ASCENSION OF SAP. 75 



constantly evaporating from them. As fast 

 as this evaporation takes place more water is 

 needed. A demand is made upon the cells 

 in the interior of the leaf which contain 

 more water than those near the stomata, and 

 as these interior cells lose some of their wa- 

 ter they in turn call upon cells still more 

 distant, and so on until the call is made all 

 through the stem and to the minute root 

 hairs which derive their water from the 

 earth. This water does not flow upwards in 

 tvibes or cells, but it is soaked up through 

 the thick walls of the wood cells, and it 

 keeps soaking upwards as fast as evapora- 

 tion pumps it out through the leaves. In 

 this manner the water from the earth, laden 

 with its food materials, finally reaches the 

 leaves ; and there, in conjunction with car- 

 bonic acid gas from the air, in the chloro- 

 phyll grains in the minute cell laboratories, 

 and with the aid of sunlight, occurs the 

 wonderful transformation into organic mate- 

 rials. These materials afterwards pass into 

 the protoplasm and are used in building new 

 cells. During the process of assimilation 

 oxygen gas is set free and given off through 

 the stomata. This oxygen is necessary to 

 the life of animals, while the carbonic acid 



