74 



VEGETABLE PHYSIOLOGY 



close and intimate relations with these particles and with 

 the film of hygroscopic water which surrounds them. In 

 some cases the pressure between the two is so close that 

 the particles become embedded in the membrane (fig. 55). 

 The hygroscopic film of water is thus separated from the 

 interior of the root-hair by a most delicate pellicle of cell- 

 wall substance, lined by an almost equally delicate layer of 

 protoplasm. The vacuole of the hair contains a somewhat 

 acid cell-sap, the acidity being due to the presence generally 

 of acid potassium phosphate, by virtue of which osmosis is 

 set up ; the osmotic pressure of the 

 sap being considerable, the cell quickly 

 becomes turgid and distended, such 

 turgescence continuing so long as the 

 conditions remain favourable. The root- 

 hairs are very numerous, and their united 

 action causes a considerable accumula- 

 tion of water in the cortex of the root, 

 for it passes into the cells of this region 

 by osmosis through the base of the hair. 

 This, being one of the cells of the ex- 

 ternal layer, impinges upon one or more 

 of the cortical cells, which have a similar 

 reaction to that of the root-hair itself. 

 FIG. 55. ROOT-HAIR Osmotic currents are thus set up from 

 eyer y }}&{? and a gradual accumulation 



J 



of water takes place in the cortex of the 

 young root, making all its cells turgescent 

 and causing a considerable hydrostatic pressure in the 

 tissue. This turgescence with its consequent pressure soon 

 extends all along the axis of the young root, though it is 

 originally set up only by the region which is clothed by 

 the absorbing hairs. 



The central portion of the axis of the root is occupied 

 by a cylindrical mass which extends throughout its whole 

 length, and which is known as the stele (fig. 56). It is 

 generally marked off sharply from the cortex, the cells of 



IN CONTACT WITH 



PARTICLES OF SOIL. 



x 300. 



