132 



ORGANOGRAPHY. 



chyma. 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 communica- 

 tion is kept up between the interior of the leaf and the external 

 air, which is essential to the due performance of its functions. 



Such is the general arrangement of the parenchyma of leaves, 

 but it is subject to various modifications in the leaves of differ- 

 ent plants. Thus in those leaves which have their margins 

 turned upwards and downwards instead of their surfaces, the 

 arrangement of the parenchyma is similar beneath both the sur- 

 faces ; while in succulent leaves the parenchyma is composed of 

 cells which are usually larger than those of ordinary leaves, and 

 closely compacted, or with but few interspaces. Other modifica- 

 tions of the parenchyma may also be found in different plants, 

 but these are of little importance. 



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

 parenchyma, the veins being composed simply of more or less 

 elongated parenchymatous cells. Such leaves are generally very 

 thin, only containing two or three layers of cells, so that all the 

 cells are nearly in contact with the water in which the leaves are 

 placed. The cells are disposed very regularly and have no inter- 

 spaces, but all contain chlorophyll. In submerged leaves however, 

 which are thickened, we find large cavities which are very regu- 

 lar intheir form and 

 arrangement {fig. 

 253, m). These con- 

 tain air, by which 

 the specific gravity 

 of the leaf is dimi- 

 nished, and it is 

 thus enabled to float 

 in the water. Sub- 

 merged leaves have 

 Fig. 253. Vertical section of a leaf of a Potamogeton, ^ onirlprTTifll 



highly magnified. ?, i. Air cavities, r. Parenchy- f^" ^^'^^ epiuermai 

 matous cells containing chlorophyll. layer, and no sto- 



mata, both of which would be useless from their being always 

 exposed to similar hygrometric conditions. 



3. INSERTION AND ARRANGEMENT OF LEAVES. 



1. Insertion. — The point by which a leaf is attached to the 

 stem or branch is called its insertion. Leaves are inserted on 

 various parts of the stem and branches, and receive different 

 names accordingly. Thus the first leaves which are developed 

 are called cotyledons {fig. 15 c, c), mirsing, or seminal; the 

 latter term however is a bad one, because it would indicate 

 that these are the only leaves that exist in the seed, which is 



