ROOTS AND THEIR WORK 93 



shows. If we fill each of the vessels with a given weight (say 100 grams 

 each) of gravel, sand, barren soil, rich loam, leaf mold, and 25 grams of 

 dry, pulverized leaves, then pour equal amounts of water (100 c.c.) on 

 each and measure all that runs through, the water that has been retained 

 will represent the water supply that plants could draw on from such soil. 



The Root Hairs take more than Water out of the Soil. If a 

 root containing a fringe of root hairs is washed off carefully, it will 

 be found to have little particles of soil still clinging to it. Exam- 

 ined under the microscope, these particles of soil seem to be ce- 

 mented to the sticky surface of the root hair. The soil contains, 

 besides a number of chemical compounds of various mineral sub- 

 stances, lime, potash, iron, silica, and many others, a consider- 

 able amount of organic material. Acids of various kinds are present in 

 the soil nitric acid, which comes from the dead bodies of plants and 

 animals as they decay and oxidize ; carbonic acid, formed by the 

 union of the carbon dioxide from the roots and the water in the 

 soil, and other acids. These acids so act upon certain of the 

 mineral substances that they become dissolved in the water which 

 is absorbed by the root hairs. 



The proportion of each of these mineral materials is very small 

 compared with the water in which they are found. A very great 

 amount of water must be taken up by the roots in order that the 

 plant may get the needed amount of mineral matter with which to 

 build its protoplasm. 



Plants will not grow well without certain of these mineral sub- 

 stances. This can be proved by the growth of seedlings in a so- 

 called nutrient solution. Such a solution contains all the mineral 

 matter that a plant uses for food. 1 If certain ingredients of this 

 solution are left out the plants placed in such a solution will not 

 live. 



1 A nutrient solution may be prepared as follows : 



Distilled water (H 2 O) 1000.00 c.c. 



Potassium nitrate (KNO 8 ) 1.00 gram 



Sodium chloride (NaCl) 0.50 gram 



Calcium sulphate (CaSO 4 ) 0.50 gram 



Magnesium sulphate (MgSO 4 ) 0.50 gram 



Calcium phosphate (CaJPOJ?) 0.50 gram 



Ferric chloride (FeCl 8 ) 0.005 gram 



(Do not put the ferric chloride into the solution in the first place, but add a drop 

 of it to each bottle when the seedlings are put in.) 



