Deserts 107 



rapidity in some plants, as will be seen presently ; but 

 when leaves revive on a dewy evening, or during a 

 shower, it is not because they have drunk in any of 

 these fresh supplies. Moisture is constantly passing up 

 to them in larger or smaller quantities from below ; but 

 they part with it nearly as fast as they receive it 

 generally, and faster than they receive it in dry weather. 

 It is the want of sufficient moisture which makes them 

 droop and renders them flabby. But when the dew 

 falls on them transpiration ceases, or nearly so ; they 

 are able to keep nearly all the moisture sent up to 

 them, and so they swell out again and stiffen, and hold 

 themselves up. 



A similar effect may be seen even in cut shoots which 

 have been allowed to fade, and are then placed in very 

 damp air. No moisture is taken up ; quite the con- 

 trary; the continued decrease in their weight shows 

 that moisture is passing off into the air, little by little, 

 all the time ; but stem and leaves are losing it very 

 much more slowly than they did in ordinary air ; and 

 as water from the lower, older parts of the stem con- 

 tinues to rise, as it did before the shoot was severed 

 from the tree, so the younger parts at the top, the 

 leaves and buds, are refreshed and revived. Of course, 

 this can go on only for a time, so long, that is, as any 

 of the original moisture is left in the stem ; and when 

 this is exhausted the leaves droop as before, and at last 

 wither entirely. 



And now to gain some idea, if we can, of the rate at 

 which water travels upwards from the roots to the 

 leaves of a plant. This of course varies enormously in 

 different plants, because some transpire so very much 

 more than others j and it also varies greatly at different 



