52 ANATOMY OF LEAVES. 



oxygen are combined. When leaves languish from disease, 

 they give off oxygen sparingly, and the chlorophyl assumes 

 some shade of yellow or red. 



LECTURE X. 



ANAT0:StY AND PHYSIOLOGY OF LEAVES. THEIR USE IN THE VEGE- 



T^U3LE SYSTEM. xiPPENDAGES TO PLANTS. 



60. The leaf, though simple in appearance, is complex in 

 structure. It is composed of a cellular systevi^ which is an ex- 

 pansion of the cellular integument ; and ^fihrO'Vascular system, 

 which arises from the medullary sheath. The outer covering, 

 or cuticle, is a continuation of that of the stem. Leaves are com- 

 pared to the lungs of animals ; they are organs for absorjptioii^ 

 exhalation^ resjnration^ and digestion: When leaves are wanting, 

 as in the Prickly Pear {Cactus)^ the green surface of the stem 

 appears to perform their office. Observe a dead leaf which has 

 for some time been exposed to the action of the atmosphere ; 

 its skeleton^ or frame-worh^ consists of various fibers, minutely 

 subdivided, which originate from the petiole. After boiling 

 tlie leaves slightly, or rubbing them in water, the cuticle easily 

 separates, and the pulp, or cellular texture, may then be w^ashed 

 out from between the meshes of the veined net-work ; thus, 

 the most minute cords of the different 

 vessels become perceptible, with their ^'^ ^^• 



various divisions and subdivisions ; 

 these form wdiat is called i\\Q fihro-vas- 

 Gular system. (See Fig. 61). Though 

 in external appearance the organs 

 which compose the vascular system of 

 plants are analogous to the bones 

 w^hich constitute the foundation of the animal system, yet they 

 are rather considered as performing the office of veins and ar- 

 teries. Tliey are found to be tiibular. In some cases, this is 

 ascertained by the naked eye ; in others, it may be beautifully 

 illustrated by immersing the fibers of the leaf in some colored 

 liquid : on taking them out, they are found to contain inter- 

 nally a portion of the liquid. This experiment proves them to 

 be iransjparent^ as well as tubular. Leaves are pulpy or juicy 

 according as they contain more or less parenchyma or cellular 

 tissue. This tissue consists of a mass of little cells, various in 



60- Structure of the leaf— Explain Fi^'. ^—Cellular texture. 



