THE TRANSPIRATION CURRENT 79 



indeed, it will be coloured quite up to the veins of the leaves, 

 while the pith and cortical tissues will remain unstained. 

 An isolated branch can be taken as the subject of the experi- 

 ment, its cut surface being placed in a solution of the dye. 



The dye in these cases passes with the current of water, 

 as may be seen by the difference in its rate of passage when 

 transpiration is vigorous, and when from severance of the 

 leaves of a branch it can penetrate only by diffusion. ^ 



A good deal of controversy has been excited with refer- 

 ence to the manner in which the transport of the water in 

 the wood takes place. ^Sachs^originally suggested that the 

 path was altogether the walls of the cells, and that their 

 cavities were empty. This view was based partly on the 

 fact that the vessels undoubtedly contain a quantity of air 

 during the period of active vegetation, and that this air is 

 at a less pressure than that of the atmosphere. Another 

 reason advanced for it was based on the nature of lignin 

 and its relation to water. While refusing to absorb much 

 water and swell as cellulose can be made to do, lignin can 

 contain a certain quantity, which it will part with very 

 easily. On this view the walls of the lignified vessels may be 

 regarded as a column of water held together by the mole- 

 cules or micellae of lignin. A very little water removed 

 from the top of such a column would be immediately 

 replaced from below so long as a supply existed there. 



Such a remarkable conductivity, however, is probably 

 not possessed by the walls of the vessels. Many observa- 

 tions made in recent years tend to negative this view, and 

 to support the hypothesis that the water passes in the 

 cavities of the vessels. Sachs's opinion that these are 

 always free from water during active transpiration has 

 been shown not to be well founded, for various observers 

 have proved that their cavities are occupied by a chain of 

 water-columns and air-bubbles, the air having been 

 originally absorbed from the intercellular space system. 

 If the end of a transpiring branch is injected for a short 

 distance with a viscid fluid, which will penetrate the 



