348 PHYSIOLOGY 



bundles, and the cortex in different specimens and compare the effect. 

 It will be found that only in the specimens whose bundles have been 

 cut do the leaves wilt, and the fact that in woody plants the bark may be 

 removed without causing wilting eliminates the phloem strands. Such 

 experiments permit the inference only that the xylem strands are the 

 chief paths of the transpiration stream, not that they are the sole path ; 

 for wilting implies merely an inadequate water supply. 



Water moves in the lumina. — But the path can be localized more 

 exactly. A shoot of a climber, such as Clematis, may be cut off under 

 water, and the end sliced very obliquely, so as to open wide the ends 

 of the ducts. If this shoot be fastened to a microscope slide, and the end 

 covered with water, into which has been introduced some finely divided 

 carbon, as from Chinese ink, one may watch the water swirling into the 

 open ends of the ducts, its course being made evident by the opaque 

 particles it carries. Under such circumstances it is evident that the water 

 enters and probably traverses the lumen of the trachea. But this was 

 for a long time a disputed point. When the extraordinary freedom of 

 movement of water in lignified tissues was discovered, it was held that 

 the water traveled in the substance of the walls and not in the lumina 

 (the chambers they enclose). This opinion, however, rested upon inac- 

 curate experimentation. 



Closing the lumina. — Attempts were made by compressing the stem in a vise to 

 collapse the tracheae, and so to close their lumina. In the earlier experiments of 

 this sort, wilting did not occur, and the inference was plain, therefore, that the water 

 traveled in the wall itself. Repeated studies showed that the difficulty of compress- 

 ing the tracheae had been underestimated, and that when they were actually 

 closed mechanically, the leaves did wilt. A better method of closing them is by 

 plugging them with paraffin or gelatin which melts at a low temperature. By 

 cutting a shoot under the melted material, it is carried up instantly to some distance 

 in the tracheae. When cooled, it solidifies and a fresh surface of wall can be exposed 

 by removing a thin slice, while the lumina remain plugged. The leaves of such a 

 shoot promptly wilt when exposed to dry air. 



Path of least resistance. — On the whole, therefore, it is fairly certain 

 that the transpiration stream traverses the xylem strands, and that it is 

 the lumina of the tracheae that form the chief conduits for the water. 

 That some travels in the walls is quite probable, especially when the 

 tracheae are partly blocked, as they often are, by gas, the path of least 

 resistance being followed here as always. Nor is it impossible that 

 some water moves in the cortex; but this is never enough to cover any 

 considerable loss by evaporation. 



