The Orderly Cycles Pursued in Growth 365 



will help to illustrate. Although 

 these stems possess separate fibro- 

 vascular bundles with intervening 

 plates of soft tissue at the start 

 (compare figure 73), the continuous 

 growth of the cambium, wliich 

 forms new duct tissue on its inner 

 and new sieve tissue on its outer 

 side, gradually fuses the bundles 

 into one woody mass, although 

 preserving, more or less perfectly, 

 the intervening plates of tissue 

 called medullary rays. The growth 

 of the cambium, however, is peri- 

 odically checked by the winter, and 

 the contrast between the small 

 compact autumn-formed cells and 

 the large loose tissue of the spring 

 (figure 139, B) gives rise to the 

 familiar phenomenon of the annual 

 rings, which appear also, though 

 faintly and in reverse order, and 

 ultimately crushed to unrecogniza- 

 bility, in the bark. Most of these 

 features show well in such a wood 

 as that of the Oak, where the 

 annual rings, and even the sepa- 

 rate ducts, are easily visible to the 

 eye, while the medullary rays be- 

 come the broad shining plates 

 which give distinction and value to 

 quartered oak. Meanwhile corky 

 waterproof layers are forming in 

 the outer part of the bark, which 



Fig. 141. — A segment, including a three 

 year old fibrovascular bundle, of a 

 typical stem, the Linden, showing 

 the annual rings of the wood, the 

 cambium cylinder, and the annual 

 rings (less prominent) in the bark. 

 Compare with a single bundle in fig- 

 ure 139, B. (Copied from a r-all- 

 chart by L. Kny.) 



