54 



Science of Plant Life 



again be checked. This time the water can pass through the 

 tube freely, but it cannot evaporate through the cup so 

 rapidly because of the wax covering. 



How plants are adjusted to maintain the water balance. 

 Plants become modified in many ways in response to the con- 

 ditions of water supply and water loss under which they grow. 

 Among the adjustments that help plants to maintain an 

 advantageous water balance under dry conditions are : 



(i) Thickened cuticle and "bloom." The cuticle of a leaf 

 checks transpiration as does the wax film in the experiment, 

 and in plants of dry climates the cuticle may be so thick as 

 to reduce transpiration through the 

 epidermis to almost nothing. There 

 are many plants which secrete, in 

 addition to the cuticle, particles of 

 wax on their leaves or other parts. 

 This is the so-called " bloom " which 

 may be seen on the leaves of the 

 houseleek and cabbage and on the 

 fruits of the grape, plum, and blue- 

 berry. The bloom consists of a layer 

 of wax particles scattered thickly over 

 the surface of a leaf or fruit. It forms 

 a layer that is nearly impervious to 

 water and helps to reduce water loss 

 E.s.ciements through the epidermis. 

 FIG. 35. Vertical sections of (2) Compact leaves. A plant may 

 leaves of Mertensia showing b adjusted to an inadequate 



differences in structure when 



growing in moist, shaded water supply by the development of 



situation (above), and when leayes ^.^ compact tlSSUCS. In SUCh 

 growing in dry, intensely 



lighted situation (below). leaves the intercellular spaces are much 



