230 ANATOMY AND PHYSIOLOGY OF PLANTS. 



tissue of the bark, with its accompanying cortical layers, the vessels 

 of which hare a reticular form. The bark is thick ; its vessels fre- 

 quently contain the proper juice, as those of the trunk, and its cells 

 are alike filled, in some roots, with concreted matter. From the 

 bark to the centre, the two orders of transverse septa are observed 

 to proceed j and are interposed between the rays of vessels, as in 

 the trunk. These vessels, in the root, are often larger than those of 

 the trunk, and instead of a pith, the central part of the root is com- 

 monly occnpied by vessels. 



4. In shrubs, the arrangement of the parts of the 

 root, is commonly distinct, and corresponds with that in 

 the trunk. 



5. The structure of herbaceous roots is very differ- 

 ent to either of the preceding, not only in form and 

 appearance ; but also in their internal organization. 



The skin of these roots is of a very different colour and thickness. 

 In the early state, it is represented by Grew, as an extension of that 

 which covered the radicle of the seed ; but in more aged plants, the 

 exterior covering is derived from the cellular tissue of the bark. It 

 is usually, if not always, compounded of vessels and cellular tissue ; 

 beneath the skin, a cortical texture is observed, making up in some 

 herbs, the greater portion of the root, while in trees it is commonly 

 thin. It is composed of cellular tissue and fasciculi of vessels variously 

 dispersed through it, and forming a net-work, the meshes of which 

 are filled with the tissue. In these vessels, various gummy andresin- 

 ous juices are frequently contained. The woody part of herbu. 

 ceous roots is described as consisting of vessels and cellular tissue. 



6. From these remarks, we learn that the roots of 

 all vegetables, in their internal structure, do not differ 

 very materially from the stem or herbage. 



7. The root being the part by which plants are 



