I 



124 A TEXTBOOK OP BOTANY [Ch. IV, 2 



and the atmosphere. Further, the outer dead bark steadily 

 weathers and falls away, either somewhat evenly as in Beech, 

 or else in great flakes cut off by the cork layers as in Elm, 

 Hickory, Oak, or in remarkably smooth layers as in Birch. 

 The inner living part of the bark consists of soft, continuously 

 growing tissue, together with the phloem parts of the bundles. 

 In the older stems, both wood and bark are greatly in- 

 creased in thickness as result of the activity of the cambium, 

 the growth layer of the stem, which continuously forms 

 new wood on its inner and new bark on its outer face. 

 This process goes on indefinitely, making the woody trunk 

 grow steadily in thickness. The bark, however, is simul- 

 taneously weathering and peeling away on the outside, and 

 there comes a time when the rate of this weathering just 

 about keeps pace with the additions within, thus holding 

 the bark thenceforth of nearly constant thickness, though 

 in constant renewal. In the wood only a few outer layers 

 forming the sap wood, distinguishable by the light color, are 

 alive, while the heart wood, usually much darker colored, 

 is all dead ; and the heart may even decay and vanish com- 

 pletely, leaving a mere shell of sap-wood, which, however, 

 suffices, on the hollow column principle, to support the tree. 

 The cambium forms the annual rings, one each year 

 (Fig. 80). It is easy to see that the appearance of the 

 rings is due simply to the contrast between the loose open 

 texture of the wood formed in spring, when large quantities 

 of water, carrying with it stored food, are needed for the new 

 growth of the herbaceous parts, and the close compact growth 

 of the autumn, when less water, and no such food, are re- 

 quired. It is these annual rings which, when cut lengthwise, 

 give the distinctive, attractive " grain" to cabinet woods. 

 Not only do annual rings appear in the wood, but they also 

 occur in the bark, though here they are difficult to see 

 (compare Fig. 87), because the tissues are soft, and soon 

 crushed, and later cut off by the cork layers. Since they 

 are formed by the cambium, the older layers of bark 



