The Structures and Processes of Roots 175 



small amount of sugar is present inside the tube, the water 

 will rise slowly and exert but little pressure. 



When a membrane permits water or other substances to 

 pass through, it is said to be permeable to that substance. 

 For example, animal membranes are permeable to water and 

 to various dyes. A membrane that allovv^s one substance to 

 pass through it, but retards the passage of another substance, 

 is said to be selectively permeable. The membrane on the 

 thistle tube is selectively permeable, because it allows the 

 passage of water but restrains the sugar that is dissolved in 

 the water. The membranes in root cells are permeable to 

 water, but they do not allow sugar and many other sub- 

 stances found inside the cells to pass out. 



The conditions for osmosis as it occurs in plants, then, in- 

 clude a selectively permeable membrane between two bodies of 

 water, one of which contains a dissolved substance that does not 

 pass through the membrane readily. 



Osmosis in roots. The cellulose walls of plant cells are 

 permeable to water and to most of the substances that dis- 

 solve in w^ater ; but the layer of cytoplasm inside the cell wall 

 forms a selectively permeable membrane about the cell con- 

 tents. The cell sap in the vacuole may contain sugar and other 

 dissolved substances, just as the water in the thistle tube con- 

 tained sugar. The outer cells of the root are in contact with 

 the soil water. Hence the water passes into these cells in 

 the same way as into the thistle tube. In a similar manner 

 water may move from one cell to another and replace the 

 water that is being carried to the stem and leaves through the 

 conductive tissue. The path of the water from the epidermal 

 cells is through the cells of the cortex to the water-conducting 

 vessels in the interior of the root. 



