TROPISMS 289 



in a horizontal position the region just behind the tip 

 bends so as to bring the apex pointing vertically down- 

 wards. This bending is carried out by the cells on the 

 upper side of the elongating region growing faster than 

 those on the lower side, but how the root perceives 

 the stimulus of gravity is not by any means fully 

 understood. The branches of the taproot do not 

 grow vertically downwards but obliquely, making some 

 angle less than a right angle with the continuation of 

 the main root. 



Many roots are also negatively phototropic, i.e. they 

 tend to grow away from a source of bright light. The 

 strongest tropism of roots is, however, positive hydro- 

 tropism, or the tendency to grow towards a region of 

 greater moisture. In the case of a taproot growing 

 in ordinary moist soil these three tropisms will clearly 

 all act in the same direction, they will all take the root 

 directly downwards, for not only is this the direction 

 of the pull of gravity, it is the direction away from the 

 light, and it is also the direction of greater moisture, 

 for the surface layers of soil will be drier than the deeper 

 ones as a result of evaporation to the air. But if the 

 soil is rather dry and the source of moisture is to one 

 side, then the roots will bend in that direction, the 

 hydrotropic tendency overcoming the others. 



Growing roots require a supply of free oxygen for 

 respiration, and the roots of most ordinary land plants 

 flourish best and develop root hairs (their absorbing 

 organs) most abundantly in well aerated soil, in which the 

 air is saturated or nearly saturated with water vapour. 



Structure. — The apical (primary) meristem of the 

 root is covered by a root cap (Fig. 47, A, r.c), thickest 

 at the apex and gradually thinning at the sides till it 

 comes to an end a short distance behind the tip. The 



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