228 STRUCTURE AND VITAL PROCESSES OF PLANTS 



how long is the soil able to retain its moisture. Moisture is evapo- 

 rated from the surface. But the rate of evaporation is very small 

 when the surface is loose or covered by fallen lear^^es or hay. 

 Another part of the water sinks deeper and deeper, and, in a 

 certain depth, forms what may be called ground-water, when a 

 layer is reached that does not allow the water to percolate. 



(b) Exploration of Soil by Roots for Moisture. — The fact that 

 roots follow up and grow towards moist places, is generally 

 known. This is strikingly illustrated by keeping a flower-pot 

 watered daily in the neighbourhood of a tree. The roots of the 

 tree will be found to penetrate the hole at the bottom of the pot 

 and invade the pot. We thus see thRt side-roots are sensitive for 

 moisture, or in other words, that they grow stronger towards 

 moist parts of the soil than in the direction of dry soil. 



There is, however, a great difference in the way in which 

 various plants explore the soil for moisture. Some grow their 

 tap-roots right down and fetch their water from the deep: in other 

 plants the tap-root does not grow vigorously, and their side-roots 

 keep near tlie surface, feeding on the surface moisture. The 

 former may be called deep-ground-feeders, and the latter surface- 

 feeders. The dift'erence between dcep-ground-feeders and surface- 

 feeders is noticed in a coffee-plantation, where coft'ee-i)lants, 

 which are surface-feeders, are not harmed in any way by the 

 presence of the large shade trees in their vicinity, as these are 

 deep-ground feeders and do not interfere with the sui)ply of 

 moisture to the coffee shrubs. Casuarina trees, to mention 

 another example, are surface-feeders, and hence the growth of 

 annuals near such trees is greatly hampered. 



The extent to which the roots of many plants spread in the 

 soil is beautifully suited to the ivay in which the rain-ivater is 

 conducted down to tlie soil by the leaves. In some cases, as in 

 the Carrot and other plants with swollen roots, tlie leaves conduct 

 the rain water towards the centre of the plant, where it runs 

 down the stem to the tai)-ro()t. Such tap-roots have only small 

 side-roots. In other plants (see Mango tree, page 27) the water 

 is conducted from leaf to leaf towards the peri})liery of the tree 

 and there falls to the ground: and in these plants the ends of 



