THE TRANSPIKATION CURRENT 85 



from the intercellular space system. If the end of a transpir- 

 ing branch is injected for a short distance with a viscid 

 fluid, which will penetrate the cavities of the vessels and 

 subsequently solidify, these passages can be occluded for 

 a distance of a few centimetres. Gelatin or paraffin can 

 be used for the experiment, being injected at a moderately 

 low temperature such as will not injure the vitality of the 

 tissue. If after it has solidified a fresh surface is made by 

 a clean cut a very short distance from the end, and the 

 branch immersed in water, the leaves very soon flag, even 

 if some pressure is applied to the water in contact with the 

 cut surface. If the path of the liquid were the cell- walls, no 

 obstacle being offered to the transfer of water to them, the 

 upper portions ought to remain turgid. The experiment 

 shows that the normal channels are blocked by the paraffin 

 or gelatin used, and flagging results from the obstruction. 



A similar demonstration that the water passes by the 

 cavities or lumina of the cells is afforded by the experi- 

 ment of compressing the stem in a vice ; if the pressure is 

 carried so far as partially or entirely to obliterate their 

 cavities, the rate of flow is materially interfered with. 



The progress of a dye injected into the surface of a cut 

 branch also points to the same conclusion. If such stains 

 as fuchsin or eosin, which colour wood very rapidly, are 

 forced up into a stem and sections made almost immediately, 

 the lignified walls will be found to be in process of staining, 

 and the colour will be seen to be deepest on the side of the 

 wall abutting on the lumen, often only penetrating partly 

 through the thickness. If the wall itself were the path 

 of the pigment solution, its thickness would be stained 

 uniformly as far as the dye penetrated at all. 



The rate at which the transpiration current naturally 

 flows varies a good deal, plants showing differences among 

 themselves as to facilities of transport. In a fairly vigorous 

 tree it may be taken to be about 1-2 metres per hour, though 

 in some plants it has been observed to be three times as 

 rapid. In other cases as low a speed as 0-2 metre per hour 



