CHAPTER XVII 

 THE ROOT 



Having now obtained some knowledge of the appearance 

 and structure of the elements of which the tissues of 

 the bodies of the higher plants are composed, we pass 

 on to consider the nature, structure and functions of the 

 main vegetative organs of the plant — the root and the 

 shoot (stem and leaves) — ^aggregations of these tissues. 



Roots are colourless axes of the plant which 

 typically descend into and grow in the soil, fixing 

 the plant and absorbing water and dissolved salts 

 from the soil. It is pretty certain that they are to 

 be regarded as specialised underground branches of the 

 thallus which formed the body of remote ancestors of 

 the higher plants. According to their origin in the 

 life of the individual seed plant we distinguish the 

 primary root system, i.e. the taproot or main descending 

 axis of the plant (already laid down in the embryo) 

 and its branches, from the adventitioms roots which 

 arise from some part of the shoot, e.g. from a rhizome 

 or from the lower part of an erect stem (see Chapter XV, 

 p. 261). These two kinds of roots do not, however, 

 differ structurally in any essential respect. 



Tropisms of Roots. — Some of the most important 

 physiological characters of roots are their distinctive 

 tropisms (see Chapter V, p. 87). The taproot is in 

 the great majority of cases positively geotropic, i.e. it 

 grows towards the centre of the earth, and if it is placed 



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