172 THE PLANT CELL. 



oxalate has been shown to be formed by the decomposition of caleiuni 

 nitrate in leaves, the calcium combining with oxalic acid, and the nitPiC 

 acid being subsequently assimilated. 



g- Fats and waxes occur at times in the cell, and should be men- 

 tioned here, although they can hardly be looked upon as being soluble in 

 the cell-sap. They are formed by the decomposition of prOtOplasm, 

 and, possibly, at times, by other methods. The waxes are excreted 

 by the epidermal cells of some leaves, and form short rods set at right 

 angles to the surface on the outer aspect of the external walls. In this 

 manner a layer of wax is formed which prevents water from collecting 

 on the leaf. 



The cell-sap, then, contains many substances of the nature of 

 raw food -materials, some elaborated food- substances (such as 

 sugar, inulin, amido-acids, and proteids), ferments, and a good 

 many excretory products, or bye-products of the breaking down of 

 the cytoplasm. Other materials are also present, such as resins 

 and gums, oils, &c., which are not soluble in the cell-sap, but 

 which > are products removed as soon as they are formed. In 

 many cases the resins, .&c., may be looked upon as products 

 of the degradation of the cell-wall, and form striking instances 

 of substances thrown out of a cell which may nevertheless 

 be of great value to the plant. The manner in which 

 raw food-materials, such as salts of potash, sodium, silica, &c., 

 enter the plant, has been partly considered already during 

 the examination of root-hairs and the young cell (Chap. iv.). 

 It was there pointed out that the ectoplasm of the root-hair 

 exercised a selective capacity over the absorption of salts in 

 dilute solution in the soil; in the one case, perhaps, salts of 

 sodium and potassium being taken in to the exclusion of others ; 

 and in the other case, possibly salts of silica being admitted 

 as well. 



A closer consideration of the phenomena of osmosis is not 

 inappropriate at this point ; the absorption of the dilute solution 

 of earthy salts by the root-hairs is dependent upon the presence 

 in the central vacuole of the hair of substances which are osmoti- 

 cally active — that is, which exert a distinct attraction upon the 

 molecules of water and salts outside the cell. At a certain stao-e 

 the so-called osmotic pressure inside the cell reaches a limit at 

 which internal diffusion or endosmosis ceases and equilibrium 

 exists. But the water in the vacuole of the root-hair is beino' 

 constantly withdrawn by reason of the suction action of the 

 transpiration current (see p. 1 74), and also by further osmosis all 



