THE WOOD. 



Fig. 133. The branching 

 vessels of the bark along 

 which the fluids are con- 

 veyed. 



as some have supposed, but is formed in the Spring season from the leares -with the 

 wood, and lies in successive layers -within the mesophlaeum. 



The more immediate use of the hark is that of giving protection to the wood. If 

 bark did not exist there would be no cambium, and without cambium there could not 

 be any deposition of woody fibre ; and thus the presence of bark is necessary to tbe 

 growth of the tree. It is also essential to the life of the tree, 

 from its connexion with the cellular system, and with the 

 undeveloped leaf-buds. 



The bark contains a large number of air vessels and vasa 

 propria, and not only conveys refuse matter from the leaves 

 to the soil, but is in almost all cases a depository of elaborated 

 secretions. This is well seen in the oak bark, yielding tan- 

 nin ; the cinchona bark, producing quinine ; and the fir-tree, 

 emitting turpentine. There are also many milk vessels ; but, 

 with the exception of the Nepenthes, there are no spiral vessels. 

 We have oftentimes found thick wall-cells, as in Fig. 40, 

 arranged in columns with great regularity. 



Wood. We now proceed to the most important division 

 of the parts formed in exogenous stems viz., the Wood a 

 substance not merely giving stability and beauty to the tree, 

 but offering the greatest service to man. We find it occupying nearly the whole 

 body of the trunk, and arranged, as a rule, in a very regular manner in this class 

 of trees. On taking up any piece of wood, but more particularly the entire section 

 of a stem, we first notice a series of circles, which incraase in diameter, and are separated 

 by wider intervals as we approach the bark. In this manner the trunk is composed 

 of numerous zones inclosed within each other. Again, in almost all trees, we observe 

 the medullary rays before-mentioned passing in straight lines from the centre to the 

 circumference ; and as the circle of the stem at the bark is much larger than any 

 circle near to the centre, it follows that the medullary rays will be wider apart at 

 the bark than at the pith. On this view of the subject we may state that the stem is 

 composed of a series of wedge-shaped blocks, which have their edges meeting at the 

 centre. The combination of these two views gives the correct idea of the arrange- 

 ment of the wood viz., a series of wedges, each divided into segments of unequal 

 width by circular lines passing across them. From this description it must not be 

 supposed that these various portions are detached, or may be readily detached, from 

 each other ; for, although the medullary rays and the circular mode of deposition both 

 tend to a less difficult cleavage of the wood, they yet bind the parts very closely and 

 firmly to each other. 



The explanation of the occurrence of distinct zones of wood is that each zone is the 

 produce of one year, and that in our climate, more so than in tropical countries, the 

 period of growth of wood ceases for many months between the seasons, and thus induces 

 a distinction in appearance between the last wood of a former and the first wood of the 

 succeeding year. This distinction is maintained throughout each year, and throughout 

 a long series of years. 



The inclosure of zone within zone is owing to the mode in which the wood is pro- 

 duced, and the position in which it is deposited. Wood is formed by the leaves during 

 the growing season, and passes down towards the root between the bark and the wood 

 of the previous year (if any}, or in the position in which cambium is effused ; and, as th 



