Deserts 83 



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 ren- 

 ders them flabby. But when the dew falls on them trans- 

 piration 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 contrary; 

 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 continues 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; and it also varies greatly at different seasons of 

 the year, according as the plant is growing, or putting 

 out buds and leaves, or not. 



Experiments made by watering plants with colored 

 solutions are not very satisfactory, because the coloring 

 matter may be caught and entangled, while the water 

 moves on without them. Still, it is interesting to learn 



