THE STEM AND ITS FUNCTIONS 111 



they may often be used to identify the plant species from which a 

 piece of wood has been derived. The great diversity which wood 

 displays, together with its abundance and the ease with which it 

 can be manipulated, have led to its use in numberless ways, and 

 there is consequently no other plant tissue, aside from those used 

 as food, which is of such great economic importance. 



The Ascent of Sap in Stems. — We can determine by experiment 

 that water and dissolved substances absorbed by the roots are 

 carried upward in the wood of the stem. As to what causes this 

 movement, however, there is still much question. To explain the 

 ascent of water in low-growing herbaceous plants might be fairly 

 simple, but the factors which bring about the lifting of water in 

 large quantities to the tops of tall trees, sometimes more than 

 three hundred feet above the ground, are very hard to determine. 

 An upward osmotic pull is of course furnished by the increased 

 sap concentration in the leaf-cells which follows the water-loss 

 therefrom in transpiration, but even granting a strong pull at 

 the leaf, the rise obviously cannot be due to simple "suction" 

 or atmospheric pressure. Nor is capillarity probably concerned 

 to any great extent in the process, for, although water may be 

 lifted very high in exceedingly small capillary tubes, its move- 

 ment is so slow under these conditions that capillarity certainly 

 could not provide the large amounts of water which we know 

 must ascend the trunk daily. Root-pressure, if it were strong 

 enough, might perhaps be important, but root-pressure is mani- 

 fest in woody plants only during the early spring and is therefore 

 lacking at the season when transpiration is most active. It has 

 been suggested that the living ray and wood parenchyma cells 

 may be concerned in the upward movement of water in some way, 

 perhaps furnishing a continuous series of osmotic pumps. These 

 cells may be of some such service, but we know that for a con- 

 siderable time, at any rate, water may ascend through a stem 

 where all the living cells have been killed. The most plausible 

 hypothesis yet put forward is based on the very high cohesive 

 power exhibited by water under certain conditions. In very thin 

 water columns, such as must occur in the conducting cells of the 



Fig. 64. — Oak wood as seen under the microscope. A, transverse section, 

 showing one conaplete annual ring and parts of two others. Note the very wide 

 vessels in the spring wood, the narrow ones in the fall wood, the wide wood 

 ray, and the many very narrow ones. B, radial section. C, tangential section. 

 Note the many differences in structure between this wood and that of pine. 

 (Courtesy United States Forest Products Laboratory). 



