LEAVES. 



99 



and scarcely elongated at all, and is known as spongy paren- 

 chyma. 



Most any flattened leaf will serve for the study of the bifacial 

 type. An excellent leaf is that of the Rubber Tree (Ficus 

 elastica), because of its toughness and thickness. Leaves 

 bleached in alcohol will be better, as the sections will be more 

 transparent. 



Make sections perpendicular to the lateral veins of the leaf 

 and mount in water or glycerin. Sections of the fresh leaf may 

 be cleared in carbolic acid or chloral-hydrate. 



The epidermis on both surfaces is composed of three layers of 

 cells. This is not common to all leaves, but is usually found in 

 tough evergreen leaves. 

 The triple layer affords 

 greater protection. Here 

 and there along the upper 

 surface, and sometimes on 

 the lower also, occur very 

 large cells with a mass 

 hanging from a stalk at- 

 tached to the cell-wall, like 

 a bunch of grapes, in the 

 cavity of the cell. The 

 hanging masses are called 

 cystoliths. They are not of 

 common occurrence. 



Next to the upper epider- 

 mis are two layers of elon- 

 gated cells, the cells of the 

 outer layer being much 

 longer than those of the 

 inner layer, and all are fill- 

 ed with chlorophyll gran- 

 ules. These are the pali- 

 sade cells. 



The rest of the space below the palisade cells is filled in with 

 spongy parenchyma. The cells are not elongated and contain 

 much less chlorophyll than the palisade cells. The cells next 

 the lower epidermis are somewhat compactly arranged. 



In the lower epidermis will be found stomata or breathing 

 pores. Some of the pores will be found cut through the middle, 

 giving a clear view of the guard cells. Each pore is seen to lead 

 into a large air space in the leaf. (See Fig. 61.) 



The cystoliths consist of a groundwork of cellulose infiltrated 

 with calcium carbonate. On adding a drop of acetic acid to a 

 section the carbonate will dissolve with effervescence, leaving 

 the cellulose mass, which stains blue with chlor-zinc-iodine. 



At intervals along the section will be found the collateral 



Fig. 61. Cross-section of portion of leaf of 

 Rubber Tree, a, e, three-layered upper and 

 lower epidermis ; c, palisade cells ; d, 

 spongy cells ; h, xylem ; i, soft bast ; k, 

 bast fibres, of a small vein ; f, stoma ; g, 

 air space (reduced, from Bastin). 



