STRUCTURE OF EXOGENOUS STEMS. 



95 



as lines diverging from the centre. They are 

 called medullary rays (or rays passing off from 

 the medulla or pith) ; and their office is to 

 maintain a constant connection, between the 

 pith and interior of the stem, and the bark or 

 exterior, for an important purpose hereafter to 

 be mentioned (. 140). The thin plates which 



, ., . ., 1 ,1 T , / F IG - 42. VERTICAL 



they form, crossing as they do the direction ot SECT ION OF A PORTION 

 the fibres of the wood, are known as the silver OF AV EXOGENOUS 



STEM, IN A DIRECTION 



grain by Carpenters. In many instances they CROSSING THAT OF THE 



add greatly to the beauty of the wood. "fSIt^ductT 8 *, ! 



129. The number of rings or layers, of woody fibres -,c,c, cut 



ends of the medullary 



which the wood of any stem or branch consists, rays, 

 is in general easily reckoned by cutting it 

 across, as shown in the accompanying series of figures. They 

 correspond exactly, in this climate, with the number of years 

 which the part has existed. But there is reason to believe that, 

 though in temperate climates, the trees of which shed their 

 leaves and renew them once a year, a layer is formed no oftener, 

 in tropical climates, where many kinds of trees have two or 

 three successions of leaves in a year, a corresponding number of 

 layers will be formed : 



FIG. 43. Piffsr YKAH MAGNIFIED. 



FIG. 44. SECOND YKAR. 



FIG. 45. THIHD YEAR. 



