DICOTYLEDONOUS PLANTS. 45 



THE WOODY PLANTS OP MASSACHUSETTS. 



FIRST GENERAL DIVISION. 



DICOTYLEDONOUS PLANTS. 



This division is far the largest and most important in the 

 vegetable kingdom. To it belong nearly all the woody plants 

 of temperate and cold regions. 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 heart- 

 wood, and the external, usually of a different color, sap-wood ; 

 and 4, of bark, consisting of the inner bark, which is somewhat 

 woody, the outer bark, composed of a green layer and a corky 

 layer, and the epidermis or skin. From the pith radiate on 

 every side, horizontally, vessels called medullary rays, the sil- 

 ver grain 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 it 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 horizontally on their way 

 to the bark. A new zone of bark is 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, ridged, and furrowed appearance common 

 in bark on the trunk of old trees. 



The leaves of dicotyledonous plants are usually jointed or 

 articulated at base to the stem, or they are composed of several 

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