53 



and consider it apart. If the humidity of the air is very great, 

 i. e., if the air is at or near its point of saturation, the amount of 

 water given off by the plant is very small, i. e., transpiration is 

 practically absent, provided always that, as is usually the 

 case, there is no important difference between the temperature 

 of the plant and that of the air outside. But in a saturated 

 atmosphere, variations in the temperature of the air may 

 occur, so that the temperature of the air rises above that 

 of the plant, and under these conditions, transpiration will 

 go on to a perceptible, though to no very great extent, al- 

 though the air is saturated with moisture. Under ordinary 

 circumstances, however, when the degree of humidity of the 

 air is moderate, the amount of transpiration is affected by the 

 degree of humidity, since if the air becomes moister the amount 

 of transpiration becomes less, and vice versa. Variations in the 

 degree of humidity of the air are, however, affected in their turn 

 by changes in the temperature, so that every change in the 

 temperature of the air produces either directly or mediately an 

 effect on transpiration. When the degree of humidity of the 

 air is high, it tends to reduce the amount of transpiration 

 in the plant, but when the temperature of the air is high, that 

 is to say, when the air is hot, it tends to promote transpiration 

 and render its amount greater. On the other hand, the cooler 

 the air is, the less will transpiration go on. 



To return for a moment to the influence of light on trans- 

 piration. We have seen that this factor has an important bear- 

 ing on the opening and closing of the stomata ; and when 

 we come to speak of their function later on, we shall see these 

 stomata exert in turn an important influence upon transpira- 

 tion ; — hence, through their mediate agency, light exercises 

 an important influence on transpiration. 



From experiments conducted by Hales more than a century 

 ago upon many plants of the Sunflower (Helianihus annuus) it 

 was found that in a day of twelve hours the maximum loss of 

 water by a plant was thirty ounces, and the mean loss twenty 

 ounces, while in a night of the same duration the mean loss was 

 three ounces. This shows us that while the amount of water 



