50 



COLLEGE BOTANY 



form one kind of tissue while others form another kind. 

 In crossrsections tiie fibro-vaseular bundles appear to be 

 wedge-shaped. As these wedges increase in size, new or second- 

 ary medullary rays are formed from the interfascicular cam- 

 bium, and, therefore, the older the stem the more numerous the 

 fibro-Tascular bundles. These bundles and rays are very evident 

 on the cut end of a woody stem. Along with the growth from 

 the fascicular cambium there is a corresponding growth from 



the interfascicular cambium. In 

 fact, the growth of the fascicular 

 and interfascicular cambium is 

 and must necessarily be uniform. 

 In addition to this growth, the 

 interfascicular cambium some- 

 times forms new fibro-vascular 

 bundles, which in crossrsection ap- 

 pear as narrow strips that split 

 the primary medullary ray. 



Annual Rings. — The cross- 

 sectionofa woody, perennial 

 stem shows a number of well-defined rings (Figs. 26 and 27). 

 Since the rings are usually formed at the rate of one per year 

 and are a good index to the age of the tree, they are called 

 annual rings. They are due to differences in growth in the early 

 and late part of the growing season. In the early part of the 

 growing season the tracheary tubes are numerous and have laige 

 cavities and thin cell walls. As the season advances they de- 

 crease in number and size, aud the walls become thicker; the 

 wood fibers undergo a corresponding increase in number 

 throughout the season. In brief the conducting cells predomi- 



FiG. 27. — Cross-section of dicotyledon- 

 ous woody stem showing annual rings, 

 medullary ray and epidermal system. 



