87 



filled with a strong solution of common salt, or some other 

 solution denser than water, and is then immersed in pure 

 water the quantity of water passing into the bladder is con- 

 siderably greater than that of the solution which passes out of 

 it. The more concentrated the solution also, the more rapid 

 is the endosmotic current. The wall of a plant cell, like other 

 organised substances, is capable of imbibing water and swelling. 

 Protoplasm is in this way able to imbibe water to the extent of 

 90 % of its entire weight, although it then becomes almost fluid. 

 That great pressures may be created by this property of im- 

 bibition is shown by the fact that the swelling of wooden 

 wedges, which have been moistened with water, will suffice 

 to split granite rocks, thus indicating how strong this 

 attraction for water is. The walls of living active cells are 

 always saturated with the water they have imbibed and in 

 this condition they are permeable to solids and gases, pro- 

 vided these are in a state of solution, and in this state these 

 substances are able to diosmose freely through the cell walls. 



In the living cell, however, another factor besides the cell- 

 wall enters into the case which is able to a certain extent to 

 control and regulate this purely physical phenomenon of dios- 

 mosis and that is the living protoplasm which lines the interior 

 of the cell-wall. It by no means follows, for example, that 

 substances which are able to readily pass through the cell-wall 

 will be able to pass through the protoplasmic lining and thus 

 penetrate to the cell-sap. The protoplasm in fact decides what 

 shall, and what shall not, pass respectively from the cell-sap to 

 the exterior of the cell and from the exterior into the cell-sap. 

 Seeing that the protoplasm of one plant differs more or less 

 fundamentally in character from that of another plant and 

 that the protoplasm of each individual thus has its own parti- 

 cular requirements to satisfy, we should natural!}' expect 

 that the materials absorbed should differ in different cases, 

 and that while one plant takes up large quantities of a particular 

 substance another may take up less, and this is actually the case. 

 Moreover, plants on account of this selective power are able 

 to collect and accumulate large amounts of substances which 

 exist only in very small quantities in their surrounding medium, 

 diffusion from outside continually replacing the small quantities 

 of any particular substance which the plant may absorb from its 

 environment. 



84. The controlling power of the Turgescence. 

 living protoplasm is also responsible for the phenomenon 

 called turgescence. In the cell-sap of the living alga, for 



