GO 



PHYSIOLOGY OP THE LEAP. 



tubular •■, in some cases, this is ascertained by the naked eye ; in oth- 

 ers, it may be beautifully illustrated by immersing the fibres of the 

 leaf in some coloured liquid ; on taking them out, they are found to 

 contain internally a portion of the liquid ; this experiment proves 

 them to be transparent, as well as tubular. 



The covering of this frame of the leaf is the cuticle, and a pulpy 

 substance, called the parenchyma, or cellular texture. Some leaves 

 contain much more of this than others, of course they are more 

 pulpy and juicy ; it is found, as its name cellular would denote, to 

 consist of a mass of little cells, various in size in different leaves ; in 

 some, with the most powerful magnifiers, the cells are scarcely per- 

 ceptible ; in others, they may be seen with the naked eye. These 

 cells are of important use in the secretion and communication of 

 substances through the leaf; and may thus be considered as a kind 

 of gland, ha\ang a communication 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 important functions of absorbing nourishment, and 

 throwing off such substances as are useless or hurtful. The cuticle 

 is sometimes covered with downy, or hairy glands, which seem to 

 afford security against changes of weather ; such plants are capa 

 ble of enduring a greater degree of heat than others. In some cases, 

 the cuticle is covered with a transparent varnish, 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 countries, 

 which are subject to long rains, anS continued moisture, are thus 

 sliielded from the injurious effects of the weather. 



When the surface of the cellular tissue is more ample than the vas- 

 cular net-work, the leaf is rugose, as seen at 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 cellulai 

 tissue, the leaf presents the appearance of lattice-work, and is sai(f 

 to be cancellated ; the leaves of an aquatic plant of Madagascar 

 (Hydrogeton Jhiestralis, Fig. 62, b,) are of this kind. Another exam- ' 

 pie of this leaf is seen in the Claudea elegans, a species of marine 

 Algse, found in New Holland, (Fig. 62, c ;) the veins are parallel to 

 tlie sides, and cross the nerves. 



Fig. 62. 



♦ The cuticle is sometimes called epidermis, from ept, around, and derma, skin i 

 the true skin being not the outer 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. 



How ascertained to be tubular and transparent— Cellular texture— Cuticle— Impor 

 taut office of the leaf— What is a rugose leaf 7— What is a cancellated leaf "?— Expiaia 

 Fig. 62. 



