40 THE PHYSIOLOGY OF PLA^SiTS 



the roots is the water, which circulates in the interstices of the 

 soil, and is there retained. 



The capacity of a soil to retain water witliout allowing it 

 to escape in the form of drops (the retentiveness of the soil) 

 increases with increasing fineness of the particles ; but the 

 attraction which the particles of the soil have one for another 

 can be overcome even in fine-grained soils with great water 

 capacity ; large quantities of water are forced into the soil ; 

 the particles then become separated, and the soil becomes soft 

 and pasty. In the case of clays, less often of loamy soils, 

 a complete dissolution of the soil may take place. This may 

 be very serious in the case of seed-beds, as the soil between 

 the young plants is washed away, and the roots are laid more 

 or less bare. Such a condition of the soil will also cause the 

 sowing of crops to be postponed, and the yield of the harvest 

 in our country often depends upon the time of sowing. For if 

 plants have to produce a good stock or a number of large leaves 

 before they can yield a good crop, a retardation of the time of 

 sowing may prevent the proper development of such growth, 

 and an early dry summer will stimulate the development of 

 flowers, which will be scanty and poor. In hot and dry 

 summers we see the peas which have been sown late attacked 

 by mildew and smut, the kolrabi becoming woody, and the 

 lettuces running to seed. 



The heterogeneous particles of the soil possess in different 

 degrees the power of attracting by physical and chemical 

 means, and of retaining the substances which are washed into 

 the soil. This can be demonstrated by a very simple experi- 

 ment. If a drain-pipe is filled with clay, the highly-coloured 

 liquid manure from a dunghill will be entirely discoloured by 

 passing through it. The organic compounds are retained by 

 physical forces of attraction, while inorganic compounds are 

 chemically fixed. This power of the soil to bind the various 

 organic and inorganic salts is termed its power of absorption. 



The root of the plant will not alone absorb the water con- 

 tained between the particles of the soil, but can also make use 

 of the salts which have been absorbed by it. 



