282 HOW OEOPS GEOW. 



kind, whose growth is entirely arrested by winter, consist 

 usually of a single ring of woody tissue with interior 

 pith and surrounding bark. Often, however, the zone of 

 wood is thin, and possesses but little solidity, while the 

 chief part of the stem is made up of cell-tissue, so that the 

 stem is herbaceous. 



Woody Stems. — ^Perennial exogenous stems consist, in 

 temperate climates, of a series of rings or zones, corre-, 

 spending in number Avith that of the years during which 

 their growth has been progressing. The stems of our 

 shrubs and trees, especially after the first few years of 

 growth, consist, for the most part, of woody tissue, the pro- 

 portion of cell-tissue being very small. 



The annual cessation of growth which occurs at the 

 approach of winter, is marked by the formation of smaller 

 or finer wood-cells, as shown in fig. 54, while the vigorous 

 renewal of activity in the cambium at S]3ring-time is ex- 

 hibited by the growth of larger cells, and in many kinds 

 of wood in the production of ducts, which, as in the oak, 

 are visible to the eye at the interior of the annual layers. 



Sap-wood and Heart-wood. — The living processes in 

 perennial stems, while proceeding with most force in the 

 cambium, are not confined to that locahty, but go on to a 

 considerable depth in the wood. Except at the cambial 

 layer, however, these processes consist not in the forma- 

 tion of new cells, nor the enlargement of those once form- 

 ed — not properly in growth — but in the transmission of 

 sap and the deposition of organized matter on the interior 

 of the wood-cells. In consequence of this deposition the 

 inner or heart-wood of many of our forest trees becomes 

 much denser in texture and more durable for industrial 

 jjurposes. It then acquires a color different from the outer 

 or sap-wood (alburnum,) becomes brown in most cases, 

 though it is yellow in the barberry and red in the red 

 cedar. 



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