148 



BOTANY: PRINCIPLES AND PROBLEMS 



sloughed off with the bark, whereas the wood, with its much firmer 

 structure, remains unchanged. Well-marked growth rings are 

 developed in the wood so that an inspection of this tissue as seen 



One Year Old Two Years Old Three Years Old 



Fig. 74. — Growth of a stem in width (diagrammatic). Transverse section 

 of three progressively older stems. Through the activity of the' cambium a new 

 layer of wood and of bast is added each year. Pith and cortex dotted, wood 

 plain, bast lined. 



Fig. 75. — Diagram of a portion of a cross section through a six-year-old twig. 

 The annual rings which are of the same age, in wood and bast, are shaded simi- 

 larly. (From Ganong, " Textbook of Botany", copyrighted by the Macmillan Com- 

 pany. Reprinted by permission) . 



in cross section makes it possible to read with much accuracy the 

 age and past history of the plant. 



Cork Cambium. — Another lateral growing-point of importance 

 is the cork cambium which product the layers of corky bark. 





