PHYSIOLOGY OF THE LEAF. 53 



size in different leaves ; in some, with the most powerful mag- 

 nifiers, the cells are scarcely perceptible; in others, they may 

 be seen with the naked eye. These cells are of important nse 

 in the secretion and communication of substances tliroiigh the 

 leaf ; and may thus be considered as a kind of gland, having a 

 commmiication with the vascular system. The covering of the 

 leaf, or the cuticle^'^ guards the vascular and cellular system 

 from injury, and is the medium by which the leaf performs the 

 imj)ortant functions of absorbing nourishment, and exhaling 

 such substances as are useless, or hurtful. The cuticle is some- 

 times covered with doiony or hairy glands^ which seem to 

 afford security against changes of weather ; such plants are 

 capable of enduring a greater degree of heat than others. In 

 some cases, the cuticle is covered with a transjmrent "uarnish^ 

 which preserves the plant from injury by too much moisture, 

 and adds to the beauty of the leaves. The trees of Abyssinia 

 and some other coimtries, which are subject to long rains and 

 continued moisture, are thus shielded from the injurious effects 

 of the weather. 



When the surface of the cellular tissue 

 is more ample than the vascular net- 

 work, the leaf is rugose (Fig. 62, a)\ 

 where, for every swelling of the upper 

 surface of the leaf, there is a correspond- 

 ent depression of the under surface : the 

 sage has a leaf of this kind. When the 

 net-work exists, but the meshes are destitute of cellular tissue, 

 the leaf presents the appearance of lattice-work, and is said to 

 be cancellated. 



The leaves of an aquatic plant of Madagascar {Hydrogeton fenestralis, Fig. 62, 6). 

 are of this kind. Another example of this leaf is seen in the Claicdea elegans (Fig. 

 62, c), a species of marine Algae, fomid in New Holland ; the veins are parallel to 

 the sides, and cross the nerves. 



61. Sjyecial functions of Lea^ves. — Leaves expose the fluids of 

 the plant to the influence of air and light ; they aid in the 

 elaboration of the various vegetable secretions, in the forma- 

 tion of wood, and in the absorption of fluids and gases. In the 

 cells of the leaves changes take place by the agency of light, 

 by which oxygen is given off, and carbon remains in a fixed 

 state. Exlialation^ or transpiration., is the process by which the 

 superabundant juices of the plant are given off; it takes j)lace 



* The cuticle is sometimes called epidermis, from epi, around, and derma, skin ; the true skin being 

 not the (fUter covering, but a cellular substance beneath : thus, the thin skin upon the back of the 

 hand, which so easily becomes rough, is the cuticle, or epidermis (sometimes called the scarf-skin), 

 while the real skin is below. 



Cuticle— Important office of theleaf— What is a rugose leaf ?— What is a cancellated leaf?— Ex- 

 ilain Fig. 62.-61. Functions of leaves. 



