STRUCTURE OF STEMS 67 



layer of bark on the inside of the old. As the stem increases in 

 diameter new bundles arise in the cambium between the old 

 ones so that the bundles are always crowded. The annual 

 rings are not marked by rings of different bundles. Each 

 bundle is thin and becomes very broad and flat radially, by the 

 constant addition of new wood on the outer edge. The annual 

 ring is due to different rates of growth of the wood during a 

 single season. In the spring growth is more rapid and the vessels 

 are larger. This gives great porosity to the new wood formed in 

 the spring. In the summer growth is slower and the vessels 

 while present are very much smaller so that the summer wood is 

 quite compact. The annual rings are then made up of alternat- 

 ing rings of porous and compact wood. A ring of each consti- 

 tutes an annual ring, since both are produced in one season. 



