580 APPENDIX C. 



has its periphery defined and its histological characters decided, 

 the appearances show that the tissue forming its outer surface 

 begins to take a leading 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 all normal and abnormal 

 growths that have not formed wood, the channels for the passage of 

 sap are the spiral, annular, fenestrated, or reticulated vessels. These 

 vessels, here included in the bundles of the medullary sheath, are, 

 in common with the tissues around them, subject, by the bendings 

 of the shoot, to slight intermittent compressions, and, especially 

 the outermost of them, are thus forced to give the prosenchyma 

 an extra supply of nutritive liquid. The thickening of the pro- 

 senchyma, 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 

 cylinder of hardened tissue, surrounding the medulla and the vas- 

 cular 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 developing wood outside of them. Clearly, too, the 

 greatest stress must be felt by the outer layer of the developing 

 wood : being further removed from the neutral axis, it must be 

 subject to severer strains 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 mechanically, they become greater carriers 

 of sap than the original vessels, and, in consequence of this, as well 

 as in consequence of their relative proximity, become the sources 

 of nutrition to the still more external 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 discharged 

 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 layer of inosculating 



