158 COLLEGE BOTANY 



a cubic yard of soil. The root system of a single wheat plant 

 is said to aggregate more than 600 yards and that of a single 

 pumpkin plant more than, fifteen miles. Although these 

 figures may appear to be exaggerated and may be exceptional, 

 those who have made a study of the root systems of plants know 

 that these organs are much more extensive than they are supposed 

 to be by those who have not studied them. Some trees send their 

 roots deep into the soil, while others spread them near the sur- 

 face for long distances. The fact that our trees are able to with- 

 stand storms is sufficient evidence that they are firmly anchored 

 by a wonderful root system. The method for measuring the root 

 system of plants was developed by Rotmistrow of Russia and 

 consists in growing plants in sunken boxes filled with soil. These 

 boxes are one inch wide, twenty to forty inches long and twenty 

 to forty inches deep. When the plants are mature, the boxes 

 can be removed, opened and the soil washed from the roots with 

 a minimum of injury. Although this method is open to objec- 

 tion it gives fairly accurate records. 



Unavailable Water. The plant cannot use all the water 

 from the soil. Under ordinary conditions every particle of soil 

 is surrounded by a film of water known as hygroscopic water. 

 When the soil contains only the hygroscopic water it is- said to 

 be " air-dry." In general, it may be said that the plant cannot 

 take water from the soil unless the amount of water exceeds 

 three times the hygroscopic water. The water of the soil, which 

 is in excess of the hygroscopic water, is usually known as capil- 

 lary water. When the water content of the soil drops below 

 three times the hygroscopic water, the plant wilts as a result of 

 the withdrawal of the water in its tissues into the surrounding 

 soil. In our ordinary, sandy, loam farm soils from 5 to 12 per 



