56 PLANT PHYSIOLOGY AND ECOLOGY 



contiguous to the bundle. In part also this effect comes from the 

 evaporation of water from cavity and wall, which facilitates the 

 rise of water in both tracheid and vessel. 



The principal cause of the upward movement of water is un- 

 known. The most divergent views are held, not one of which has 

 proved capable of satisfactory demonstratioti. In the account 

 just given of probable factors in the movement, it is hardly possible 

 to deny the existence of root pressure, capillarity, and the lifting 

 power of evaporation and osmotic pressure. The relative impor- 

 tance of these, the manner in which theywork, and the existence 

 of other factors are points that it is impossible to settle at present- 

 It seems certain, however, that the so-called "vital properties" 

 of the plant, apart from the physical forces already mentioned, 

 have no part in the movement. The effect of transpiration upon 

 the rate of niovement would seem to bespeak great importance for 

 this factor, though it is hard to prove this. In fact, while it is 

 difficult to discover forces of strength sufficient to carry water 

 to the tops of the tallest trees, the forces already discussed seem 

 entirely adequate in the case of all herbaceous plants. 



Experiment 14. Pathway and rate of movement. Cut three sunflower 

 plants in such a way as to obtain three leafy stems ^, 1, and 2 dcm. long 

 respectively. Cut similar leafy shoots from the young growth of a 

 tree or shrub. For use as a check, cut a 2 dcm. stalk from sunflower 

 and tree, and remove the leaves. Fill a flat dish with a 1% solution 

 of erythrosin, and place the lower ends of the cut stems in the stain. 

 Arrange the sunflower stalks in one row, and the shoots in a second 

 one to permit ready comparison. Note the time that elapses before 

 the stain appears in the petioles of the uppermost leaves. Compare the 

 rates in herbaceous and woody stems, and in the leafy and leafless ones. 

 Cut cross-sections near the base and tip of stalk and shoot, and also 

 across the leaf. Draw a stem bundle, shading the elements in which 

 the stain moved. Make a diagrammatic drawing of stem and leaf 

 section, showing the number of pathways. 



TRANSPIRATION 



72. The structure of a representative leaf. The usual type 

 of leaf is flat, broad, and thin, and is ordinarily more or less hori- 

 zontal in position. The leaf is an organ peculiarly modified for 

 the reception of light and the absorption of gases. Consequently, 

 it becomes also the main seat of water loss or transpiration. The 



