84 VEGETABLE PHYSIOLOGY 



colouring matter will be found to have stained the wood 

 for a considerable distance ; in the case of a small plant, 

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

 leaves, while the pith and cortical tissues will remain un- 

 stained. An isolated branch can be taken as the subject 

 of the" experiment, 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 lower pressure than that of the atmosphere. Another 

 reason advanced for it was founded on the nature of lignin 

 and its relation to water. While refusing to imbibe 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 molecules 

 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 observations 

 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 



