324 THE MICROSCOPE. 



fusion of cells ; hence they do not constitute a true cellular 

 tissue. 



Vascular tissue is formed by the fusion of perpendicular 

 rows of cells ; by the absorption of their contiguous walls 

 they become converted into continuous tubes of more or 

 less considerable length. Then we have a combination of 

 tissues destined for particular purposes in the economy of 

 the plant, divided into three primary systems — the Cellular, 

 the Fibro-vascular, and Cortical. The 1st, Cellular, forms 

 the great mass of the living structure of plants ; and it is 

 in this system that the vital processes of vegetation are 

 chiefly carried on. The 2d, Fibro- vascular, forms all the 

 woody structures, which in all cases are composed of a 

 quantity of conjoined portions of cellular and vascular 

 tissue arranged in a peculiar manner ; differing in their 

 modes of growth in different classes of plants, and which 

 in consequence present considerable differences in the 

 structure of their mature stems. The 3d, Cortical, also 

 termed the epidermal, exists in the form of a simple flat 

 layer of cells united firmly together by their sides, and 

 forming a continuous coat over the surface of a plant. 

 Such a layer clothes all the organs of plants above the 

 Mosses, and, as stems grow older, the epidermal layer 

 gives place to the bark or rind. Stomata are orifices 

 between the meeting angles of the epidermal cells ; most 

 abundant usually on the lower surface of leaves, often 

 wanting on the upper surface. On the leaves of aquatic 

 plants they are only found on the upper surface, and are 

 absent where the leaf touches the water. 



Hairs and scales of all kind depend on the development 

 of the epidermal cells. Simple hairs are merely single 

 epidermal cells produced in a tubular filament, and when 

 cell-multiplication occurs in them they present a number 

 of joints ; see hairs of nettle, fig. 188, No. 2. Thorns, such 

 as those of the rose, are aborted branches, in which the 

 cells become thickened by woody secondary deposits. In 

 leathery or hard leaves, and in the thick tough leaves of 

 succulent plants, such as the aloes, the secondary layers 

 acquire great thickness. The aerial roots of the Orchi- 

 dacese exhibit a curious structure, the growing extremities 

 being clothed by a whitish cellular tissue composed of several 



