62 



no water is present in the cavities of the vessels ; — conse- 

 quently it is erroneous to compare them with the blood-vessels 

 of animals, and another explanation must be sought. It is only 

 in special cases that the vessels do contain water, for, as a rule, 

 they contain nothing but air. 



By introducing various staining fluids into the plant, such as 

 chloride or sulphate of aniline, we apparently get some evidence 

 that the course of the current of liquid is confined to the 

 wood. For all the woody elements of the younger portion 

 of the wood of the plant through which this staining solution 

 passes will be stained a bright yellow or deep brown colour by 

 it. This solution, then, stains the wood-vessels, and thus affords 

 a characteristic test for lignified tissue. But though this evi- 

 dence appears at first satisfactory, we must not overlook the 

 fact that the solution of aniline might pass up through other cells 

 as well as through those of the wood, but yet not stain them, be- 

 cause they are not lignified ; and, consequently, if we relied on 

 it for our conclusions alone, we might be led into error. 



The pith, which seems to be, next to the cavities of the vessels, 

 the most feasible channel, cannot afford a means for its convey- 

 ance, because in all cases it remains exceedingly small, and the 

 larger the plant becomes, the smaller proportionately is the 

 amount of pith ; so that except in the very young condition the 

 consistence of its cells becomes either dried up or inert ; and in 

 many stems, after a few years, it ruptures and either completely 

 disappears or remains in the form of interrupted thin shreds of 

 tissue. Further, some observers have removed the pith, and 

 shown that after its removal the plant flourishes as well as it 

 previously did. 



The medullary rays, which are strands of parenchymatous tis- 

 sue, running radially from the pith to the bast layer of the bark, 

 only occur at intervals, and do not extend along the stem, con- 

 sequently they are of no value as a channel for the upward 

 current of water ; and the same thing may be said of the inter- 

 cellular spaces, which habitually contain air, and of which there 

 are very few in the wood. Does the upward current then ascend 

 by the bast layer of the bark [phloem)^ or by the fibres of the 



