76 



General Botany 



for transpiration and their best development is available in the 

 upper layers of the soil. It has been shown by experiment that 

 for production of every pound of solid matter in the above-ground 

 parts of crop plants, from 300 to 500 pounds of water are required 

 in the central United States, and that from 400 to 1000 pounds 

 are needed on the plains of Colorado. The amount of water 

 used in transpiration is, therefore, many times the amount used 

 in the manufacture of food. It is estimated that an acre of corn 

 uses 1700 tons of water in transpiration and 4 J tons in photo- 

 synthesis. 



Substances and structures modify transpiration. Most leaves 

 possess certain structures that reduce the rate of transpiration. 

 The possession of these structures enables the plants to hve under 

 somewhat drier conditions than they otherwise could. 



(i) Thickened cuticle and " hloom.^^ The cuticle of a leaf checks 

 transpiration, and in plants of dry cli- 

 mates the cuticle may be so thick as to 

 reduce transpiration through the epi- 

 dermis 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 

 blueberry. 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 



Fig. 42. Vertical sections of ^ ^ ^ 



leaves of Mertensia, showing thrOUgh the Cpidcrmis. 



gJo:r;S Z£^:iZ I^ transpiration, however, most of the 

 ation (above), and when water is lost through the stomata. So we 



growing in dry, intensely i . -n r, 



lighted situation (below). may have a heavy cuticle and stiil nave a 



E. S. Clements 



