THE FORMATION OF WOOD IN PLANTS. 407 
On making longitudinal sections of the part traversed by it, the dye is found to have 
penetrated extensive tracts of the woody tissue; and on making transverse sections, 
the openings of the ducts appear as empty spaces in the midst of a deeply-coloured pros- 
enchyma. It would thus seem that the liquid is carried up the denser parts of the 
vascular bundles: neglecting the cambium layer, neglecting the central pith, and neg- 
lecting the spiral vessels of the medullary sheath. Apparently the substance of the 
wood has afforded the readiest channel. When, however, we examine these appearances 
eritically, we find reasons for doubting this conclusion. If a transverse section of the 
lower part, into which the dye passed first and has remained longest, be compared 
with a transverse section of the part which the dye has but just reached, a marked 
différence is visible. In the one case the whole of the dense tissue is stained; in 
the other case it is not. This uneven distribution of stain in the part which the 
dye has incompletely permeated is not at random; it admits of definite description. A 
tolerably regular continuous ring of colour distinguishes the outer part of the wood from 
the inner mass, implying a passage of liquid up the elongated cells next the cambium 
layer. And the inner mass is coloured more round the mouths of the pitted ducts than 
elsewhere : the dense tissue is darkest close to the edges of these ducts; the colour fades 
away gradually on receding from their edges; there is most colour where there are 
several ducts together; and the dense tissue which is fully dyed for some space, is that 
which lies between two or more ducts. These are indications that while the layer of 
pitted cells next the cambium has served as a channel for part of the liquid, the 
rest has ascended the pitted ducts, and oozed out of these into the prosenchyma around. 
And this conclusion is confirmed by the contrast between the appearances of the lowest 
part of a shoot under different conditions. For if, instead of allowing the dye time for 
oozing through the prosenchyma, the end of the shoot be just dipped into the dye and 
taken out again, we find, on making transverse sections of the part into which the dye 
has been rapidly taken up, that, though it has diffused to some distance round the ducts, 
it has left tracts of wood between the ducts uncoloured—a difference which would not 
exist had the ascent been through the substance of the wood. Even still stronger is 
the eonfirmation obtained by using one dye after another. If a shoot that has ab- 
sorbed magenta for an hour be placed for five minutes in the logwood decoction, transverse 
sections of it taken at a short distance from its end show the mouths of the ducts sur- 
rounded by dark stains in the midst of the much wider red stains. ; : 
Based on these comparisons only, the inference pointed out has little weight; but its 
Weight is increased by the results of experiments on quite young shoots, and shoots that 
develope very little wood. The behaviour of these corresponds perfectly with the ex- 
pectation that a liquid will ascend capillary tubes in preference to simple cellular tissue 
or tissue not differentiated into continuous canals. The vascular bundles of the me- 
dullary sheath are here the only channels which the coloured liquid takes. ne 
of the parts up to which the dye has but just reached, the spiral, fenestrated, scalarılorm, 
or other vessels contained in these bundles are alone coloured ; and lower down it is only 
after some hours that such an exudation of dye takes place as suffices partially to colour 
the other substances of the bundle. Further, it is to be noted that at m he 
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