56 THE BIOLOGY OF FLOWERING PLANTS 



Now wilting is an important if not quite definite 

 physiological state, but we are here concerned with 

 the attempt to use it to indicate a critical point in the 

 relation of soil moisture to the plant. This can be done only 

 with great caution, for it seems clear that wilting must occur 

 at different moisture contents as the shoot of the plant is 

 subject to conditions of drought or of humidity, and 

 transpiration is rapid or slow. It is certain, too, that a 

 limitation of water supply to the leaves is sometimes the 

 result of the condition of the conducting vessels, and this 

 may occur more frequently than is yet suspected. The 

 necessity for using the wilting coefficient with caution will 

 appear from the investigations we must now consider, 

 on the effect of soil composition and of the forces retaining 

 water in the soil. 



We have already seen that the amount of water retained 

 is a function of soil constitution, and the same is true of the 

 proportion of water which the plant cannot readily utilise. 

 The finer particles of the soil are active in retaining water 

 against gravity ; they also hold it against the plant. This 

 was demonstrated as long ago as 1 865 by Sachs (quoted from 

 Jost) for the tobacco ; his results are given in Table XII. 



TABLE XII 



Supply of Water to the Tobacco by Two Different Soils 



The soil with humus withholds more water from the 

 plant than the sand, though, as it has a greater water 

 capacity, it also supplies much more. Recently much exact 

 work has been done on this relation. Some of the results 

 obtained by Crump (igi^b), for moorland plants, are given 

 in Table XIII. 



