Roots 



129 



as fast as the oil is consumed, more rises to take its 

 place. 



In time of drought the soil may appear quite dry 

 even to some depth, but, as we have seen in the case 

 of the tobacco planted in sand, plants can continue to 

 draw moisture from the soil long after all trace of 

 moisture has vanished, so far as can be seen. To find 

 it at all we should have to dry the soil by artificial 

 heat ; but it is there, and the plant manages to draw it 

 out. 



In very long droughts, even the springs near the 

 surface may fail, all their water being drawn away 

 from them by degrees ; but still, at a greater or less 

 depth, some water there is, for the deeper wells do 

 not fail though the shallow ones may; and, unless 

 this water be buried under some bed through which 

 it cannot pass, it will continue to rise to the sur- 

 face. 



The water which plants need, therefore, for growth, 

 and to supply the small loss by evaporation and the 

 large loss by transpiration, comes to them from rain and 

 dew ; it is also very largely absorbed from the air by 

 the soil; and the large stores accumulated in the 

 ground are also drawn upwards as they are needed^ 

 chiefly, of course, during the spring and summer. Ift 

 autumn, when growth ceases, transpiration is less ; in 

 winter, when the trees are bare, there is next to none, 

 so that they have little need of water. In spring and 

 summer, on the other hand, they need much, and 

 receive it, in part, from the accumulated stores of the 

 other months. 



All the water which plants transpire — in many cases 

 a very large quantity, as we have seen — is taken up by 



9 



