ANATOMY OF PLANTS. 183 



pith is formed when the root is laid open to tlic air. The bark 

 of the root is richly stocked with peculiar juices, which the 

 more readily are collected in it, as the descent of the sap from 

 the stem into the root is favoured by the size of this latter part. 

 This direction of the root towards the centre of the earth, 

 is, without doubt, an effect of the common law of gravitation, 

 to which plants are partly subject, as they are fixed by their 

 lower extremity in the earth. This tendency, however, is 

 considerably modified by other circumstances, which originate 

 in the organization of the plant, so . that in many trees we 

 perceive fewer roots proceeding downwards into the soil, than 

 those which we observe running horizontally. 



288. 



TuberdeSy or tuberculous roots, are distinguished by their 



greater thickness, and by their fleshy appearance, {Q5.^ 



They enclose within a cellular covering certain parts, in which 



the cellular texture is much crowded, and from which the hio-h- 



o 



er forms, the sap-vessels and the spiral-vessels, take their orioin, 

 as being the beginnings of the future shoot. A root has, 

 therefore, the greater means of production, the more tuber- 

 culous it is ; and in many tubercles we can distinguish very 

 accurately, at fixed periods, the harder kerael, from which 

 the future shoots are to arise, from the surrounding soft cellu- 

 lar texture. Even in the higher parts of the stem similar 

 thickened parts occur, in which the power of propagation re- 

 poses ; since, universally, wherever the cellular texture is 

 much crowded, new sap-vessels and spiral-vessels arise as 

 the foundation of future shoots ; whence, in the stem and 

 branches, the transition from tubercles to buds is obvious. 



289. 

 Bulbs appear above and upon the root, like more |x?rfectly 

 formed tubercles. They have, as their foundation, a solid 

 substance, consisting of extremely compressed cellular tex- 

 ture. From this substance spring the germs of the leaves 

 between scales, which are a continuation of that solid matter ; 

 and in the middle of these arises perpendicularly the futiu*e 



