ROOTS AND THE SOIL. I i 5 



vapour-laden air to ascend to those colder levels of the 

 atmosphere, in which it is obliged to condense on any 

 convenient dust particle, and fall as rain. After the 

 shower has fallen, most of the water sinks into the earth, 

 though much of it is hurried off by streams to the sea. 

 The sinking water eventually reaches some impervious 

 layer, probably of clay, and there accumulates in under- 

 ground lakes, or gently flowing streams. As soon as 

 the surface begins to dry by the action of sun and 

 wind, the water rises from this underground reserve, or 

 subsoil water. If there are no plants on the soil above, 

 it ascends, and is evaporated very quickly, so that 

 drought soon begins to set in ; but, if there are live 

 plants of any kind, their root hairs fix themselves on 

 the wet earth particles and absorb the moisture, which is 

 eventually given back to the atmosphere by the leaves ; 

 only, however, after it has performed all the duty which 

 the plant requires of it, and only just as rapidly as the 

 live cells of the leaf choose to let it go. 



The plant world is then a sort of temporary passage 

 in the general water circulation of the globe. 



Much of the surface drainage is hindered in its 

 passage to the sea, and often absorbed by vegetable life. 

 A wet rock is often covered by Algae, such as the dark 

 purple Rivularia or by Liverwort cushions. Mosses and 

 Lichens also are kept fresh and living by occasional 

 showers. In any place where water tends to gather, 

 marsh plants accumulate; sometimes they even delay the 

 escape of water for a whole vegetation period. This 

 is conspicuously the case in Uganda, where the Papyrus 

 swamp is a sort of natural water reservoir, allowing tall 

 grasses and even forests to flourish in the valleys, in 

 spite of the nine months' drought. Forests on the hills 

 in a dry country, such as India, have a most important 



