WORK OF THE SHOOT 95 



The food of plants, taken up by the roots, is absorbed 

 as very weak solutions, and one of the great functions of 

 leaves is to get rid of surplus water. The rate of transpira- 

 tion varies in different plants and under different conditions ; 

 there are many peculiarities and exceptions, but generally 

 the circumstances which favour transpiration are : (1) 

 winds ; (2) warm air ; (3) height of the plant above 

 ground, the upper layers of the atmosphere being drier 

 than those near the ground ; (4) numerous leaves, large 

 leaf-surface, and many stomata. On the other hand, 

 transpiration is reduced : (1) in calm weather ; (2) when 

 the air is cold ; (3) in plants which are low-growing ; and 

 (4) in plants which have fewer leaves, smaller leaf-surface, 

 and fewer stomata. 



Protection of stomata. For a shoot to perform its func- 

 tions in a satisfactory manner it is important that the 

 stomata should not become blocked by rain or dew, and 

 it is interesting to determine how leaves are protected 

 from this danger. Dip the leaves of several common plants 

 in water, then take them out and examine both upper and 

 lower surfaces. To what extent are they covered with 

 water ? Has immersion resulted in blocking a considerable 

 proportion of stomata ? What are the peculiarities of 

 surface-structure which prevent the surface from becoming 

 wetted ? Not only is the leaf practically non-absorbent, 

 but surface coverings such as hairs and wax render wetting 

 difficult or impossible. Repeat these experiments with 

 leaves of Pinks or Carnations ; note the blue-grey coating 

 of wax or ' bloom ' and see how easily it may be wiped 

 off. Dip a leaf in cold water, note the silvery-looking air- 

 cushion on the surface ; then remove it. Is it wet or dry ? 

 Dip another leaf into hot water, note what happens to 

 the bloom ; remove the leaf and compare it with a fresh 

 one. Bring the flame of a match near an uninjured leaf 

 and note the effect of heat on the bloom. Other leaves 



