270 Internal Modifications. 



being deposited each year as long as the plant continues to 

 live. As each new zone is formed over that of the previous 

 year, the latter undergoes no alteration of structure when once 

 formed ; wood is not subject to distension by a force beneath it as 

 the bark is, but, whatever may be the first arrangement or direction 

 of its tissues, such they remain to the end of its life. The forma- 

 tion of the wood is therefore the reverse of that of the bark ; the 

 latter increasing by addition to its inside, the former by succes- 

 sive deposits upon its outside. It is for this reason that stems 

 of this kind are called exogenous (from two Greek words, as we 

 mentioned before, signifying to grow outwardly). According to 

 M. Dutrochet, each zone of wood is in these plants separated 

 from its neighbor, by a layer of cellular tissue, forming part of the 

 system of the pith and bark. After wood has arrived at the a<>e 

 of a few years, or even sometimes sooner, it acquires a color 

 different from that which it first possessed when deposited, be- 

 coming what is called heart wood or duramen. For instance, 

 in the beech it becomes light brown, in the oak deep brown, in 

 Brazil wood and Guiacum green, and in ebony black. In all 

 it was originally colorless, and owes its different tints to matter 

 deposited at first in the ducts, and subsequently in all parts of 

 the tissue. That part of the wood in which no coloring matter 

 is yet deposited, and consequently that which being last formed 

 is interposed between the bark and duramen is called alburnum ; 

 a distinction between the two physiologically important. As 

 each of these zones is the result of a single year's growth, it 

 should follow that to count the zones would be to delermine the 

 age of the individual under examination ; this, however, is a nice 

 and difficult operalion, and has occasioned much dispute; judg- 

 ing from fragments, the diameter of the stem being known, is 

 almost always sure of leading to erroneous results, so that when 

 we hear of the Baobaeb trees of Senegal being 5150 years old, as 

 computed by Adanson, and others still more aged, it is impossi- 

 ble to avoid suspecting that some errors have vitiated the calcu- 

 lations. This same natural division is also called Dicotyle- 

 donous, because the embryo has two seed leaves or cotyledons. 

 Such plants may generally be recognized by their leaves having 

 netted veins; so that it is neither necessary to watch the stem as 

 it grows, nor to examine the seed under a microscope in order 

 to count the cotyledons, to see whether it be exogenous or not • 



