Buds and Stems. 61 



the roots cannot provide water fast enough to supply the 

 loss by transpiration. Neither does the lifting power 

 appear to be due to living cells in the roots or stem acting 

 after the manner of a heart, or in any other way ; for after 

 these cells have been killed by poisonous solutions the 

 water continues to rise and evaporate from the leaves. It 

 seems probable that the leaves themselves assist in lifting 

 the water, perhaps by osmotic action between the tracheal 

 tubes and the parenchyma cells in the leaves; but this 

 has not been satisfactorily demonstrated. The problem of 

 the ascent of water, although apparently simple on the 

 face of it, has been one of the most elusive in plant physi- 

 ology. The forces concerned in the rapid movement of 

 food materials through the sieve tubes also remain undem- 

 onstrated. Diffusion would account for slow movements, 

 but the sieve tubes are evidently designed for conducting 

 more rapid hydrostatic currents. ^ 



44. Direction of Growth. We have seen that the shoot 

 of the young seedling employs gravity to direct its course 

 upward. After it appears above the ground, however, it 

 does not necessarily continue in a vertical direction, but 

 may grow more or less nearly horizontal, and may even 

 turn downward. If the stem becomes more intensely 

 illuminated on one side than another, it usually grows 

 toward the region of greatest illumination, but in some 

 climbing plants it may grow in the opposite direction and 

 thus keep in close contact with its support. 



45. Gravity as Guide. While the plant employs grav- 

 ity as a guide in bringing the shoot out of the ground, it 

 also uses the same force in conjunction with light to guide 

 it in placing its branches at various angles to the vertical, 

 as its needs may require. Although it may be stated as a 

 rule that shoots which are formed beneath the surface of 



