ROOTS AND THE SOIL. 



117 



following amounts of water (in cubic centimetres) to 

 sink through it in 24 hours. 



The amount of water in saturated soil is also very 

 different according to its nature : 1 00 cubic centi- 

 metres of peat will contain 82 grammes of water, of 

 loam 43, of sand 34, and of clay 51. Then, again, if 

 the particles of soil are more than 2-3 millimetres in 

 diameter, it is not possible for water to rise by capil- 

 larity from below. Hence, on the clinkers of a railway 

 track, for instance, the plants are reduced to what they 

 can extract from the ground, so long as it is kept 

 moist by rain, which, in such places, dries up very 

 rapidly. Therefore the railway line will possess 

 southern and dry-climate or sun-loving species, such as 

 Linaria viscida and L. rep ens. Plants such as Tragopogon, 

 and even Viola cornuta, only find it possible to exist, in 

 such wet and cold districts as Glasgow and Dumfries- 

 shire, upon the railways. (See p. 334.) 



Generally speaking, the finer the particles of soil the 

 more easily water can be carried up to the thirsty 

 roots. And, in fact, in clays and loams, the danger is 

 often that the plant receives not too little but too much 

 water. The. soil will tend then to bear rushes and moss, 

 and the more useful plants, whose roots object to water 

 in excess, will become sickly and languish. This can 

 often be seen in Glasgow in small garden plots or 

 areas, where the grass will often die, forming a sodden 

 and unsightly felt, on which a few green Algae begin 



