140 



GENEKAL BOTANY 



on the average to from .0001 to .03 per cent, to pass into the 

 water vacuoles of the root cells from the soil by osmosis. In 

 general, absorption by root-hair cells is undoubtedly to be 

 explained as an osmotic .process following the laws already laid 

 down regarding the movement of solvents and solutes through 

 a parchment membrane. The actual phenomena of absorption 

 and of movement of water and soil salts through the root will 

 be more readily understood by reference to Fig. 69, which indi- 

 cates a portion of a long section of a root, showing root hairs, 



a portion of the 

 cortex, and water 

 ducts. The lower 

 root hair is repre- 

 sented surrounded 

 by the soil, which is 

 made up of soil par- 

 ticles (solid black), 

 water films (concen- 

 tric lines), and air 

 (light spaces sur- 

 rounding the soil 

 particles). If now 

 nitrogen in the form 

 of a nitrate is in 

 solution in the soil 

 water in greater concentration than it is in the water vacuole of 

 the root hair, the laws of osmosis already enunciated will insure 

 the inflow of the needed nitrogen salt into the root-hair cells, and 

 thence, by the same physical law, into the cortex cells which 

 surround the duct. Water will likewise tend to flow from the 

 soil water into the water vacuoles of the root hair and root cortex 

 cells as long as these water vacuoles contain more solutes, and 

 so less water per unit of volume, than the soil water outside. 

 The result will be a continued flow of certain soil salts into the 

 root hairs from the soil, and a great pressure developed inside 

 of the root hair and cortex cells by the forcible inflow of large 

 quantities of water into the water vacuoles of these cells. This 



FIG. 68. The structure of root hairs 



A, a transverse section of a root with hairs; B, a single 

 hair with adhering soil particles 



