62 



THE SCIENCE OF LIFE. 



FIG. 16. a. Transverse stem of Endogen, (Palm.) b. Of Exogen, (Buckthorn.) c. Trans- 

 verse and longitudinal section of Maple in the beginning of the second year 



and medullary rays. (Fig. 16.) The pith is the cellular 

 tissue of the center ; the medullary sheath a ring of 

 spiral vessels round the pith, which sends projections 

 through it to form the medullary rays ; the wood con- 

 sists of concentric layers of woody and vascular tis- 

 sue ; and the bark is trade of cellular materials, some- 

 times containing branching vessels (laticiferous tissue) 

 conveying milky juice. 



