ITS STRUCTURE AND GROWTH. 



85 



Wliile some of these remain slender and serve merely for absorption, 



others, tliickened by this deposit, may become tuberous (as in Fig. 



139) ; and buds, formed on the stem 



just above, draw upon this store when 



they start into growth in the spring. 



These particular roots perish when 



exhausted of their store; but new 



accumulations have meanwhile been 



formed in some of the roots of the 



season, which serve the same purpose 



the following spring ; and so the plant 



survives, year after year. 



147. Some less ordinary modifica- 

 tions of roots remain to be noticed. 

 It has already been stated that they 

 may spring (as secondary roots, 142) 

 from any part of the stem, although 

 they commonly do so only when this 

 rests on or is covered by the soil, which affords the darkness and 

 moisture congenial to them. Some stems, however, strike root freely 

 in the open air, forming 



148. Aerial Roots. The Ivy of Europe, our own Poison Ivy (Rhus 

 Toxicodendron), and the Trumpet Creeper climb by such roots, in 

 the form of small rootlets, whicli attach themselves to the bark of 

 trees, &c. These serve merely for mechanical support. Other 

 plants produce larger aerial roots, which, emitted from the stem in 

 the open air, descend to the ground and establish themselves in the 

 soil. This may be observed, on a small scale, in the stems of In- 

 dian Corn, where the lower joints often produce roots which grow 

 to the length of several inches before they reach the ground. More 

 remarkable cases of the kind abound in those tropical regions where 

 the sultry air, saturated with moisture for a large part of the year, 

 favors the utmost luxuriance of vegetation. The Pandanus or 



• Screw-Pine (a Palm-like tree, often cultivated in our conserva- 

 tories) affords a well-known instance. Here (Fig. 140) strong roots, 

 emitted in the open air from the lower part of the trunk, and soon 

 reaching the soil, give the tree the appearance of having been par- 

 tially raised out of the ground. The fiimous Banyan-tree of India 

 (Fig. 142) affords a still more striking illustration. In this the 



5IG. 139. Fascicled tuberous roots of the Dahlia. 



8 



