539 



IB 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 confirmation 

 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 log- 

 wood decoction, transverse sections of it taken at a short distance 

 from its end show the mouths of the ducts surrounded 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 experi- 

 ments 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 medullary 'sheath are here the only 

 channels which the coloured liquid takes. In sections of the parts 

 up to which the dye has but just reached, the spiral, fenestrated, 

 scalariform, 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 the 

 terminations of shoots, where the vessels are but incompletely formed 

 out of irregularly-joined fibrous cells which still retain their original 

 shapes, the dye runs up the incipient vessels and does not colour in 

 the smallest degree the surrounding tissue. 



Experiments with leaves bring out parallel facts. On placing in 

 a dye a petiole of an adult leaf of a tree, and putting it before the 

 fire to accelate evaporation, the dye will be found to ascend the 

 midrib and veins at various rates, up even to a foot per hour. At 

 first it is confined to the vessels ; but by the time it has reached the 

 point of the leaf, it will commonly be seen that at the lower part it 

 has diffused itself into the sheaths of the vessels. In a quite young 

 leaf from the same shoot, we find a much more rigorous restriction 

 of the dyp to the vessel*. " On making oblique sections of its petiole, 

 iiiculb, and veins, the vessels have the appearance of groups of 



