WATER AND WATERING. 



87 



The following table by Werner shows the leaf surface 

 of rufca-baga turnips both of plants grown directly from 

 the seed, and of those subsequently transplanted. 



CHAPTEE IX. 



WATER AND WATERING. 



Water is the medium by which the soluble matters of 

 the soil are conveyed, through the roots, into the in- 

 terior organism of plants. We know that the earth, 

 only apparently dry, cannot support vegetation. The more 

 advanced the state of growth, so long as the foliage re- 

 mains young and succulent, the more moisture does a 

 plant need. An important fact in the relation of vege- 

 tation to moisture is seen in the effect the humidity of 

 the atmosphere has upon its temperature. Without 

 more or less vapor in the atmosphere, radiation would 

 cool the surface of the earth so rapidly as to destroy the 

 life of all tender plants. The hottest rays of the sun 

 pass through the air, even when that is saturated with 

 moisture, without heating it; but the heat radiated from 

 the earth, and every object upon it, ts intercepted and 

 absorbed by the humidity in the air; and the atmospheric 

 warmth is therefore in proportion to the heat of the sun's 

 rays and the moisture of the air. Like the covering of a 

 cold frame, the moist air admits the heat by day and 



