STRUCTURE OF THE SHOOT 71 



continues, bundles are introduced, and ultimately a compact 

 ring of wood is formed with thin medullary rays between 

 the bundles (Fig. 38) . 



The formation of new tissue and especially of woody 

 tissue goes on actively in the spring and summer ; less is 

 formed in the autumn ; and little or none in the winter. 

 On the return of spring, the process is repeated ; and as the 

 wood formed in spring consists of elements with much 

 larger cavities than those formed in the late summer and 

 autumn, the successive zones stand in strong contrast with 

 each other, and may be clearly seen in a transverse section. 

 These annual rings (Fig. 39, a) are often irregular, and some- 

 times more than one ring may be formed in a season, but 

 their number enables us to obtain a fairly accurate idea of 

 the age of a tree. By such increase some stems may grow 

 to a great age and size, and, unlike animals, they may add 

 to their body-substance year by year. 



These different tissues may be determined by dissecting 

 a piece of Elder stem. Outside is the thin dying epidermis 

 with a layer of cork below ; next the green cortex, followed 

 by the slimy tissues of bast and cambium. The firm wood 

 is easily discovered surrounding the central pith. In some 

 stems, e. g. the Laburnum, the wood in the centre becomes 

 dense and dark-coloured and is known as ' heart-wood ' ; 

 while the newer, outer wood is soft and light in colour, and 

 is known as ' sap-wood'. The heart- wood usually serves 

 chiefly for the storage of water, the main ascending current 

 passing along the sap-wood, whence its name. 



Cork 

 Cork and lenticels. During the early stages in the de- 

 velopment of the woody tissue of Dicotyledons, the outer 

 cells of the cortex and epidermis keep pace with it, but 

 eventually they lose their power of increasing and tend to 

 give way under the internal strain. Meanwhile, provision 



