228 BOTANY PART i 



in the root-hairs. A plant can extract water even from a soil which 

 appears dry. As absorption from such a soil continues the plant 

 begins to wilt, but even in this state absorption is still taking place, 

 though it does not go so far as to obtain the last traces of water from 

 the soil. The process continues further in desert plants according to 

 FITTING, since their cell sap is highly concentrated and can develop 

 a very strong osmotic suction ( 12 ). 



Following SACHS, BRIGGS and SHANTZ have determined the water-content of 

 the soil at the moment of wilting. This they term the coefficient of wilting and 

 express it as a percentage of the dry weight of the soil. They find that it has 

 nearly the same value in different plants, but differs widely in different soils. 

 Thus the coefficient of wilting in coarse sand is 0'9, in fine sand 2 '6-3 '6, in sandy 

 loam 9 '9, and in clayey loam up to 16'5. 



Other Types of Absorption of Water. Some plants do not obtain their water 

 from the soil. They belong chiefly to two distinct ecological groups, the 

 EPIPHYTES and the WATER PLANTS. The morphological and anatomical pecu- 

 liarities found in relation to the absorption of rain and dew by -the sub-aerial 

 organs have already been dealt with on p. 184. 



Movement of Water in the Plant 



That a movement of water from the roots to the aerial parts of the 

 plant must take place follows from the fact that water is required in 

 the development of new cells in the growing regions. The plant, 

 however, requires far more water than is needed for its construction, 

 because it gives off large quantities of water in the form of vapour, 

 and a less amount in the liquid form from its aerial portions. The 

 former process is known as TRANSPIRATION, the latter as EXUDATION. 



Transpiration ( 13 ) 



The vegetable cell, like every free surface of water or substance 

 swollen with water (e.g. gelatine, mucilage), must give up water to the 

 air so long as the latter is not completely saturated. Under certain 

 conditions the loss of water from some parts of plants (e.g. roots, sub- 

 merged portions, shade plants) is very great. Such objects exposed to 

 dry air, especially in the sun, lose so much water that they become 

 collapsed, limp, and wilted, and ultimately dried up. The leaves borne 

 on ordinary land plants behave otherwise. At first sight no loss of 

 water is perceptible from them ; but they also wilt during a drought, 

 which renders absorption of water from the soil difficult. If the supply 

 of water to them is interrupted completely, as by cutting them off, 

 the wilting occurs more speedily. That they as a rule do not wilt 

 when in position on the plant evidently depends on the fact that water 

 is supplied from below in equal amount to that evaporated from above. 

 The giving off of water can be demonstrated by simple methods. 



