The Extent of Root Surface ' 59 



roots usually have an acid reaction, and so much so that if they 

 are allowed to develop in contact with blue litmus paper, it is 

 changed to red along the lines of contact with the root. Further 

 than this, if the roots of a plant are allowed to develop in con- 

 tact with a polished surface of marble, the lines of root contact 

 with it will be plainly etched into its surface. Such observations 

 as 'these lead to the belief that the delicate root-hairs, at their 

 innumerable places of contact, hasten the solution of plant-food 

 from the difficultly soluble ingredients of the soil by the acids 

 which permeate their -walls being exuded upon the soil grains, 

 and there, in conjunction with the water, being able to dissolve 

 materials much more rapidly than water alone could do. 



When we reflect upon the many wide leaves with which most 

 land plants are provided, we are impressed with the great extent 

 of surface through which the sunshine and the air may come into 

 touch with the plant. But broad as these leaf surfaces are, they 

 only in the smallest way express the real magnitude of the sur- 

 face of contact, for the air actually enters the leaf and passes 

 around and between and in contact with the millions of loosely 

 packed cells in every leaf, and the number of times the extent of 

 the internal surface of the leaf exceeds that of its outer sur- 

 face is more than one would dare to express. Then, too, to in- 

 crease the contact surface for sunlight, the chlorophyll grains 

 which are scattered through the interior of the cells around 

 which the air can pass provide an enormous surface for the 

 absorption of light. 



In the root system under ground, the extremely numerous 

 root-hairs, small as they are, yet provide a surface for the con- 

 tact of soil and moisture with the plant which is quite commen- 

 surate with that furnished by the leaf. 



That we may the more clearly appreciate the great need 

 there is for the vast extent of root surface spread out by agri- 

 cultural crops, and how important it is that there shall be a 

 deep, well-drained soil in which the roots may expand, let me 

 give the measured amounts of water used by four stalks of corn, 

 and withdrawn by their roots from the soil, between July 29 and 

 August 11. Two of the maize plants were growing in each of 



