174 BOTANICAL GAZETTE [September 



fibrovascular bundles, whose finest ramifications are fine trach-^ 



eides. 



IV. So far, the mechanism of the movement of water is a 

 very simple problem. The difficulty is in the next step, in 

 explaining how suction in the tracheides and vessels of the 

 leaves can be propagated so as to cause an upward movement 

 along the stem, and even from the roots of lofty trees. Sum- 

 marizing a part of our empirical knowledge may facilitate a 

 subsequent analysis of the theories. The path of the transpira- 

 tion stream is, speaking for trees, in the younger wood. The 

 number of years' growth of gymnosperms and dicotyledons 

 which share in conducting it is very variable. Beside this radial 

 limit, the stream supplying any particular part of the crown 

 as a single branch — is also limited tangentially. A well-known 

 experiment by Th. Hartig (1853:313) illustrates this. He bored 

 five holes in a tree, meeting in the axis, and filled the cavities with 

 " holzsaurem Eisen ;" the tree was afterward cut forty feet higher 

 up, and the figure of a star, one ray corresponding to each hole 

 found stained. Experiments on the lateral movement of water 

 by overlapping cuts from opposite sides date back to Hale s. 

 As on various other points, the most valuable collection of data 

 is Strasburger's (1891 : 595 seq.\ 1893: 34, 37). Other more 

 notable work on transverse movement is by Darwin and PhillipSr 

 Vesque(i883, 1891 : 576), VViesner (1875). A limit to the ease 

 of lateral movement is necessary in order that any store of 

 water can be retained, while other water moves past.it from the 

 ground. It is also necessary for the maintenance of any pro- 

 portion of air and water in the individual channels. 



With the exception of a time in spring when the lumen of 

 some trees becomes full of water forced there by root pressure^ 

 the tracheae always contain a considerable amount of air. Scheit 

 maintains that the spaces in the lumen not occupied by liquid 

 water are filled with its vapor, and Sachs in his latest work 

 (1892) inclines toward the same view. But before Scheit's time 

 there was abundant proof of the presence of air, including anal- 

 yses by Bohm (1878), Faivre and Dupre, and others ; and more 



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