110 PHYSIOLOGICAL VIEWS. 



with tlie principle of life. As the liber is formed annually from 

 the cambium or descending sap, new layers of alburnum are 

 supposed to have the same origin, and to be formed during the 

 same intervals of time. Most of the sap ascends through the 

 albm-num, though some passes through the perfect wood. The 

 sap which nourishes the buds passes through the center of the 

 stem, and from thence is conveyed in appropriate vessels to 

 the buds. 



129. "YSi^ ])erfect wood is also called the Iteart-ioood^ or dura- 

 men ; its color is usually darker than that of the sap-wood, and 

 its texture is firmer and more compact ; it is also more durable 

 for timber. It is formed by the gradual concentration and 

 hardening of the alburnum. The wood constitutes the greater 

 part of the bulk 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 bark of trees 

 that were growing, and the wounds carefully bound uj) ; after 

 some years, on cutting them across, as many layers of new 

 wood have been found on the outside of the metal, as years 

 had elapsed since its insertion. The strength and hardness of 

 wood is owing to woody fibers extending longitudinally ; these 

 fibers 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. 125, B, which represents a 

 section of a woody stem of three years' growth ; i h, next the pith {i k), is a layer 

 of the first year's growth, and the hardest part of the wood ; 'h ^ is a layer of the 

 second year's growth ; and g b, of the third ; the next to tliis is the sap-wood re- 

 cently formed from the cambium. 



130. Pith. The pith (see Fig. 125, B, Jc i) is situated in the 

 center of the trunk and branches of plants, and is a soft, si3ongy 

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

 of cellular tissue. Tlie cells, which are very large in some 

 plants, are filled with fluids when young, but in old branches 

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

 eral, herbs and shrubs have a greater 2:>roportion of pith than 

 trees ; it is also more abundant in yomig than old vegetables ; 

 it extends from the root to the summit of the trunk or stem of 

 the plant. 



131. The MEDULLARY RAYS are lines which diverge from the 

 pith toward the circumference ; they are fibrous textures in- 

 terwoven in the wood, the alburnum, and the difierent layers 

 of the bark. The new buds o^-iginate from the points at which 



129. Perfect wood — How has it been proved that wood is deposited externally? — Strength and hard- 

 ness of wood, to what owing 1— Illustrate the formation of wood by a reference to Fig. 125.— 130. Pith. 

 -!31. Medullary ray.s. 



