164 THE VEGETABLE KINGDOM. 



severely by the first class on the bark, by the second on the 

 inside, while the inside of the exogenous may be removed, 

 or the outside of the endogenous may be cut, without 

 stopping the growth in the least. The Mowana possesses 

 the power of both. The reason is, that each of the laminae 

 possesses its own independent vitality ; in fact, the Baobab 

 is rather a gigantic bulb^run up to seed than a tree. The 

 roots, which may often be observed extending along the 

 surface of the ground forty or fifty yards from the trunk, 

 also retain their vitality after the tree is laid low, and the 

 Portuguese now know that the best way to treat them is 

 to let them alone, for they occupy much more room when 

 cut down than when growing." 



On examining the wood of the exogenas, it will be found 

 that in the very centre is a small column of cellular tissue, 

 called the pith, and that from this, fine layers of the same 

 substance radiate towards the circumference. These are 

 called the medullary rays, or silver-grain; they form an 

 exterior layer or ring of cellular substance on the outside of 

 the wood, which is called the cambium. The lightness of 

 wood is owing to its porosity, and, on examining a trans- 

 verse section under the microscope (figs. 17 and 19), it will be 

 seen how little of real space is occupied in wood by solid 

 substance. The spaces between the wall-work of woody 

 matter in the tree are all filled with sap, and hence it is 

 that "green wood" is so much heavier than that which is 

 well seasoned. It is its lightness and strength, together 

 with the ease with which it can be cut and fashioned, which 

 renders wood so exceedingly useful; but its inflamma- 

 bility and liability to decay are great barriers to its more 

 general use. 



The leaves of the exogense have their veins always in the 

 form of a network, and not running parallel with each other, 

 as in the endogenae (fig. 21). 



Of the three divisions of the vegetable kingdom, the 

 exogense alone furnish building timber, properly so called, 

 and it is doubtful whether nature has supplied a more gene- 

 rally applicable substancs than wood ; being a bad con- 

 ductor, it can be handled in the coldest weather, which 



