122 WATER-ABSORPTION AND TRANSPIRATION. 



155. How Leaves are Protected. We shall see later 

 tLat young leaves nearly always have some mode of protec- 

 tion against loss of water ; they are often covered with hairs, 

 which may disappear as the leaf grows to its full size, or 

 they may be bathed in a gummy or resinous liquid ; or they 

 may be rolled or folded up at first. These protective arrange- 

 ments are especially necessary when the leaf is young and its 

 tissues tender and liable to lose water readily, or to be injured 

 by strong light or by cold. 



In many plants the mature leaves show similar protective 

 arrangements, which are especially marked in plants which 

 grow in places where they are exposed to strong light, to 

 drying winds, cold, and other injurious conditions, such as 

 a scanty or precarious supply of water. Most evergreen 

 plants have a thick cuticle, or a layer of clear cells below the 

 upper epidermis, and the stomates are often sunk below 

 the level of the epidermis. Transpiration is also diminished 

 by the rolling up of the leaf (Heather, etc.), by a thick cover- 

 ing of hairs (Woolly Mullein, etc.), and by a covering of waxy 

 " bloom " which makes the surface unwettable (Broad Bean, 

 Tropaeolum, etc.). 



(a) To imitate the effect of a hairy covering, use the apparatus of 

 Fig. 41 A or Fig. 42, after tying with thread a piece of cotton-wool 

 over each leaf, so as to cover the lower surface of the leaf (vary the 

 experiment by covering both surfaces, or only the upper surface), and 

 note the effect this has on the rate at which the leaves give off water 

 vapour. 



(6) The effect of rolling can be observed by folding each leaf or 

 rolling it up longitudinally to form a tube, with the lower epidermis 

 on the inside, and tying it with threads, and noting the reduced rate 

 of transpiration. 



(c) Take two leaves from an Indiarubber Plant, and remove the 

 "bloom" from one by sponging its surface carefully with warm 

 water ; weigh both leaves, and hang them up ; the bloom-less leaf will 

 lose water more rapidly than the untouched one. 



156. Boot-Pressure. We have studied in some detail 

 the current of water, with dissolved salts, which passes up- 

 wards from roots to leaves. How is this transpiration current 

 set up and maintained ? What part does the root play in the 

 process ? 



In the first place the evaporation from the leaves tends to 



