58 Plant Physiology 



38. Leaves poorly fitted for water absorption. In 



general leaves are of little practical value in the absorption 

 of water. On a hot day a wilted plant recovers after a 

 shower, not because it absorbs water rapidly through the 

 leaf parts, but because (1) the atmospheric conditions then 

 generally reduce the amount of transpiration, and (2) the 

 roots are able promptly to get the water needed. Never- 

 theless, partially wilted lettuce or peach leaves will be 

 revived if the blades are dipped into water, even though 

 the cut ends of the petioles are exposed to the air. A 

 flax plant or a cabbage leaf would show no perceptible 

 effect from the immersion for a long time. 



On the other hand some plants are capacitated for the 

 absorption of water through leaves or pitcher-like vegeta- 

 tive organs. Among these are certain of the Bromeli- 

 aceae (a family to which the pineapple belongs) possessing 

 leaves the bases of which sheathe the stem so closely as 

 to form reservoirs for precipitation water. In this 

 family, moreover, the absorption of water is more abun- 

 dant by means of certain cells in the peculiar shield- 

 shaped scales, and Tillandsia usneoides, the Florida moss, 

 is an extreme form in this respect. It is an epiphyte, 

 consisting of thread-like stems and narrow leaves, very 

 common on trees in the far South. This plant is provided 

 with much the same type of water-absorbing hairs which 

 give the entire surface a glistening appearance. The 

 aerial roots of orchids and some other tropical plants are 

 provided with velamen, a chambered epidermal tissue 

 which may absorb water like a sponge. 



