150 PHYSIOLOGICAL VIEWS. 



Fig. 93. 



i n g a e a 



Fig. 93, at A, represents a young dicotyledonous stem, cut 

 transversely ; the inner circle surrounds the pith ; the wood 

 extends to the bark, which at a appears darkly shaded. 



At B is a section of the same stem magnified ; a b, is the 

 bark ; b i, is the wood, and i k, the pith. 



The divisions of the bark may be seen as follows : a c, rep- 

 resents the cuticle, or the dry, disorganized part ; at c d, is the 

 cellular integument; at d b, the cortex, the extreme part of 

 which, as at b, is the liber. 



Wood. The wood (lignum) consists of two parts, alburnum 

 or sap-wood, and perfect wood. 



The alburnum is so called from albus, white, on account of 

 the paleness of its colour. This is the most newly formed wood, 

 and constitutes the outer part of the woody substance of the 

 plant. It is at first soft and tender, and in this state appears 

 to be active with the 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 alburnum, though some passes through the 

 perfect wood. The sap which nourishes the buds, passes 

 through the centre of the stem, and from thence is conveyed 

 in appropriate vessels to the buds. 



The perfect wood, is sometimes called the heart ; its colour 

 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 



Describe a dicotyledonous or exogenous stem Wood Alburnum Perfect 

 wood. 



