PLANT NUTRITION. 11 



be the result of the displacement say of one molecule of 

 water by one molecule of spirit, and so, throughout the 

 whole quantity of liquid, there is displacement and re- 

 placement until at length equilibrium is restored and a 

 thorough diffusion results. This power of diffusion does 

 not always exist. The molecules of water and of oil will 

 not mix or diffuse freely through each other. "Water 

 containing carbonic acid gas will not mix, in this sense 

 of the term, with water containing acetate of lead ; and 

 when the attempt is made, chemical change is set up, 

 the heretofore clear solutions that containing the gas 

 and that containing the lead -become when combined 

 opaque and milky, owing to a chemical change, resulting 

 in the formation of a white insoluble lead carbonate. 



It may be a truism to say, that for the process of dif- 

 fusion the liquids must be diffusible, but the fact must 

 be carefully borne in mind in all questions relating to the 

 feeding of plants. In the case of plants, the phenomenon 

 of diffusion, or the gradual admixture of two liquids of 

 different natures, is complicated by the presence of a 

 membrane in the shape of the cell-wall. The water from 

 the outside has to pass through the membrane to reach 

 the protoplasm on the other side. Speaking broadly, 

 there are no holes in the membrane through which the 

 water can pass. Ingress is secured by that process of dif- 

 fusion to which reference has just been made, and by 

 virtue of which the molecules of the membrane and the 

 molecules of the water shift and change places ; the 

 space that was occupied by a molecule of membrane is 

 now occupied by a molecule of water, and vice versa. 

 The access, therefore, of water into the interior of a 

 closed cell is the result of the process of diffusion. 

 Where two liquids mix without any intervening mem- 

 brane, the mixture is called diffusion simply ; where 

 there is an intervening membrane, the diffusion process 



