320 TEXTBOOK OF BOTANY 



WILTING FIELD 



PERCENTAGE CAPACITY 



LITTLE OR NO MOVEMENT OF 



FREE MOVEMENT OF 



WATER IN SOIL ' WATER IN SOIL 



WATER CONTENT 



N^o^GRowTH POOR GROWTH BEST GROWTH poo" growth or death 



Fig. 133. Diagram illustrating water relations of a clay loam. 



innumerable branches and root hairs penetrate to all parts of the moist 

 layer. Oxygen is abundant in the soil air and respiration is not restricted. 



Let us now assume that no more water is added. After a few weeks, 

 depending on the temperature, the young plants begin to wilt in the 

 middle of the day, but recover their turgidity at night. The rate of absorp- 

 tion of water is not equal to the rate of transpiration during midday, but 

 at night it is greater than the water loss. After several more days, how- 

 ever, the plants wilt and do not recover at night. In spite of the large 

 root surface, water no longer moves from the soil to the plants, and they 

 become permanently wilted. The wilting percentage is the amount of 

 water present in a soil under these conditions. It varies from 1 to 10 per 

 cent in sandy loams, from 15 to 20 per cent in fine clay loams. 



Absorption of water by osmosis. Thus far we have visualized osmosis 

 as occurring in each individual cell, in which the water inside the vacuole 

 is separated from the more concentrated water outside the cell by the 

 differentially permeable cytoplasmic membrane. It is possible also to 

 visualize a whole root as one osmotic unit; that is, the water in the xylem 

 vessels in the central cylinder of the root is separated from the water in 

 the soil by the cambium and bark ( phloem, pericycle, cortex, epidermis ) , 

 which may be considered as one complex differentially permeable 

 membrane. 



These relations may be demonstrated by means of a thickened root, 

 such as that of carrot. To simplify the problem, one may remove most 

 of the xylem of the root with a cork borer and regard the cavity thus 

 formed as one large xylem vessel, in lieu of the many original vessels 

 removed by the cork borer. A glass tube may be securely attached to 

 the top of the root to represent the continuation of a xylem vessel into 

 the stem of the plant. If the cavity in the root made by the removal of 



