4 2 



would produce that condition of the protoplasm called plasmo- 

 lysis, and so upset the entire regulation of the cell ; consequently 

 it becomes a necessity that the substances to be taken up should 

 be in the form of a very dilute solution. The difference, then, 

 in the passage of fluids holding substances in solution through 

 the cell-wall from a case of ordinary osmosis, is due to the living 

 layer of lining protoplasm, which can modify or prevent the 

 entrance of substances in solution from the exterior; thus it 

 allows the various solutions of salts, if sufficiently dilute, to 

 pass, while it prevents the entrance of sugar and coloring 

 matters. Let us consider more exactly how this process of 

 absorption is accomplished — that is to say, how the substances 

 absorbed pass into the cell. The cell-wall, according to the 

 researches of Nageli, consists of micellae surrounded by water, 

 and the water intervening between the various micellae takes 

 up from the surrounding water a certain proportion of salts, 

 which proportion is passed on from micella to micella through 

 the cell- wall till it reaches its interior, then from micella to 

 micella through the protoplasm to the vacuole, so that a process 

 of what is called fluid diffusion goes on from the exterior to the 

 interior of the cell ; but in the course of time a condition of 

 equilibrium is set up, and the cell-sap will contain as much per 

 cent, of the salts as the solution outside it, and then no more is 

 taken up. 



In the case of ordinary plants, every portion of the plant is 

 not capable of absorption, and this function is performed in 

 them by certain special organs. The only organs of such a 

 plant which are capable of absorbing are the leaves and the 

 roots. The leaves, as we shall see, absorb the gaseous nutriment 

 from the air ; while the roots absorb water from the soil, or if 

 roots are absent, their place is taken by hairs, shoots, or 

 branches of the thallus or plant-body. The roots are not capable 

 of absorption over their entire surface, but only by a particular 

 part. The tip of the root is covered by a rather loose external 

 sheath or root-cap, and this was formerly believed to be the 

 absorptive portion of the root, and hence received the name of 

 the spongiole, and this idea still lingers in many botanical books. 



