208 WITHERED LEAVES. 



through the transparent cells of the epidermis. In addition to the 

 chlorophyll, there are many other substances both fluid and solid. 

 Among these may be enumerated starch, sugar, oil, tannic acid, 

 crystalline formations, and albuminoid or proteinaceous com- 

 pounds. The starch granules, and raphides or plant crystals, form 

 exceedingly interesting and beautiful objects under the micro- 

 scope. The internal mass, or diachyma, of the leaf is also tra- 

 versed by the midrib and branching veins, which are composed of 

 fibro-vascular tissue, serving the various purposes of imparting 

 strength and durability of form to the leaf, and of the transmis- 

 sion of sap from the root through the branches, and its re- 

 transmission downwards after the various chemical changes have 

 taken place in the leaf, thus fitting it for the further develop- 

 ment of the vegetable structure. These changes are partly effected 

 by the action of the atmospheric air, which is brought into contact 

 with the cell contents, by the presence in the diachyma of 

 numerous cavities between the cells, and communicates with the 

 outer air by means of the stomata in the epidermis. The number 

 of these pores or stomata varies greatly in different leaves, from a 

 dozen or two up to 160,000 on a square inch. The leaf of the 

 Lilac has been found to contain 708,750 ; while the Lime tree has 

 1,053,000 on each entire leaf. The important part which the 

 stomata are intended to fulfil in the vegetable economy may be 

 manifest, if we glance at the methods by which the Hfe of the plant 

 is sustained. 



We have already stated that the leaf is composed of various 

 cells, and have enumerated some of the substances contained 

 within them, but it must be remembered that the essential 

 constituent of all living cells is protoplasm, a mucilaginous, semi- 

 fluid, transparent, hyaline substance, which, at some period of its 

 existence, secretes out of itself an enveloping membrane, more or 

 less solid and elastic, which is known as the cell-wall ; and the 

 process which we term " growth," consists in the formation of new 

 cells, and the production of new chemical combinations ; and 

 these manifestations of the mysterious power denominated " Hfe," 

 can only take place by the agency of these cells containing pro- 

 toplasm. Cells in which the living protoplasm has become 

 changed into lifeless organic substances, serve a useful purpose in 



