282 HOW CROPS GROW. 
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- 
sponding in 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 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 spring-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 locality, but go on toa 
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 
purposes. It thea 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, 
