CIRCULATION AND FORMATION OF WOOD IN PLANTS. 569 



lately been experimenting on the absorption of dyes by plants. So 

 far as I can learn, experiments of this kind have most, if not all of 

 them, been made on stems, and, as it would seem from the results, 

 on stems so far developed as to contain all their characteristic 

 structures. The first experiments I made myself were on such 

 parts, and yielded evidence that served but little to elucidate 

 matters. It was only after trying like experiments with leaves of 

 different ages and different characters, and with undeveloped axes, 

 as well as with axes of special kinds, that comprehensible results 

 were reached ; and it then became manifest that the appearances 

 presented by ordinary stems when thus tested, are in a great degree 

 misleading. Let me briefly indicate the differences. 



If an adult shoot of a tree or shrub be cut off, and have its lower 

 end placed in an alumed decoction of logwood or a dilute solution 

 of magenta,* the dye will, in the course of a few hours, ascend to a 

 distance varying according to the rate of evaporation from the 

 leaves. 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 

 prosenchyma. 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 neglecting the spiral vessels of the 

 medullary sheath. Apparently the substance of the w r ood has 

 afforded the readiest channel. When, how r ever, we examine these 

 appearances critically, 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 sec- 

 tion of the part which the dye has but just reached, a marked 

 difference 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 



* These two dyes have affinities for different components of the tissues, 

 and may be advantageously used in different cases. Magenta is rapidly 

 taken up by woody matter and other secondary deposits ; while logwood 

 colours the cell-membranes, and takes but reluctantly to the substances seized 

 by magenta. By trying both of them on the same structure, we may guard 

 ourselves against any error arising from selective combination. 



