VEGETABLE PHYSIOLOGY. 



Starch, gum, sugar, oil, resin, colouring matter, 

 air, crystals, and other nitrogenous substances, are 



0, Papyrus : pith-like tissue, composed of cells arranged 

 in form of a network, so as to leave large intercellular 

 spaces (ad nat.}. 



also found in the cells of plants. Starch is a very 

 abundant substance in cells, serving purposes of 

 nutrition, and may be readily seen by placing 

 under the microscope a thin slice of potato upon 

 which a drop of tincture of iodine has fallen ; this 

 induces a beautiful blue colour in the starch- 

 granule, and is a constant test for the occurrence 

 of starch. Starch usually occurs in the form of 

 minute grains, many of which are contained in 

 each cell. Their form is often constant and char- 

 acteristic in certain plants for example, the very 

 minute angular starch-granules of rice are quite 

 different from those of the other cereal grains, 

 which, again, are easily distinguished from those 



c, Starch-grains of Potato (ad nat.). 



of the potato, shewn in the above figure (c) ; while 

 those of leguminous plants have a somewhat 

 common character (d). The peculiarities of 

 starch-granules have been successfully employed 

 in detecting adulterations of food and drugs, and 

 for other purposes of practical utility. 



The cell in many cases displays a thickened 

 opaque wall and an empty cavity within. This 

 arises from the deposit of woody matter on the 

 interior surface of the cell-wall, which is seen in 

 the hard stone of the cherry and in the shell of the 

 coco-nut, both of which consist of cellular tissue 

 equally with the softest parts of their respective 

 fruits, and owe their solidity to the indurated 

 woody matter. Instead of having the whole inner 



surface of the cell covered in this way, the de- 

 posited matter may be so arranged as to cover 



d, Lentil : cells of seed containing Starch-grains 

 (ad nat.). 



certain parts only : it is often deposited in the 

 form of rings or spiral coils ; and in the latter case 

 gives rise to the beautiful spiral cells seen in the 



e, Spiral tissue from the root of an Epiphytal Orchid 

 (ad nat.). 



leaves and roots of epiphytal orchids (<?), and in 

 the leaves of bog-mosses (Sphagna). 



With respect to their general form, cells are 

 exceedingly variable, and are usually greatly modi- 

 fied according to the position in the plant which 

 they occupy. They very commonly present the 

 hexagonal appearance represented in the wood- 

 cut (a) ; but in the bark and young stems, as well 

 as in roots and other parts, they often assume an 

 oblong form (fig. e) ; sometimes resembling the 

 bricks of a wall set on end, and thus indicating 

 the manner in which plant-structure is built up, as 

 it were, by separate cells cemented together. 



The rounded or spherical form may be regarded 

 as the normal form of the cell, the two modifica- 

 tions to which we have alluded arising from the 

 pressure of spherical or oval cells in contiguity, or 

 special modifications intended to fit them for their 

 place in the structure. In many cases, however, 

 we find cells of a very different form, whose modi- 

 fications obviously arise from certain special func- 

 tions which they are designed to perform. The 

 pith-like substance of the rush, used as wick for 

 'rush-lights,' consists of cells of a most beautiful 

 stellate form ; the free cells or spores of certain 



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