74 IMPORTANT TIMBER TREES 



are connected with the pith, although broken more or less 

 as they recede therefrom. The microscope shows, however, 

 that all are not connected with the pith. They show inde- 

 pendently in the bark of some species. The common names 

 given them are "mirrors" and "silver sheens." It is well 

 known that most woods will split more evenly and readily 

 on lines radiating from the centre than tangentially. This 

 is undoubtedly brought about by the medullary rays. It is 

 also known that in seasoning, woods begin to check in these 

 rays. 



The Bark. It has already been stated that when the di- 

 gested food travels back from the leaves and puts on a layer 

 of wood on the stem, branches, and roots, it also gives a 

 layer of suitable material to the inside of the bark on both 

 stem and root development. This is necessary because the 

 size of all parts is constantly increasing and more expanse 

 of bark is required each year to cover the whole. As the 

 tree grows the distance around it increases, and as the bark 

 is only slightly elastic it necessarily cracks, and new bark 

 must be grown or the tender wood will be exposed and the 

 cambium layer so interfered with that a new layer of wood 

 cannot be grown through its good offices. As the inner 

 rings of the sapwood practically die as they change to heart- 

 wood and perform no further functions in the life of the 

 tree than that of protection, so a similar change takes place 

 in the bark, except that the outer layer dies and a new inner 

 one is formed, the latter serving the purpose for which it is 

 designed, which is that of aiding in the distribution of food 

 and protecting the parts it covers from injury. It must be 

 admitted that some of the functions of the bark are not yet 

 fully understood, but enough is known to determine pretty 

 well what takes place. As already stated, there is an an- 

 nual addition to the inside of the bark, and for a time in the 

 life of a young tree there is no death of any bark layers ; 

 but as the stem, branches, and roots grow the bark fails 

 to expand with the growth, and the outer or oldest layer 

 cracks, and when that occurs the death of such layer fol- 



