PHYSIOLOGY. 267 



tions to this definition, some roots, as shown at the commence- 

 ment of this volume, not being fixed to any thing, as those 

 of Duck-meat, (Lemna,) which float in the water. 



For the forms of different roots, see page 12 14. The 

 roots of trees so closely resemble the stems in structure, that 

 some have called them descending trunks, and subterranean 

 branches. The structure of the roots of trees so nearly re- 

 sembles that of their stems, that a description of their ves- 

 sels would only be a repetition of that already given of their 

 stems and branches. In herbaceous plants, this analogy is 

 not so close, but it is hardly necessary to describe the differ- 

 ence in this epitome of the subject. 



The roots of herbaceous plants are generally at first spin- 

 dle shaped, but they sometimes become forked, in which case 

 the sap vessels of the caudex are divided according to the 

 number of forks, in the same manner as if a skein of thread 

 was divided into two or three parcels. When any thing op- 

 poses the descent of a fusiform root into the ground, or de- 

 stroys a portion of it, these branches shoot out sideways in 

 search of nourishment. 



Rootlets, (radicena,) or little roots. These are small roots 

 which project from the sides of the larger ones, or under fa- 

 vorable circumstances, from the stem. Thus if the branch 

 of a tree, or shrub, be bent down arid covered with earth, 

 rootlets will shoot out from its surface, after which this branch 

 may be separated from the parent tree, and will become a tree 

 itself. This method is much employed by gardeners, and is 

 called propagation by layers. 



Solid, fusiform roots, as the parsnip, are covered with root- 

 lets, by means of which the plants draw their nourishment 

 from the earth. 



Fibrils. These are the ultimate divisions of the rootlets. 

 They consist of the capillary, or hair-like ramifications of 

 the rootlets, and the Spongelets, or Spongioles, which are 

 seated on their sides, and ends. The latter are oblong bo- 

 dies, of a spongy texture, very minute, and are found only on 

 the fibrils. They are so small and delicate, that after a few 

 seconds exposure to the air, on taking the root out of the 

 ground, they shrivel and lie so close to the fibril, as scarcely 

 to be visible. But if the fibril be placed in water, they rap 

 idly expand again, and project from its sides. 



Fig. 235 represents the extremity of a fibril with its spon- 



