CHAPTER IV 



THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



SECTION 15. General. 



EVERY plant and every cell must be supplied with not only water, 

 but also other necessary substances, while at the same time certain 

 products of metabolism, such as carbon dioxide, must be excreted. Plants 

 absorb all substances offered to them in soluble form, whether useful or 

 not. The importance and mode of utilization of the absorbed food 

 material will be dealt with later ; at present we have to consider the ways 

 and means by which food material is obtained and accumulated, as well 

 as the conditions which influence absorption and translocation. It is clear 

 that in the cell itself only those fluids and dissolved substances which are 

 imbibed by the cell-wall and protoplasm are able to penetrate to the 

 interior of the cell. The cell-wall, saturated with water, is in general much 

 more permeable than the protoplasm, and hence many substances can pass 

 through the cell-wall, but not through the living layer of plasma within. 

 Thus the plasma while living is impermeable to the coloured cell-sap which 

 it may enclose (beetroot, &c.), but when the cell is killed, the red sap 

 quickly diosmoses through the dead and altered plasma, and also through 

 the cell-wall which it was previously unable to reach. In the same way, 

 when a cell is plasmolysed by a coloured saline solution, the latter penetrates 

 the cell-wall but not the plasma, and hence accumulates in the space left 

 vacant between them. A substance imbibed by the cellulose cell-walls 

 may reach the centre of a tissue without having penetrated a single proto- 

 plast. It is indeed possible that the water and salts absorbed by the roots 

 pass mainly if not entirely through either the walls of living cells or the 

 walls and cavities of dead wood fibres, &c., so that only on reaching the 

 crown of a tree do they penetrate the protoplasts of the actively growing 

 tissues localized there. Thus, when watery solutions of indigo-carmine, 

 or aniline-blue are absorbed by the root, or by the cut surface of a stem, 

 they may be traced by the colouration they produce, and can be shown 



