194 General Botany 



are in contact with only a small part of the soil solution. Let us 

 suppose that transpiration is active and that all the water in 

 immediate contact with the roots passes into the plants. When 

 this occurs, water moves from adjoining spaces to replace it and 

 all the spaces again have about the same relative amount of water 

 in them. This movement of water to the spaces near the roots 

 continues in the soil as long as the capillary columns of water 

 among the particles near the plant do not break. If this happens, 

 the movement of water in the soil toward the root is stopped and 

 the soil no longer delivers water to the root. If transpiration 

 continues, the plant wilts. 



The freedom with which water moves by capillarity varies 

 greatly in different soils. Since it is the lateral and upward 

 movement of water that determines the continued supply to the 

 root, plants wilt in some soil sooner than in others. Wilting is 

 determined by the availabiHty of water, and this in turn depends 

 upon the conditions in the soil that maintain continuous water 

 movement toward the root. Consequently all kinds of plants 

 show wilting when the water content of a given soil is reduced to 

 a certain point. 



Summary of absorption. The roots and root hairs form a 

 mechanism into which water and mineral salts move readily by 

 diffusion, or by those forms of diffusion known as imbibition and 

 osmosis. Outside the root is another mechanism, the soil, in 

 which water moves freely or with difficulty, according to its 

 texture. If the water moves freely by capillarity, a large part of 

 the water in a given body of soil will pass to the water-absorbing 

 root, even though the root is in contact with only a small part 

 of the soil water. If the soil spaces are very minute, the water 

 is held more tightly by the soil particles, movement is impeded, 

 the continuous capillary columns of water break, and the water 

 ceases to move toward the root. Under these conditions absorp- 

 tion is stopped and the plant wilts, although there may still be a 

 considerable amount of water in the body of soil as a whole. 



