32 ESSENTIALS OF BOTANY 



roots which are adapted to live in water only. These 

 water-roots often show large and distinct sheaths on the 

 ends of the roots, as, for instance, in the so-called water- 

 hyacinth. This plant is especially interesting for labora.- 

 tory cultivation from the fact that it may readily be 

 transferred to moderately damp soil, and that the whole 

 plant presents curious modifications when made to grow 

 in earth instead of water. 



42. Parasitic Roots.^ — The dodder, the mistletoe, and a 

 good many other parasites live upon nourishment which 

 they steal from other plants called hosts. The pai'asitic roots^ 

 or haustoria, form the inost intimate connections with the 

 interior portions of the stem or the root, as the case may be, 

 of the host-plant on which the parasite fastens itself. 



In the dodder, as is shown in Fig. 14, it is most inter- 

 esting to notice how admirably the seedling parasite is 

 adapted to the conditions under which it is to live. Rooted 

 at first in the ground, it develops a slender, leafless stem, 

 which, leaning this way and that, no sooner comes into 

 permanent contact with a congenial host than it produces 

 haustoria at many points, gives up further growth in its 

 soil-roots, and grows rapidly on the strength of the sup- 

 plies of ready-made sap which it obtains from the host. 



43. Forms of Roots. ■ — The primary root is that which 

 proceeds like a downward prolongation directly from the 

 lower end of the hypocotyl. In many cases the mature root- 

 system of the plant contains one main root much larger than 

 any of its branches. This is called a taproot (Fig. 15). 



Such a root, if much thickened, would assume the form 

 shown in the carrot, parsnip, beet, turnip, salsify, or radish, 

 and is called a fleshy root. Some plants produce multiple 



1 See Kerner and Oliyer'a Natural History of Plants, Vol. I, pp. 171-213. 



