88 FOUNDATIONS OF BIOLOGY 



In the stem, the course taken by the food depends upon the 

 immediate needs of the plant. It may pass either up or down 

 through the phloem, or some may be transferred to the xylem 

 and carried to the growing tip or the developing flower and 

 fruit for immediate use. When growth is not active, most 

 of the food passes downward through the phloem to supply 

 the cambium and to be stored, chiefly as starch, in stem and 

 root. In brief, all the living cells of the plant directly or 

 indirectly draw upon the supply of food circulating through 

 the phloem, so we may look upon the phloem as primarily a 

 food-distributing system from the leaf, just as we have seen 

 that the xylem is the system for carrying water and solutes 

 from root to leaf. The raw materials pass up through the 

 wood and the finished products pass down through the bark. 



2. Dynamics of Circulation 



The question will naturally arise in the mind of the reader: 

 what is the force which brings about the circulation of all 

 these fluids in the plant body? We have mentioned that 

 water containing solutes enters the root hairs and passes to the 

 cortical cells and ducts of the xylem by the physical process 

 known as osmosis. It is now believed that osmosis in the 

 leaves draws water from the ducts of the stem into the cells 

 of the leaf. It is also thought that osmotic forces operating 

 in leaf cells are adequate to lift the water to the tops of the 

 tallest trees, where, in turn, the water is removed from the leaf 

 cells by evaporation through the stomata. 



The outgo of water by evaporation is termed TRANSPIRA- 

 TION, and is brought about by heat energy from the sur- 

 rounding atmosphere. In the last analysis, if the explanation 

 suggested above is correct, the energy of heat, resulting in 

 evaporation from the leaves, is chiefly responsible for the 

 movement of the column of water which is continually pass- 



