ROOTS AND THE SOIL. I I9 



few are absolutely essential to plant life. The most 

 important are : Nitrates, Sulphates, Phosphates, and 

 Salts of Potassium, Soda, Lime, Iron, Magnesium, 

 Manganese, Chlorine, and Silica. The first three are 

 essential, as they are essential parts of the living matter 

 protoplasm. The others are utilised in the chemical 

 changes that go on within the plant : lime, for instance, 

 is taken in, in the form of calcium sulphate (see p. 106) ; 

 iron has something to do with the formation of green 

 chlorophyll ; and potash is of great importance in the 

 process of assimilation. Certain kinds of plants make 

 special demands in the way of certain minerals : thus 

 Potatoes and Turnips prefer potash ; Cereals require much 

 silica ; and Leguminosae take in a large proportion of 

 lime. It is upon this fact that the advantage of a 

 rotation of crops depends. 



As the roots are intended to exploit this mineral 

 water in the soil, it is only natural to expect that they 

 should exhibit in every detail of structure, and in their 

 method of development, an exact adaptation to their 

 duty. 



Their method of growth shows this distinctly, for the 

 main root, in most cases, works towards the centre of 

 the earth, yet not in a straight line, for it often 

 revolves with the tip moving in a circle or ellipse, so 

 that it is able to take advantage of any convenient 

 crack or crevice, e.g. a worm burrow. The side branches 

 go off horizontally, and their branchlets go off both 

 directly upwards, and downwards. Thus every year 

 the sphere of influence is extended and new ground is 

 broken. A very old comparison of the root system to 

 a river is absolutely correct : the branchlets of the root 

 correspond to the mountain streams ; the tree trunk or 

 stem to the main river ; and the leafy branches to the 



