6o 



GROWTH AND WORK OF PLANTS 



however, for the pith ray is between. But 

 the parenchyma of the pith rays is squeezed 

 into thin plates by the crowding of the bundles. 

 This brings the bundles of the first year's 

 growth of the stem so near together that they 

 form a ring visible to the eye. Since the 

 bundle is an open one the cambium grows 

 each year, adding on new wood on the inside 

 and new bast on the outside. This causes 

 the growth in thickness of woody stems since 

 each successive year the cambium places on 

 a new layer of wood on the outside * of the 

 old and a new 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 radi- 

 ally, 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 and early sum- 

 mer growth is more rapid and the vessels are 

 larger. This gives great porosity to the new 

 wood formed in spring and early summer. 

 In late summer growth is slower and the 

 vessels while present are very much smaller 

 so that the late summer wood is quite com- 

 pact. The annual rings are then made up 

 of alternating rings of porous and compact 

 wood, and rings of each, constituting an 

 annual ring, since both are produced in one 

 season. 

 * For this reason dicotyledons are called by some exogenous (growing 



on the outside) , while the monocotyledons are endogenous (growing on the 



inside). 



Fig. 58. 



Section of oak tree show- 

 ing annual rings and med- 

 ullary rays. 



