58 BOTANY. [CHAP. n. 



many of the lower plants, where stomata are absent, 

 diffusion takes place through the cell- wall ; this takes 

 place to some extent in the higher plants also. 



The roots of plants growing in damp soil, and not 

 exposed to desiccation, are not furnished with an epi- 

 dermis, and in many plants certain of the external cells 

 of the youngest rootlets grow out into very delicate, 

 one-celled hairs known as roof-hairs; these are for the 

 purpose of absorbing water from the surrounding soil 

 that contain food substances in solution. In some plants 

 root-hairs are not developed, when the superficial cells 

 of the root perform their function. 



The mode by which water is taken up by the root- 

 hairs or cells of the root from the soil is due to the working 

 of a physical law called osmosis, which may be stated as 

 follows. When two liquids of different densities are 

 separated by a pervious membrane, the denser liquid 

 will attract a large proportion of the rarer liquid to itself 

 through the membrane the act of endosmose a very 

 small proportion of the denser liquid will at the same 

 time pass through the membrane and mingle with the 

 rarer liquid the act of exosmose. The above law can 

 be demonstrated by a simple experiment. If the bladder 

 of a sheep that has been well washed in a weak solution 

 of potassic hydrate, to remove the fat, be half filled with 

 a fairly strong solution of salt and water, and then com- 

 pletely submerged in a bucket of pure water, it will be 

 found after a while to be quite full of liquid, the dense 

 salt and water having drawn through the membrane a 

 large quantity of the rarer water. The cell-walls of 

 root-hairs are permeable to liquids, and the contained 

 cell-sap is normally much denser than the water in the 



