162 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 32. 



the same layer, and which appears in a transverse section as a 

 dark dot on the radial walls of the cells. 



In some cases the cells of the exodermis are prosenchymatous 

 and sclerenchymatous (e.g. species of Carex, aerial roots of a 

 species of Philodendron). 



When the exodermis is invested by tegumentary tissue (as in 

 aerial roots of Orchids, for instance) some of its cells retain their 

 thin unaltered walls, and are the passage-cells, by means of which 

 water can penetrate into the interior of the root. 



3. The general ground-tissue of stems, leaves, and roots, lying 

 within the hypoderma, consists mainly of parenchymatous tissue, 

 with, frequently, a considerable differentiation of masses of fibrous 

 sclerenchymatous stereom. 



In the root, where the extra-stelar ground-tissue is entirely cor- 

 tex, it can generally be distinguished into two regions, an external. 

 in which the arrangement of the cells is somewhat irregular, and 

 an internal, in which the cells are arranged in radiating rows. 



In aerial stems and foliage-leaves, the more external, at least, 

 of these cells frequently take part in the assimilatory processes of 

 the plant; the cells contain chloroplastids and constitute assimi- 

 latory tissue. Towards the most highly illuminated surface of the 

 member, the cells are frequently so arranged that their longer 

 axes are perpendicular to the surface, that is, are parallel to 

 the incident rays of light; assimilatory tissue of this structure is 

 termed palisade-tissue ; the whole of the internal ground-tissue of 

 a leaf-blade is termed generally mesophyll. 



In view of its great physiological importance a somewhat detailed account of 

 the structure of the mesophyll of the leaf-blade seems necessary. 



The mesophyll consists of parenchymatous thin-walled cells of various form. 

 When the blade is thin, the whole mesophyll consists of assimilatory tissue ; 

 but when it is more or less fleshy and succulent, the more central part consists of 

 cells without chloroplastids, the assimilatory tissue being confined to the surface. 



When the mesophyll is altogether assimilatory, the arrangement of the cells 

 is correlated with the symmetry of the leaf-blade. In a dorsiventral lamina 

 (Fig. 125) the structure of the mesophyll is different in relation with the upper 

 (ventral) and the lower (dorsal) surfaces. Towards the upper surface, which is 

 more directly exposed to light, the somewhat elongated cylindrical cells form 

 a compact palisade-tissue one or more rows in thickness ; whereas, towards the 

 lower shaded surface, the cells are less regular, frequently somewhat stellate 

 in form, leaving large intercellular spaces between them, constituting what is 

 known as the spongy parenchyma. The loose structure of the mesophyll 

 towards the lower surface of the blade is correlated with the presence of 

 numerous stomata in the epidermis of that surface (see p. 156). 



