74 



BOTANY 



When a tree gets old the central part of its wood 

 usually dies and becomes very hard. The only living 

 wood is a narrow area close to the cambium. This is 

 known as the sap-wood, or alburnum, the dead centre 

 being the heartwood or duramen. 



This substitution of a solid core for a hollow cylinder 

 of wood is necessary for the strengthening of the trunk 

 and branches. In this well-developed form the mass of 

 the body of both shoot and root is made up of hard wood. 

 As the trunk and branches gradually thicken, their 

 outer regions are strengthened by the production of bark. 

 This begins in the stem as in the root by the forma- 

 tion of sheets of cork. In 

 the root these layers arise 

 in the pericycle ; in the stem 

 they begin in the cortex just 

 below the epidermis (Fig. 

 30). As the years pass, 

 more and more layers of 

 cork are formed deeper and deeper in this region. The 

 outer ones are pierced by lenticels (Fig. 31). They are 

 all formed in the same way, by the formation of meri- 

 stematic layers, which 

 produce cork in their out- 

 side and add to the cortex 

 on their inner faces, be- 

 having in much the same 

 way as the cambium, 

 though the cells they 

 form do not give rise 

 to bast and wood. The n 



, , FIG. 31. Section of a lenticel, /; per, 



cork is impermeable to cork layer. 



water and consequently 



all the cells outside the innermost layer of it die ; the 



tissue thus formed constitutes the bark (Fig. 32). As the 



years go on it becomes thicker and thicker and much 



FIG. 30. Commencement of cork 

 formation in stem. 



