278 THE STRTJCTUKE OP TIM BEE. 



not occupying so mucli space and not being so deep-coloured as 

 the same portion of the wood of Larix. Moreover, in some 

 pines the heart- wood does not become dark till the tree has been 

 felled for some time, whereas in the larch it is always dark even 

 in growing trees. 



And now we come to the great division of the dicotyledons, 

 which all agree in possessing vessels. In many of these trees 

 we find that the vessels are so very much larger in the spring- 

 wood of a ring than in the autumn-wood, that we require to 

 examine a cross section somewhat carefully to detect their pre- 

 sence in the outer portion of the wood-ring at all. Amongst 

 trees possessing a well-marked porous zone of spring-wood we 

 find such important examples as the oak, elm, Spanish chestnut, 

 and ash. In order to classify the wood of these trees we re- 

 quire to call to our aid the important organs in trees that are 

 known as medullary rays. These are met with in all trees, and 

 consist of bands of cells which radiate from the centre, or from 

 some other point in the wood, out to the bark. In some trees 

 they are so fine as to be observed with difficulty on a cross sec- 

 tion with the naked eye. This is the case with all conifers, and 

 also with such dicotyledons as the willow, poplar, pear, etc. In 

 other trees, such as the plum and true plane tree, they are com- 

 paratively large and striking, while in others, such as the oak 

 and beech, we find both broad and narrow medullary rays. 



As regards the disposition of the vessels the oak and Spanish 

 chestnut bear a close resemblance to each other, and the timbers 

 of these trees are often confounded. In both cases we find a 

 porous zone of large vessels in the spring- wood, while in the 

 autumn-wood the smaller vessels are arranged in somewhat 

 radially running lines. But whenever we come to examine the 

 medullary rays the points of similarity in the woods of these two 

 trees vanish. In the oak many of these organs appear like thick 

 lines running out towards the bark, while in the Spanish chest- 

 nut all are barely recognizable. In the ash the medullary rays 

 are also very thin, but here there is no appearance of a radial 

 arrangement in the vessels of the autumn-wood. The very dark 

 and large duramen of the elm is of great service in distinguish- 



