562 PHYSIOLOGY. 



the mineral elements of sea-water : and plants growing near the 

 sea derive a certain amount of the salts of sea-water from the 

 atmosphere, brought by the winds ; the salt spray is shown to be 

 carried great distances by its being injurious and destructive to 

 many kinds of plants growing exposed to sea- winds. 



Sect. 4. ABSOEPTICXN-. 



Since the lower plants consist of closed cells, in the interior of 

 which their vitalized substance resides, and the membrane of their 

 cells, so far as our investigations can reach, is, in general, desti- 

 tute of orifices, the food of these plants can only be taken up in 

 a liquid or gaseous condition by the still mysterious process of 

 imbibition. 



In plants of more complex organization, although loose parenchymatous 

 tissues exist, and the interspaces become concerned in at least secretion, 

 the external surface of the plant, by which food must penetrate, is care- 

 fully guarded by a continuous epidermis, entirely devoid of orifices in the 

 roots, the principal absorbing organs ; and though perforated by stomatal 

 orifices in the leaves and other aerial organs, these are carefully guarded 

 by special contrivances to prevent the entrance of solid matter, and in all 

 cases lead merely to intercellular passages, external to the membranes of 

 the vegetable cells. 



Absorption in Cellular Plants. In the Fungi and Algae ab- 

 sorption appears to take place freely at all points of the thallus to 

 which gases and liquids have access. The structure of Mosses, 

 Hepaticse, and the smaller members of the higher groups of Cryp- 

 togams are likewise so simply cellular that they appear to be little 

 dependent on root-structures. 



Absorption in Vascular Plants. In the higher Cryptogamia 

 and the Phanerogamia the absorption of liquids appears to be 

 confined to the roots and the root-hairs, the epidermis of the 

 leaves &c. being in general so organized as to oppose the entrance 

 of water, while the stomatal cells which guard its orifices, swelling 

 up so as to close the slit between them when filled with fluid, 

 concur to prevent the absorption of water or other liquid. Gases, 

 however, penetrate freely through most cell-membranes, and hence 

 may be absorbed by leaves, and can pass freely through the stomata 

 into the intercellular passages. 



Osmosis. The physical phenomena of diffusion and osmose are the most 

 important agents in the acquisition, by the cell-contents, of material from 

 without. These phenomena depend, 'first, on adhesion of the liquid to 

 the solid, and then on any circumstances which cause movements in the 

 molecules of the liquid, such as the attraction one for the other of two 

 fluids of different natures and densities. 



We may say, in g-eneral terms, that when two liquids of different den- 

 sities (the* one "colloidal," or little diffusible, the other "crystalloid" or 



