ROOT STRUCTURE AND GROWTH. 



65 



bium of that bundle. At the same time faces of the cambium. This constitutes the 

 the cells lying in contact with the outer secondary structure of the root-stele, and 



any further growth which may occur is 

 merely a continuation of the process de- 

 scribed as secondary growth. When an 

 annual resting period in growth occurs 

 the ducts of the xylem produced toward 

 the close of the year's growth will be con- 

 spicuously smaller than those produced at 

 the beginning, so that conspicuous An- 

 nual Rings are produced in many woods. 



After a tree has attained a certain age 

 the wood at the centre dies, and becomos 

 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;" the outer is called the Al- 

 burnum, or "Sap-wood," It is the dura- 

 men only which yields the most of our 

 colored cabinet lumber. 



The effect of secondary growth upon 

 the structures external to the bast cylin- 



surfaces and with the sides of each xylem- 

 bundle similarly become a cambium for 

 that bundle (y), and sometimes produce 

 secondary xylem, upon their inner faces, 

 in contact with the primary xylem there, 

 and secondary phloem upon their outer 

 faces. By these processes each bundle, 

 previously consisting of one kind of tis- 

 sue, therefore an incomplete bundle, 

 comes to consist of both kinds of tissue 

 and becomes a complete bundle. Con- 

 necting the cambium arcs of the adja- 

 cent bundles a cambium arc (z) forms in 

 the intervening medullary ray, and this 

 produces secondary medullary ray tissue 

 on both its inner and its outer face. 

 There is thus formed a continuous cyl- 

 inder of cambium (x, y, z), though a some- 

 what irregular and wavy cylinder, stand- 

 ing between- the zone formed within by 

 the primary and secondary xylem-bundles 



and their intervening portions of the me- der is extremely variable, according to 

 dullary rays, and the outer primary and the extent of such growth and the rela- 

 tions of the phellogen and its structures 

 and the individual habit of the plant. It 



secondary phloem-bundles with their in- 

 tervening portions of the medullary rays. 



Although this cambium forms a cylinder, has been stated that the phellogen may 



as stated, it is usually referred to as the 

 "Cambium-ring," or "Cambium-circle," 

 because it presents this appearance in 

 transverse section. Provision is now made 

 for the growth of all portions of the stele. 

 Additional eompleta fibro-vascular bun- 



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 



dies are now developed in the medullary case of Bork-casting by the root will be 



ray spaces between the others, fed by a 

 portion of the cambium in a similar man- 

 ner. New medullary rays also develop in 

 the substance of the bundles. We thus 

 have developed upon the inside of the 

 cambium-cylinder a cylinder of xylem, 

 solid except for the blades of medullary 



cast off. 



In all the classes which yield our medi- 

 cinal roots the branches start from the 

 pericycle outside of a xylem-bundle at the 

 point h in Pig 353, as it is first developing 

 and grows through the surrounding tis- 

 sue to the surface. If a root-section has 



ray tissue penetrating it nearly to the cen- passed through branches these will ap- 



tre, and outside of the cambium-cylinder 

 a hollow 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 the inner face and secondary 

 phloem upon their outer. While this does 

 take place in some roots, it usually does 

 not only medullary ray tissue forming at 



pear upon the older part as mature sec- 

 ondary roots, which are successively less 

 developed downward, appearing at length 

 upon the younger portion as not having 

 made their way through the overlying 

 tissues to the surface. As the root first 

 formed is called the Primary, so (its 

 branches are called Secondary. Their 

 structural development is a repetition of 

 that of the primary. 

 The continuity of growth in the root is 



those points on both the inner and outer uniform— that is, there is no division of it 



