MOISTURE 103 



the stem, as is so often done in gardens, instead of 

 over a wide area, the roots will concentrate them- 

 selves there, and in consequence have but a small 

 volume of soil from which to draw both their mineral 

 food-material and water; and if the latter fails, the 

 plant will suffer. Whereas, if the water is poured in 

 a wide circle round the plant, the roots are induced 

 to spread out widely, thus having a much larger 

 volume of soil from which to draw their food and 

 water, and are in consequence less liable to be killed 

 by drought. In nature this is provided for by the 

 outward sloping of most leaves, so that rain drips off 

 them some little distance from the main stem. With 

 a shady tree, whose almost rain-proof crown of foliage 

 is highest in the middle and slopes towards the sides, 

 this is even more the case, for the water drips from 

 leaf to leaf till it drops to the ground in a wide circle. 

 This is why though near the trunk there is shelter, 

 towards the sides the drops are heavier than in the 

 open and the ground underneath wetter. It is here 

 that the youngest roots are in consequence concentrated, 

 and, since they alone possess root-hairs, it is here that 

 the chief absorption takes place. As the branches 

 grow and extend the crown wider, the circle of 

 wetness is widened too, and with it the rootlets 

 invade new ground. Exactly the opposite is how- 

 ever the case with some plants, especially certain 

 monocotyledons, such as ALOCASSIA, CANNA and the 

 Traveller's palm (p. 398), whose leaves slope to- 

 wards the centre and are channelled so that rain 

 water is directed inwards. This hydrotropism of roots 

 is responsible also for the invasion of well watered 



