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 fdm 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 



I may be seen on the leaves of the 



p n houseleek and cabbage and on the 



('I' ii fruits of the grape, plum, and blue- 



;' berry. The bloom consists of a layer 



of w^ax particles scattered thickly over 



) - > the surface of a leaf or fruit. It forms 



: a layer that is nearly impervious to 



- i^^^;i^zi^xx^3^ water and helps to reduce water loss 



E.S.Clements through the epidcmiis. 



Fig. 35- Vertical sections of (2) Compact Icavcs. A plant may 



di^Lirr/ulrfwt:^ become adjusted to an inadequate 



growing in moist, shaded water supply by the development of 



situation (above), and when j^^^^^ ^^j^j^ compact tisSUCS. In SUCh 

 growing in dry, intensely ^ 



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



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