-surface, there is found to be a pretty constant cor- 

 respondence between the absorbing surface of the roots and that of 

 the surface of the leaves. The increase in the absorbing surface 



of the roots is brought 

 about by the ramifications 

 of the roots and the de- 

 velopment of absorbent 

 root-hairs. 



Modifications of Roots. 

 Roots are sometimes quite 

 absent, as in the aquatic 

 Salvinia and certain sapro- 

 phytic plants; e.g., Coral- 

 lorhiza. In the first case 

 slender submersed leaves 

 function as roots, in the 

 second rootlike subter- 

 ranean stems. 



Very commonly, espe- 

 cially in biennial plants 

 like the common Thistle, 

 Carrot, Turnip, etc., the 

 root is much enlarged, stor- 

 ing up during the first sea- 

 son food which is drawn 

 upon by the plant in its 

 rapid growth in the second 

 year, when flowers and 

 fruit are developed. These 

 enlarged roots may be a 

 tap-root, as in the Carrot 

 and Dock (Fig. 20), or 

 they may be lateral roots, as in the Sweet Potato. 



Aerial Roots. In the Tropics it is very common to find roots 

 developing from the aerial parts of plants. Such aerial roots are 

 occasionally met with in plants of temperate regions e.g. the root- 

 tendrils of Ivy and the Trumpet-creeper ; but it is in the moist for- 

 ests of the Tropics that these aerial roots are best seen. In many 

 species of Fig, for example, they are formed upon the branches 

 and grow downward until they reach the earth, when they fasten 

 themselves and finally develop into a stout trunk, which functionally 

 is a stem. The many trunks of the Banyan Fig are of this nature, 

 and there are numerous similar species. Very much like these roots 



FIG. 20. A, young plant of Rumex crispits, 



th enlarged tap-root. J5, y 

 Ivy climbing by root-tendrils, r. 



