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A leaf consists of a flattened out sheet of parenchyma tissue, 

 composed of cells of various form, through which at intervals 

 run fibro-vascular bundles, constituting what we know as the 

 venation of the leaf ; in some leaves we find a central large 

 fibro-vascular bundle, or midrib, from which the other bundles, 

 called veins or nerves, are given off, like the barbs from the 

 quill of a feather. These again give off smaller branches or 

 veinlets in their turn, in a like manner, which are connected 

 with one another by minute branches, so that we get finally a 

 very complex reticulum or network formed. In other leaves 

 there is no such central midrib, but several large bundles termed 

 nerves enter the leaf, and traverse it running parallel to one 

 another. These are simply connected by cross branches at 

 intervals. This sheet of parenchyma, known to botanists as the 

 mesophyll, and the cells of which contain green coloring matter, 

 is covered both on the upper and lower surface, by a sheet of epi- 

 dermis, which is usually only a single layer of cells in thickness, 

 to protect it. The cell walls of these epidermal cells have become 

 chemically changed ; they have become cuticularized, that is to 

 say, they still retain their elasticity, but they can no longer be 

 permeated by water, nor will they swell up under its influence ; 

 when this change takes place all distinction between the outer 

 boundaries of the constituent cells ceases, and a continuous sheet 

 of protective cuticle covers the whole surface of the leaf. 



This sheet is broken at certain intervals by apertures which are 

 known as stomata or mouths. These stomata are as a rule 

 much more frequent on the lower surface of leaves than on the 

 upper. They consist of a central aperture or space surrounded 

 by two or sometimes four cells, which are firmly united at their 

 extremities. This central space can be opened or closed ac- 

 cording as these cells, which are called the guard cells of the 

 stoma, and contain chlorophyll, are turgid or flaccid. If the 

 guard cells are flaccid, with no considerable quantity of fluid, 

 that is to say, if they do not contain as much water as they 

 can, their tendency is to come together, and so close the stoma. 

 If the guard cells take up more water, so as to become tense 

 and turgid, they must increase in size, and so tend to separate 



