144 GENERAL STRUCTURE OF ROOT AND STEM 



cylinder of phloem tissue or bast tissue, continuous except for similar, 

 but of course much shorter, medullary rays. 



It has been said above that the portions of the cambium-circle 

 opposite to the primary wood-bundles "may" produce secondary 

 wood upon their inner faces and secondary phloem upon their outer. 

 While this does take place in some roots, it usually does not, only 

 pericycle tissue forming at those points on both the inner and outer 

 faces of the cambium. 



The above constitutes the secondary structure of the root-stele, and 

 any further growth which may occur, except for the development of 

 branches, considered hereafter, is merely a continuation of the process 

 described as secondary growth. 



The Annual Rings. — When an annua! resting-period in growth occurs, 

 the ducts of the xylem produced toward the close of the year's growth 

 will be conspicuously smaller than those produced at the beginning, so 

 that conspicuous Annual rings are produced in many woods. 



The Duramen and Alburnum. — ^After a tree has attained a certain age, 

 the wood at the center dies, and becomes dryer and harder and of a 

 different color from the living wood outside of it, and this dead portion 

 becomes thicker year by year. It is called the Duramen, or "Heart- 

 wood," and it often contains medicinal or coloring matters. The outer 

 is called the Alburnum, or "Sap-wood." It is the duramen only which 

 yields the most of our colored cabinet lumbers. 



Effects of Secondary Growth upon the Superficial Structure. — The 

 effect of secondary growth upon the structures external to the bast- 

 cylinder is extremely variable, according to the extent of such growth 

 and the relations of the phellogen and its structure and the individual 

 habit of the plant. It has been stated that the phellogen may develop 

 in any part of the cortex. It may now be stated that it may, and, in 

 fact, usually does, in the root develop in the bast-cylinder itself, so 

 that all the parts external to it, and even portions of itself, will belong 

 to the periderm, or in the rare case of Bork-casting by the root, will 

 be cast off. 



Origin of the Branches of the Root. — In all the classes which yield our 

 medicinal roots, the branches start from the pericycle outside of a 

 xylem-bundle at the point h (Fig. 421), as it is first developing, and grows 

 through the surrounding tissue to and from the surface. If cross- 

 sections have been cut through a root so as to pass through its branches 

 also, the branches on the older part will appear as mature secondary 

 roots. Those lower down will be successively less developed, appearing 



