DICOTYLEDONOUS PLANTS. Ab 
THE WOODY PLANTS OF MASSACHUSETTS. 
FIRST GENERAL DIVISION. 
DICOTYLEDONOUS PLANTS. 
Tuts division 1s far the largest and most important in the 
vegetable kingdom. 'To it belong nearly all the woody plants 
of temperate and coldregions. Dicotyledonous plants are distin- 
guished by the structure of the wood, the structure and arrange- 
ment of the vessels and leaves, and especially by the structure of 
the seeds. The trunk in woody plants is composed of 1, a cen- 
tral pith or medulla, which does not extend into the root; 2, of a 
ligneous medullary sheath, enclosing the pith; 3, of wood arrang- 
ed in circles or zones, the inner ones of which are called heazi- 
wood, and the external, usually of a different color, sap-wood ; 
and 4, of bark, consistmg of the mner bark, which 1s somewhat 
woody, the outer bark, composed of a green layer and a corky 
layer, and the epidermis or skin. Fiom the pith iadiate on 
every side, honzontally, vessels called medullary rays, the sul- 
ver giain of wood, which extend through the wood and bark. 
The wood is formed by the annual addition of a new zone or 
layer outside the older wood and between 1t and the bark. ‘This 
new zone consists essentially of woody vessels extending from 
the leaves to the extremities of the roots, and of the silver grain 
or medullary rays which traverse it honzontally on their way 
to the bark. A new zone of bark 1s at the same time formed 
between the new wood and the previous bark. The former 
exterior coats of bark are forced to expand, to make room for 
the newly formed wood and bark; and, when expanded to their 
greatest capacity, the external layers yield, crack, and open, 
causing the rugged, ndged, and furrowed appearance common 
in bark on the trunk of old trees. 
The leaves of dicotyledonous plants are usually jomted or 
articulated at base to the stem, or they are composed of several 
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