THE ROOT. 7 



the stimulation of contact with a supporting surface, in the same manner 

 as the adventitious (true) roots of the strawberry runner are called forth 

 by contact with the damp earth. Of this kind of adventitious roots many 

 illustrations might also be adduced ; one more will suffice, the common 

 ivy (Hedera Helix L.) (Fig. 12). 



Some plants jDroduce a kind of adventitious roots by which they attach 

 themselves to other herbs or shrubs and draw their nourishment from 

 them. The common dodder (Fig. 13) will serve as an illustration. The 



Fig. 13.— Dodder {CusctUa), parasitic upon another plant. 



seeds of this plant germinate in the ground ; the stems grow until some 

 suitable support is reached, then twine about it and put forth a sort of 

 rootlets by means of which nourishment is drawn from the supporting 

 plant. Then direct communication with the earth is cut off by the death 

 of the stem below the adventitious rootlets, and the j^lant thenceforth lives 

 as a true parasite. 



The general structure of the root does not differ greatly from that of 

 the stem. In both there is bark, woody tissue, and pith, though the last- 

 named is seldom very distinct in the root. In their modes of growth 

 there is, however, considerable difference. As already shown, the stem 

 branches according to a fixed and regular plan, and, as will be seen later, 

 while still young elongates between the branches. In the root, on the 

 contrary, bi-anches issue irregularly and elongation is limited to the grow- 

 ing extremity. In other words, a root of a given length, once laid in the 



