ORGANIZATION OP THE STEM. 53 



small proportion in thickness to the wood; the amount of bark 

 which remains deposited about the wood is exactly propor- 

 tionate to the stretch or tension to which it will submit, vary- 

 ing greatly in different species. In the old trunks of some 

 pines and firs, it sometimes attains the thickness of from eight 

 to twelve inches, whilst in the Platanus occidentalis, or com- 

 mon plane-tree, after the eighth or tenth year, all the epi- 

 phlceum or old and outer layers of bark fall away entirely in 

 the form of brittle plates. 



The duramen or heartwood. The sap chiefly circulates in 

 the inner bark and alburnum where growth is going on, the 

 new and fresh tissues being most active in its transmission. 

 The walls of the cells soon begin to thicken by the internal 

 deposition of mineral matter or sclerogen imbibed through 

 the pores of the roots with the sap, and what was once sap- 

 wood is every year, by the development of new rings of wood 

 removed farther and farther from the region of growth ; after 

 a few years, therefore, it ceases to take part in the vital opera- 

 tions of the plant, its color changes, and it becomes what is 

 called duramen or heartwood 



As the duramen or heartwood doe's not assist in maintaining 

 the functions of the tree, it may decay without injury to the 

 vitality of the plant. Hence it is that we sometimes see old 

 trees covered with the most luxuriant foliage, although their 

 inside is totally gone. 



Having taken a cursory view of the development of an 

 exogenous stem, from the period when it first emerges from its 

 cotyledons or seed leaves, to that term of ifs existence when it 

 begins to show signs of decay in its interior, we shall now 

 attempt a more careful analysis of the different layers of bark, 



