78 MORPHOLOGY OF STEMS. 



their strength and toughness. They are like wood-cells except 

 in their greater length and flexibility, and in the thickness of 

 their walls, which greatly exceeds the calibre. This is the 

 material which gives to the bast or inner bark of Basswood, &c., 

 the strength and pliability that adapts it for cordage and for 

 making mats : it is the material of linen, and the like textile 

 fibres. (For a view of the whole composition and structure of a 

 woody stem at the close of the first year's growth, and immedi- 

 ately before that of the second year begins, see Fig. 135 a .) 



143. Annual Increase in Diameter. An herbaceous stem does 

 not essentially differ from a woody one of the same age, except 

 that the wood forms a less compact or thinner zone ; and the 

 whole perishes, at least down to the ground, at the close of the 

 season. But a woody stem makes provision for continuing its 

 growth from year to year. As the layer of wood continues to 

 increase in thickness throughout the season, by the multiplication 

 of cells on its outer surface, between it and the bark, and when 

 growth ceases this process of cell-multiplication is merely sus- 

 pended, so there is always a zone of delicate }~oung cells in- 

 terposed between the wood and the bark. This is called the 

 CAMBIUM, or, better, the CAMBIUM-LAYER. It is charged with 

 organizable matter, which is particularly abundant and mucila- 

 ginous in spring when growth recommences. This mucilaginous 

 matter was named Cambium by the older botanists : they sup- 

 posed as is still popularly thought that the baik, then so 

 readily separable, really separated from the wood in spring, that a 

 quantity of rich mucilaginous sap was poured out between them 

 and became organized into a tissue, the inner part becoming new 

 wood, the outer, new bark. But delicate slices show that there 

 is then no more interruption of the wood and inner bark than 

 at any other season. The bark, indeed, is then very readily 

 detached from the wood, because the cambium-layer is gorged 

 with sap ; but such separation is effected by the rending of a 

 delicate forming tissue. And if some of this apparent mucilage 

 be scraped off from the surface of the wood, and examined under 

 a good microscope, it will be seen to be a thin stratum of young 

 wood-cells, with the ends of medullary rays here and there in- 

 terspersed. The inner portion of the cambium-layer is therefore 

 nascent wood, and the outer is nascent bark. As the cells of 

 this layer multiply, the greater number lengthen vertically into 

 woody tissue : some are transformed into ducts ; and others, 

 remaining as parenchyma, continue the medullary rays or com- 

 mence new ones. In this wa}*, a second layer of wood is formed 

 the second season over the whole surface of the former layer 



