WATER RELATIONS OF PLANTS 293 



hardy plants, as for instance in winter cereals, low temperatures 

 interfere less with the intake of water. These plants grow and 

 develop perfectly well in late fall and early spring, in spite of 

 periodic frosts. Their unhindered absorption of water from the 

 soil is indicated by an abundant guttation. 



Oxygen of the soil air is another external factor influencing 

 the rate of water absorption by the root. In order to develop 

 exudation pressure and to absorb mineral salts, as well as for 

 their growth, roots require a supply of oxygen. On too compact 

 or submerged soils, therefore, plants develop poorly or die early. 

 This is observed frequently in spring in small depressions of the 

 ground where water stagnates. It is not the excess of water 

 itself that is injurious to the plant, since it develops perfectly 

 in water cultures. It is rather the lack of oxygen resulting from 

 submersion that is harmful. Plants in water cultures develop 

 successfully only when an air current is passed through the 

 solution or the latter is changed often. Experiments with 

 potometers have shown that a delay in absorption results when 

 the water is saturated with carbon dioxide or nitrogen, the 

 oxygen thereby being removed. The inhibiting influence of 

 carbon dioxide is more marked than that of nitrogen, the former 

 apparently being toxic. 



The root system is the special organ of water absorption. 

 This does not mean, however, that the other organs of the plant 

 are unable to absorb water. Every unsaturated cell imbibes 

 water if brought into contact with it. Thus leaves, especially, 

 may absorb water, in spite of their cutinized epidermis. Experi- 

 ments have shown that a dry cuticle is almost impermeable to water. 

 When moistened, it swells and becomes much more permeable. 



Leaves moistened by rain or dew are able, therefore, to take 

 up water. The dormant branches and buds of trees likewise 

 absorb water through their surface during thaws and spring 

 rains. This absorption, however, plays no important role in the 

 general water regulation of the plant. The amount of water 

 absorbed always remains small in comparison with the total 

 amount of water lost. Moreover, owing to the moistening of the 

 cuticle, the water absorbed in this way easily evaporates again 

 and in genei-al increases the rate of transpiration. This explains 

 the well-known rule that the watering of plants in direct sunlight 

 must be avoided, as it induces more rapid wilting. 



