98 ORGANIC STRUCTURE. 



of dense cellular matter encloses it, and to this the 

 first concentric layer of wood is attached. In hollow 

 stems pith is only found at the articulations ; and in 

 jointed stems which are solid, the pith is interrupted 

 at each joint. It is also somewhat interrupted at the 

 base of every branch of a simple stemmed tree. 



The Wood. This member is simultaneously pro- 

 duced with the pith which it surrounds. It appears 

 in three different states during its growth. At first 

 it resembles a semi-transparent mucus : next an 

 inspissated jelly showing faint signs of organisation ; 

 and last as alburnum, possessing all the fibrous 

 structure, tubes, sap vessels, and other components of 

 perfect wood. It is the lateral expansion of this mem- 

 ber, and that of the pith, which increases the diameter 

 of the seedling stem. During its growth it is the 

 seat of the vitality ; but ceases to be so as soon as the 

 summer growth is over. This is demonstrable by 

 the fact, that the first formed concentric layer of wood 

 ever remains of the same dimensions it acquired in 

 the first year. 



If this first layer of wood be examined in the 

 autumn, we find its exterior side formed of denser 

 cellular matter than the interior ; the latter being- 

 more porous, owing to its containing larger and a 

 greater number of tubular openings. We can also 

 discover radiating partitions of close cellular sub- 

 stance perpendicularly placed, diverging from, or con- 

 verging to, the pith, and dividing the ligneous layer 

 into segments. These partitions are not so conspi- 



