80 VEGETABLE PHYSIOLOGY 



cavities of the vessels, and subsequently solidify, these 

 passages can be occluded for a distance of a few centi- 

 metres. (Jelatin or paraffin can be used for the experi- 

 ment, 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. 



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 a stain 

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

 forced up into a stem and sections made almost immedi- 

 ately, 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 goo</ 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 -2 metre 

 per hour has been found. It is a little difficult to measure 

 in most cases ; the plan generally adopted has been to 



