230 PART III. VEGETABLE PHYSIOLOGY. 



solution of gum, albumen or other colloidal substance, and on 

 the other an aqueous solution of some crystalline substance, the 

 latter will pass through to the colloidal side with great facility, 

 while but little, if any, of the colloid will traverse the membrane 

 in the opposite direction. To phenomena like these the term 

 osmose is applied. The membrane contains no visible pores, 

 even though viewed with the highest powers of the microscope. 

 How, then, is the phenomenon to be accounted for ? There is 

 reason to believe that organic membranes consist of a minute 

 network of solid organic particles or threads, the meshes of 

 which are filled with constituent water, or water which is held 

 by an attractive force so strong that it cannot be expelled with- 

 out destruction of the membrane, and yet which still retains 

 essentially the properties of water. Such a membrane would 

 permit the diffusion through it of any liquids capable of mixing 

 with water, very much as though only a film of water were stretched 

 between the liquids. There would, however, be this difference : 

 the interchange would be somewhat impeded by the network, and 

 it is evident also that the denser liquid, other things being equal, 

 would be more impeded than the less dense one. We are thus 

 able to explain the phenomena of osmose. 



Now, the cell-walls of root-hairs and root-cells are organic 

 membranes which separate a colloidal solution within the cells 

 from a saline solution without, namely, the water that, with its 

 dissolved mineral matters, permeates the soil. An interchange 

 of liquids therefore takes place, and much the stronger current 

 is toward the interior of the cells. 



The imbibition of liquid through the roots is aided in various 

 ways : (1) By the branching of the root into numerous fine divi- 

 sions which develop near their tips great numbers of root-hairs. 

 (2) By the fact that the root-hairs are in intimate contact with 

 numerous earth particles, and even grow fast to them. (3) Each 

 earth particle, except when the soil is excessively dry, is envel- 

 oped in a closely adhering film of water, and as each particle is 

 in contact with adjacent ones, the whole virtually forms a compli- 

 cated network of capillary tubes through which water is drawn 

 from a distance, in proportion as that adjacent to the root-hairs 

 is absorbed. (4) The liquid of the exosmotic, or lesser current, 

 from the interior of the cells outward, is of indirect service, on 



