THE RELATION OF THE PLANT TO WATER 167 



Plants should be pruned during the early fall or winter 

 while dormant, or late in summer after they have passed the 

 period of most active spring growth so as to prevent bleeding. 

 Maple sugar is made very early in the spring when bleeding is 

 greatest and the sugar content of the sap at its highest. 



The Effects of Plants on Soil. From a study of the fore- 

 going it will be readily seen that growing plants are continually 

 taking water from the soil and we will learn later that this water 

 is given off by the foliage of the plant through the process of 

 transpiration (Chapter XIV). Therefore, the plants are gradu- 

 ally reducing the water supply of the soil, which must be re- 

 plenished by rainfall or other natural agencies. But this is 

 only one of many influences which the plants exert on the soil. 

 Plants also take up materials (Chapter XVII), which in a state 

 of nature may be returned to the soil in the same or in different 

 compounds, when the plants decay ; or in the manure from ani- 

 mals, which have fed on the plants, or by decaying animals. 

 The farmer removes a great deal of plant materials in harvest- 

 ing his crops and this also brings about important changes result- 

 ing in a soil of reduced fertility. Plant roots are said to give off 

 carbon dioxide and other compounds which no doubt react on the 

 compounds already in the soil. The relative importance of this 

 action on growing plants is not well understood. However, it 

 appears that the carbon dioxide unites with the water, forming 

 carbonic acid, which dissolves certain soil minerals, thus making 

 them available for the growing plants. It is also claimed by 

 some students that the roots give off enzymes which oxidize 

 poisonous soil compounds and render them harmless. Decaying 

 plant material is recognized as exerting a most important influ- 



