192 TRANSPIRATION 



portions of plants lose water more quickly than similar living 

 portions. 



The amount of water transpired by a sunflower 3^ feet high on 

 a warm day was found by Hales to be 20 ounces in twelve hours, 

 and an ordinary cabbage gave off 15 ounces in the same time. 

 At this rate an average crop ol cabbages would give off between 

 3 and 4 tons of water per acre per day. As the loss by the upper 

 parts of the plant must be compensated by absorption of water 

 from the soil, it will be readily understood that land bearing a 

 crop is always drier than bare fallow. 



If transpiration goes on at a greater rate than the absorption 

 by the root the turgid state of the cells is more or less decreased 

 and ' wilting ' appears. This ' wilted ' condition of plants not 

 unfrequently happens in bright hot weather, in dry soils contain- 

 ing too little water, but it may occur in ordinary soils even when 

 the roots are actively taking in what would be a sufficient 

 quantity of water for the needs of the plants, if the brightness, 

 high temperature and other conditions encouraging excessive 

 transpiration were reduced. 



' Wilting ' does not necessarily imply that no water is entering 

 the plant : it is merely an indication that the plant is losing more 

 than it is taking in. 



Unavoidable mechanical injury to the absorbing region of the 

 root when plants are transplanted, injuries from the attack of 

 insects, and reduction of the temperature of the soil below that 

 at which the root is able to carry on its work satisfactorily, are 

 responsible for inadequate absorption of water and consequent 

 ' wilting ' : moreover, an insufficient supply of air to the root 

 which happens when the latter is growing in water-logged soil 

 prevents proper absorption and may result in flagging of the 

 leaves of the plant. 



Among all kinds of plants, and especially among those species 

 living in dry situations, various adaptations are observable which 

 tend to prevent a too rapid loss of water. 



