1 06 Deserts 



to check transpiration and allow them to retain most 

 of the moisture they draw up. But, when once a plant 

 has thoroughly flagged, the case is different. Then, 

 nothing short of water supplied to the roots will be 

 sufficient to revive it. Damp air will be of no use ; 

 neither will the heaviest dew avail anything. The 

 roots, and the roots only, can furnish the necessary 

 supply. 



Of course every substance — even, as we have seen, 

 the hardest rocks — will absorb some amount of water 

 when actually steeped in it ; and so, if a withered shoot 

 is kept soaking in water, it will absorb a certain quantity 

 in time, as any piece of dead wood does. But leaves 

 and stems have little or no power of absorbing moisture 

 from the air. 



This is the general rule, to which there are a few, 

 but only a few, exceptions ; lichens, which have no 

 roots, do draw moisture from the air, and would be 

 badly off if they could not, considering the bare rocks 

 upon which they grow. Mosses, too, which grow 

 where there is little or no soil, also supply themselves 

 with moisture from the air to a great extent ; and so it 

 is believed do plants, such as the mistletoe, which grow 

 upon others. 



But still the general rule holds good ; leaves have 

 little or no power of absorbing moisture either from the 

 air or from water poured upon them. 



And yet, how the drooping leaves revive on a dewy 

 evening, or in a shower of rain, or even under the 

 influence of a shower from the watering-pot ! The 

 water cannot surely have had time to reach the roots, 

 and then to travel up the stem. 



Water certainly does travel upwards with amazing 



