2l6 MOVEMENTS AND EXCHANGES OF FLUIDS 



cending current undoubtedly takes place so that it may not be 

 considered as a simple, upwardly moving stream of water. 1 In a 

 series of experiments by Dr. C. C. Curtis in the laboratories of 

 the New York Botanical Garden, a number of manometers were 

 attached to the lateral branches of small trees of Populus Simoni 

 at distances of 20 cm. to 5 m. from the roots. Regions of posi- 

 tive and negative pressure were found variously distributed in 

 the stems, and in some instances the only positive pressures found 

 were at the extreme tip of the shoot. It is evident, therefore, that 

 the conduction of water from the roots to the leaves may be most 

 seriously influenced by the osmotic absorption, and exudation 

 pressure, of the layers of living cells lying along the pathway of 

 the ascending current, and the probability is by no means ex- 

 cluded that these may be principal factors irf the conduction of 

 water from the roots to the crown : a probability by no means 

 lessened by the fact that the upward current may traverse long 

 portions of the stem, in which all of the living cells are dead. 2 



1 Dixon and Joly. Phil. Trans. Roy. Soc. 186 : 1895. 

 Dixon, H. R. Proc. Roy. Soc. 4 : 1898. 



2 See, Pfeffer, W. Physiology of Plants, i : 220-227. *9oo. And Ward, H. M. 

 Timber and some of its diseases. 59-141. 1897. 



