549 



pljcry defiued and its histological characters decided, the appearances 

 show that the tissue forming its outer surface begins to take a lead- 

 ing part in the transmission of liquid. What now is the explanation 

 of these changes, mechanically considered ? In the young soft part 

 of the shoot, as in aU normal and abnormal growths that have not 

 formed wood, the chamiels for the passage of sap are the spiral, 

 nnnular, fenestrated, or reticulated vessels. These vessels, here in- 

 daded in the bundles of the medullary sheath, are, in common with 

 the tissues around them, subject, by the bendings of the shoot, to 

 shght intermittent compressions, and, especially the outermost of 

 them, are thus forced to give the prosenchyma an extra supply of 

 nutritive hquid. The thickening of the prosenchyma, spreading 

 laterally as well as outwards from each bundle of the medullary 

 sheath, goes on until it meets the thickenings that spread from the 

 other bundles ; and there is so formed an irregular cyhnder of har- 

 dened tissue, surrounding the meduUa and the vascular bundles of 

 its sheath. As soon as this happens, these vascular bundles become, 

 to a considerable extent, shielded from the effects of transverse 

 strains, since the tensions and compressions chiefly fall on the de- 

 veloping wood outside of them. Clearly, too, the greatest stress 

 must be felt by the outer layer of the developing wood : being fur 

 ther removed from the neutral axis, it must be subject to severer 

 strain^ at each bend ; and lying between the bark and the layer of 

 wood first formed, it must be most exposed to lateral compressions. 

 Among the elongated cells of this outer layer, some unite to form 

 the pitted ducts. Being, as we see, better circumstanced mechani- 

 cnUy, they become greater carriers of sap than the original vessels, 

 and, in consequence of this, as well as in consequence of their rela- 

 tive proximity, become the sources of nutrition to the still more ex- 

 ternal layers of wood-cells. The same causes and the same effects 

 hold with each new indurated coat deposited round the previously 

 indurated coats. 



This description may be thought to go far towards justifying the 

 current views respecting the course taken by the sap. But the 

 justification is more apparent than real. In the first place, the im- 

 plication here is that the sap-carrying function is at first discharsred 

 entirely by the vessels of the medullary sheath, and that they cease 

 to discharge this function only as fast as they are relatively incapaci- 

 tated by their mechanical circumstances. And the second implica- 

 tion is, that it is not the wood itself, but the more or less continuous 

 canals formed in it, which are the subsequent sap-distributors. This, 

 though readily made clear by microscopic examination of the large 

 pitted ducts in a partially lignified shoot that has absorbed the dye, 

 is less manifestly true of the peripheral layer of sap-carrying tissue 

 finally formed. But it is really true here. For this layer, though 

 nominally a layer of wood, is practically a laver of inosculating 



