128 BIOLOGY 



causes oil to rise in the wick of a lamp. To what extent this 

 contributes to the flow of sap is uncertain. 



These factors combine to produce a lack of water at the 

 top, and an excess in the roots, which produces a conse- 

 quent tendency of the liquids in the plant to flow upward; 

 the total result being a flow of the liquids from soil to root, from 

 root to stem, and through the stem to the leaf and bud. The 

 rapidity of this ascent of sap is directly proportional to the ac- 

 tivity in the leaves and buds, since this determines the extent 

 to which the water is used up. In warm bright sunshine the 

 life processes in the leaves are vigorous, the stomata open, and 

 the sap rises rapidly. At night the current is decreased, and in 

 winter the processes practically cease, to be revived again 

 when the warm sun of spring makes it possible for the cells in 

 the leaves and buds to resume their activity. It is known that 

 the water rises chiefly in the large ducts of the fibrovascular 

 bundles, the spiral and ringed ducts serving for this purpose. 

 It does not flow, however, in the cavities of these ducts, but 

 rather in their walls, passing from cell to cell within the thick, 

 but evidently porous, walls. 



While these factors partly account for the rise of sap, they do 

 not explain the actual force which lifts the water, rising as it 

 does to the tops of the tallest trees. This is difficult to explain. 

 It is generally thought to-day that the three forces above men- 

 tioned are sufficient for the process: (1) Osmosis: this forces 

 the water from the soil, through the root hairs into the roots, 

 and probably from cell to cell within the plant, up through the 

 root and stem to the top of the plant. (2) Capillarity: this 

 force causes liquids to rise inside of small spaces, and 

 must play some part in the rise of water in the plant. 

 (3) Avidity for water: the demand for water of the protoplasm 

 at the top of the plant, above explained, is doubtless an 

 active agent also in producing the flow of water from cell to 

 cell up the plant. Whether these forces are sufficient to explain 

 the ascent of sap we do not know; but at all events the plant 



