xxi] TRANSPIRATION STREAM 263 



chosen as the subject of the experiments, which were made in the 

 River Cam during July and August. The method adopted was 

 to attach a small glass bulb of aqueous eosin solution to the cut 

 end of a submerged branch. A flourishing, leafy stem was 

 selected, cut under water and left submerged for a short time. 

 A little cotton-wool was then wrapped round the stem near the 

 cut end, the small bulb of eosin brought down to the surface of 

 the water, and the cut end lifted for a moment above the surface 

 and inserted in the bulb. The plant was held beneath the water 

 for a recorded time, and, at the end of the experiment, the bulb 

 was removed and the stem at once examined. The rate of trans- 

 mission of the eosin solution was found to be surprisingly rapid 

 the eosin travelling, in one case, at the rate of nearly 10 cms. 

 per minute. In these cut shoots, root-pressure is obviously 

 eliminated, and the upward stream was found to be due to the 

 action of the leaves ; the entire removal of the leaves rendered 

 the current almost negligible, while, when some were cut off, 

 the diminution in the rapidity of flow was roughly proportional 

 to the number removed. 



Such experiments as these seem to leave little room for doubt 

 that an active water-current from base to apex, corresponding 

 to the 'transpiration' current of land plants, occurs even in 

 entirely submerged aquatics, or, in other words, that the absorp- 

 tion of water is polarised in the plant. Those who have denied 

 the existence of the transpiration stream, have been led to do so 

 rather on the a priori ground that such a current would be 

 a superfluous feature in the economy of a plant surrounded by 

 " water, water everywhere." This would in any case be a dan- 

 gerous method of argument, and it is based moreover upon a 

 misconception of the value of the transpiration current. Its use 

 is not merely to supply the tissues with water, but also to convey 

 to the assimilating and growing regions certain important ele- 

 ments of their food supply. Even the soil-water contains salts 

 in solution in quantities that are relatively minute, and the 

 only method whereby an adequate salt supply can be ensured 

 is by the passage of a proportionately large volume of water 



