CHAPTER III. — ORGANS OF PLANTS. I 79 



aceous forms of the higher plants. In woody plants its propor- 

 tion is much smaller ; but in all plants it contains tissues of 

 great importance to the life of the plant. To this belong the 

 parenchyma or pulp of leaves, the cells of the medullary rays, 

 pith, and exterior layers of the bark in the stems of Dicotyledons, 

 and most of the soft cellular tissue in all plants. 



Besides parenchyma, it may include collenchyma, cork, scle- 

 rotic, laticiferous, and sometimes even fibrous tissues. The 

 region of fundamental tissue just beneath the epidermis, often 

 consisting of collenchyma, sclerotic or fibrous tissue, and there- 

 fore usually firmer than the interior tissues of this system, is 

 frequently called the hypoderma. 



HlSTOLOGV OF THE ORGANS OF PLANTS. 



In the same manner that tissues are grouped into systems, so, 

 also, the latter are grouped to form the organs of the plant. 

 These, as has already been seen, consist of roots, stems, leaves 

 and plant-hairs, together with their various modifications. Occa- 

 sionally an organ is so simple as to be composed of but one kind 

 of tissue, as the leaves of many mosses and the great majority 

 of plant-hairs, but usually they are complicated structures. 



Histology of the Root. The general structure of a root 

 may be best understood by reference to an illustration. Fig. 440 

 represents a thin longitudinal section of the tip of a root of 

 Buckwheat : a is the growing point ; bb, the root-cap ; cc, the 

 epidermis imperfectly developed ; dd, the partially formed cortex 

 or bark ; and e, the imperfectly developed central cylinder. 

 Counting the root-cap as belonging to the epidermis, there are 

 thus three layers of cells, and this is the usual number, although 

 in a few cases four are distinguished, and in some cases but two. 



The Root- Cap. — This is formed of parenchyma cells, which 

 increase in number at the growing point,, hence the older cells 

 are nearer the surface of the root and the whole forms a kind of 

 cap, which covers and protects the growing apex. 



In most roots the cap is inconspicuous, but in some aquatics 

 like the Duck-weed, Fig. 28, Part I, it is well developed. In 

 many aerial roots it is no less conspicuous, and is also composed 

 of firm-walled cells. This is the case with the roots of many of 

 the epiphytic Orchids. 



