VEGETATIVE OEGANS OF PLASTTS. 305 



kind, whose growth is entirely arrested by winter, consist 

 usually of a single" ring 'of woody tissue with interior 

 pith and sui-rounding bark. Often, however, the zone 

 of wood is thill, 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, cor- 

 responding iu number with 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 

 proportion 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, e, while the 

 vigorous renewal of activity in the cambium at spring- 

 time is exhibited 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 locality, but go on 

 to a considerable depth in the wood. Except at the 

 cambial layer, however, these processes consist not in the 

 formation of new cells, nor the enlargement of those 

 once formed — not properly in growth — but in the trans- 

 mission 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 purposes. It then acquires a color differ- 

 ent 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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