4 ‘ PHYSIOLOGY. 206 
The stem of herbs and “shrubs, as weil. as the 
trunk, the scape and the stalk, in short all the va- 
rieties of the stem, have a channel full of pith, sur- 
rounded by cellular texture, in which the reducent 
vessels lie. The adducent and air vessels form a 
circle round this, or according to the plant, a trian- 
gular, pentagonal, or hexagonal assemblage of many 
joined bundles, which run in a straight direction. 
A thin layer of cellular membrane, and another 
membrane full of lymphatics, incloses the whole. 
The same happens in the growth of the stems of 
trees and shrubs during the first year. Every year a 
new bundle of adducent and air vessels in a circular 
form is added externally to the old ones. The in- 
nermost bundles of vessels are more and more com- 
pressed, till the pith at last, except where this is na- 
tural to some shrubs and trees, entirely disappears, 
or at least is compressed to a very small point. The 
interior vascular circles become annually more dense, 
and at last get so hard, as to form what is called 
wood. ‘The less, or half indurated external circles, 
constitute the alburnum, and the outermost one,.which 
is just newly formed, is now called the inner bark. 
This then is a circle round the stem of the trée, con- 
sisting of numerous, young, new formed vascular 
bundles. It commonly consists of two parts, the 
exterior layer changing into bark, the interior first 
forming the alburnum, and then the wood. The 
bark, in ligneous plants as well as in herbs, is green 
and vascular; but as soon as it grows older, its 
green colour changes into brown; still however the 
lymphatics retain their power. ‘The more the tree 
advances 
