362 



MANUAL OP BOTANY 



which are formed in autumn have smaller diameters than those 

 of the spring wood, so that the extent of each year's formation is 

 usually very evident. The successive zones of wood are known 

 as annual rings {fig. 745). 



The activity of the cambium on its external face is marked by 

 a similar intermittence. Usually, however, the limits of each 



Fig. 745. 



Kij. 746. Section ol twig of Lime, three years olil. 1 j-, 2^, S.i;. The suooeasiTe 

 annual rings of wood. ^A. Phloem, c. Cortex, w.?-. Primary medullary 

 ray. pe. Layer of periderm. After Kny. 



year's formation of bast are not distinct. The bast is further 

 much interfered with by secondary formations of periderm. 



In an old stem the internal zones of wood are frequently very 

 different in appearance from the more external ones. As the 

 wood gets older the cells lose their protoplasm and die, becoming 

 hard and dry, and frequently much darker in colour. The 



