[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 313 



ured in detail. Some of the results as reported are given here. The plant 

 grew for 4 months in a fertile soil contained in a wooden box a foot 

 square at the top and 22 inches deep; that is, somewhat less than 2 cubic 

 feet of soil. The plant produced 80 shoots with an average of 6 leaves 

 per shoot, and was about 20 inches tall at the end of the experiment. The 

 total exposed surface of the top of the plant was 51.4 square feet. The 

 number, length (if placed end to end), and area of the roots are best 

 seen from the accompanying table. 



These figures show that the total surface of the roots is about 130 

 times that of the shoots. The presence of nearly 14 million roots and 14 

 billion root hairs with a total surface of 6800 sq. ft. in less than 2 cu. ft. of 

 soil shows how completely every part of the soil may be penetrated. The 

 roots are thus in contact with much of the water surrounding the soil 

 particles and with the salts and other compounds dissolved in the soil 

 water. 



If we assume an average daily rate of growth during the four-month 

 experimental period, 3 miles of new roots and 50 miles of new root 

 hairs ( 100,000,000 ) were added each day. In nature the daily invasion 

 of new soil masses might be of great importance if the water content of 

 a soil were decreasing, or if the soil solution of inorganic salts were very 

 dilute. 



In the chapter on transpiration it was shown that the principal evapo- 

 ration surface is the mesophyll when the stomates are open. The meso- 

 phyll surface of winter rye grass is perhaps 6 times that of the epidermis 

 of the shoots. Even when the stomates were open, the plant described 

 in this experiment had an absorbing surface 22 times that of the evapora- 

 tion surface. 



