190 THE PLANT CELL. 



cell-sap, and on the other side, either a dilute solution of earthy 

 salts, or of a gas) is by no means an inert one like parchment, 

 but is composed of several parts — viz., a layer of cellulose, then 

 a layer of ectoplasm, then a layer of endoplasm, and, finally, 

 lining the central vacuole, another very delicate pellicle of 

 ectoplasm (hyaloplasm); consequently, the osmotic phenomena 

 observed in the case of the cell are not quite equivalent to the 

 purely physical processes observed to take place when experi- 

 menting with the parchment membrane. 



The endosmosis of earthy salts into a root-hair is, as has been 

 seen, governed to a certain extent by the osmotic properties of 

 substances in the cell-sap; during the metabolism of the cell, 

 bye-products are formed, which it is found exert an attraction 

 upon the salts and water outside in the soil, and consequently, 

 these are drawn in by endosmosis, whilst a small amount of the 

 above bye-products (chiefly organic acids) escapes by exosmosis. 

 The condition of tnrgfidity thus set up is always present in such 

 a cell as a root-hair, and, in fact, in any cell which is growing 

 to any extent, this condition being, indeed, essential to the 

 growth in extent of the cell-wall. 



A state of absolute equilibrium is, of course, rarely reached, 

 since the salts absorbed are as constantly removed by dififusion 

 into adjacent cells, and by the osmotic effect of the transpiration 

 of water from the leaves which leads to a progressively decreasing 

 concentration of the sap in the cells below, and in the tracheides 

 of the wood in stem and root (c/. Transpiraton). The passage of 

 the soluble elaborated foods (sugar, amido-acids), from the leaf- 

 cells to other parts of a plant is effected mainly by osmosis ; the 

 elaborated food is being constantly used up either for formative 

 processes or storage, and this removal of soluble foods from the 

 cell-sap leads to a corresponding intake of these substances from 

 adjacent and remote cells. The bye-products of metabolism are 

 also, as was noted above, useful aids in promoting osmosis in 

 this respect. 



The gases CO^ and 0^ must, before they are taken into a cell, 

 be dissolved in water; the cell- walls of the mesophyll cells in a 

 leaf are saturated with moisture, and the gases, entering by 

 means of the stomata are led to intercellular spaces surrounded 

 by the cells of the mesophyll (spongy parenchyma). If, fcHp 

 instance, the percentage of CO2 in a cell is smaller than that 



