PHYSIOLOGY OF THE LEAF. 81 



are wanting, as in the Prickly Pear, (Cactus,} the green sur- 

 face of the stem appears to perform the office 01 leaves. If you 

 will observe a dead leaf which has for some time been exposed 

 to the action of the atmosphere, you may see its skeleton, or 

 frame work ; this consists of various fibres, minutely subdivided, 

 which originate from the petiole. This skeleton of the leaf 

 may be examined to advantage, after boiling the leaves slight- 

 ly, or rubbing them in water ; the cuticle or skin easily sepa- 

 rates, and the pulp, or cellular texture, may then be washed 

 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 ; this forms what 

 is called the vascular system. (See Fig. 42.) 



Fig - 42 ' In external 



appearance the 

 organs which 

 compose the 

 vascular system 

 are analogous 

 to the bones 

 which consti- 

 tute the founda- 

 5tion of the ani- 

 mal system, but 

 are considered 



as performing the office of veins or arteries. They are found 

 to be tubular ; in some cases, this is ascertained by the naked 

 eye ; in others, beautifully illustrated by immersing the fibres 

 of the leaf in some coloured liquid ; on taking it out, they are 

 found to contain internally a portion of the liquid ; this experi- 

 ment proves them to be transparent as well as tubular. 



The covering of this frame-work 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 perceptible ; 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 glands, having a com- 

 munication with the vascular system. 



The covering of the leaf, or the cuticle,* guards the vascular 



* The cuticle is sometimes called epidermis, from epi, around, and derma, 



Skeleton of the leaf Vascular system How ascertained to be tubular and 

 transparent Cellular texture Cuticle, 



