WOOD AND PITH. 151 - 



of trees and shrubs ; when cut across, it is found to consist of 

 numerous concentric layers. It is supposed that one of these 

 circular layers is formed every year. 



To prove that the wood is deposited externally from the 

 cambium, pieces of metal have been introduced under the 

 barks of trees that were growing, the wounds carefully bound 

 up, and after some years on cutting them across, the layers of 

 new wood have been found on the outside of the metal. 



The strength and hardness of wood, is owing to woody fibres 

 extending longitudinally ; these fibres are chiefly of vascular 

 texture, and contain sap, and the various secreted juices ; some 

 contain only air. 



For illustration of the formation of wood, see Fig. 93, B, 

 which represents a section of a woody stem of three years' 

 growth ; i h, is a layer of the first year's growth, and the 

 hardest part of the wood ; h g, is a layer of the second year's 

 growth ; and g 6, of the third ; the latter is the sap wood re- 

 cently formed from the cambium. 



Pith. The pith (see Fig. 93, B, at k andz,) is situated in the 

 centre of the trunk and branches of plants, and is a soft spongy 

 substance, analogous to the marrow of animals. It is composed 

 of cellular texture. The cells, which are very large in the 

 elder, are filled with fluids when young, but in old branches the 

 fluids disappear, and the cells are filled with air. In general, 

 herbs and shrubs have a greater proportion of pith than trees. 

 It is also more abundant in young than old vegetables ; it ex- 

 tends from the root to the summit of the trunk or stem of the 

 plant. 



The medullary* rays are lines which diverge from the pith 

 towards the circumference ; they are fibrous textures interwo- 

 ven in the wood, the alburnum, and the different layers of the 

 bark. The netv buds seem to originate from the points at which 

 they terminate. 



The pith has been compared to the spinal marrow in ani- 

 mals ; it appears to be an important part of the vegetable sub- 

 stance, though its offices are perhaps less understood than those 

 of the other parts. The letters e, Fig. 93, represent the me- 

 dullary rays as proceeding from the pith and terminating in 

 the cellular integument. 



You are not to expect that every stem or branch of a dico- 

 tyledonous plant will present you with all the various parts 



* So called from medulla, marrow, a name often given to the pith. 



How has it been proved that wood is deposited externally ? Strength and 

 hardness of wood Pith Medullary rays Pith, to what compared Various 

 parts not always distinct in different plants. 



