ORGANS OF NUTRITION. l49 



whicli a communication is kept up between the external air 

 and the interior of the leaf. The cells beneath the epidermis 

 of the lower surface ei, are loosely connected pi, and have 

 numerous large spaces between them; they are also frequently- 

 very irregular in form, thus presenting commonly two or more 

 projecting rays, Avhich become united with similar projections 

 of the cells next them, and thus leave between them numerous 

 spaces /, which communicate freely with each other, and form 

 a cavernous or spongiform parenchyma. These spaces are also 

 connected with the stomata, which, as we have already seen, are 

 generally most abundant on the epidermis of the lower surface, 

 and thus a free communication is kept up between the interior 

 of the leaf and the external air, which is essential to the due per- 

 formance of the functions of the leaf 



Such is the general arrangement of the parenchyma of leaves, 

 but it is subject to various modifications in different plants. 

 Thus in those leaves which present their margins to the earth 

 and heavens instead of their surfaces, the arrangement of the 

 parencliyma is similar beneath both of those surfaces. In suc- 

 culent leaves again, the parenchyma is composed of cells usually 

 larger than those of ordinary leaves, and closely compacted, 

 or with but few interspaces ; the cells in the centre of such 

 leaves are also commonly colourless. Besides the above, vari- 

 ous other modifications of the parenchyma occur in different 

 plants, Avhich it would be out of place to describe here, we 

 therefore proceed to notice the structure of submerged leaves. 



2. Submerged Leaves. — These are entirely made up of 

 parenchyma, the veins being composed simply of cells more 

 or less "^elongated. The cells of which the parenchyma is 

 composed contain chlorophyll granules, and the leaves which 

 are generally very thin, only contain two or three layers of 

 them, so that they are all nearly in contact with the sur- 

 rounding fluid; in such leaves the cells are disposed very regu- 

 larly and have no interspaces. In submerged leaves however, 

 which are thickened, we find large cavities which are very 

 regular in their form and arrangement (fig. 287, i). These con- 

 tain air, by which 

 the specific gravity 

 of the leaf is di- 

 minished, and it is 

 thus enabled to float 

 in the water. Sub- 

 merged leaves have 

 no true epidermal 

 layer, and no sto- 

 mata, both of which 



11 Ko iicoloco Fig. 2^7. Vertical section of a leaf of a Potamogeton 

 WOUia Oe useless iii^hly magnified. J. Air cavities. ?•. Parenuliymatous 

 from their being cells containing chlorophyll granules. 

 l3 



