Water 



79 



its wax coating against evaporation, so that almost the 

 whole of this amount is given off by the plant's own action. 

 The camellia is much less thirsty; it has fewer pores, and 

 its thick, glossy leaves are so efficiently protected, that 

 half an ounce of water, one-fortieth pint to the square 

 foot of foliage, was all that one plant gave up in a day 

 and night. 



These calculations are comparatively simple; but 

 when we come to trees, who would venture to guess at the 

 extent of surface exposed to the air and sun by the leaves 

 upon an elm? We look up at the quivering multitudes, 

 and feel as if it were hopeless for any one even to attempt 

 to count them; it is too bewildering! 



Yet the calculation has been made, and the leaves on 

 a not very large elm tree are said to be about seven mil- 

 lion, which would give a surface of about two hundred 

 thousand square feet, or five acres! 



From the whole of these five acres of green surface, 

 water passes off into the air in the form of vapor, to the 

 amount of seven tons and three-quarters during each 

 twelve hours of clear, dry weather. 



But if this is the quantity returned to the air by a 

 single tree of only moderate size, how large must be the 

 amount received from a wood or forest, containing hun- 

 dred or thousands of trees! 



QUESTIONS FOR REVIEW 



1. Give the proportion of water in the weight of different 

 plants. 



2. Why do vegetables live and plants die without water? 



3. What is sap? 



