124 THE STEM. 



222. The limitation of the Annual Layers results from two or more 



causes, separate or combined. In oak and chestnut wood, and the 

 like, the layers are strongly defined by reason of the accumulation of 

 the large dotted ducts, here of extreme size and in great abundance, 

 in the inner portion of each layer, where their open mouths on the 

 cross-section are conspicuous to the nalved eye, malcing a strong con- 

 trast between the inner porous, and the exterior sohd part of the 

 successive layers. In Maple and Beech wood, however, the ducts 

 are smaller, and are dispersed throughout the whole breadth of the 

 layer ; and in coniferous wood, viz. that of Pine, Cypress, &c., there 

 are no ducts at all, but only a uniform woody tissue of a peculiar 

 sort (46, 54). Here the demarcation between two layers is owing 

 to the greater fineness of the wood-ceUs formed at the close of the 

 season, viz. those at the outer border of the layer, while the next 

 layer begins, in its vigorous vernal growth, with much larger cells, 

 thus marlcing an abrupt transition from one layer to the next. Be- 

 sides being finer, the last wood-ceUs of the season are often flattened 

 laterally, more or less. 



223. Each layer of wood, once formed, remains unchanged in posi- 

 tion and dimensions. But in trunks of considerable age, the older 

 layers generally undergo more or less change in color and density, 

 distinguisliing the wood into two parts, viz. 



224. Sap-wood and Heart-wood. In the germinating plantlet and in 



the developing bud, the sap ascends through the whole tissue, of 

 whatever sort ; at first through the parenchyma, for there is then no 

 other tissue ; and the transmission is continued through it, especially 

 through its central portion, or the pith, in the growing apex of the 

 stem throughout. But in the older parts below, the pitli, soon 

 drained of sap, becomes filled with air in its place, and thenceforth it 

 bears no part in the plant's nourislmient. As soon as wood-cells and 

 ducts are formed, they take an active part in the conveyance of sap ; 



locliia Sipho, and Menispermum Canadense, tlie annual layers are rather obscure- 

 ly marked, while the medullary rays are unusually broad ; and the wood therefore 

 forms a series of separable wedges disposed in a circle around the pith. In the 

 stem of one of our Trumpet-creepers (the Bignonia capreolata) the annual rings, 

 after the first four or five, arc interrupted in four places, and here as many broad 

 plates of cellular tissue, belonging properly to the bark, are interposed, passing 

 at right angles to each other from the circumference towards the centre, so that 

 the transverse section of the wood nearly resembles a Maltese cross. But these 

 are all exceptional cases, which scarcely require notice in a general view. 



