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 famished 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 root-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 osni,osis, 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 



