CONTINUOUS MULTIPLICATION OF THE STRUCTURES 143 



structure, an account of which will follow. With the production of the 

 primary structure, growth and increase in thickness may cease (most 

 Monocotyledons), in which case the periderm changes which we have 

 recorded will not occur. On the other hand, secondary growth may 

 take place, in which case those changes are more or less completely 

 induced. 



Development of the Cambium. — In such case, the cells touching the 

 phloem-bundles upon their inner faces and upon their sides become 

 meristematic and proceed to produce xylem-tissue upon their inner 

 faces and secondary phloem upon their outer, in contact with the 

 primary tissue of that kind. Each such arc of meristem (Fig. 422, x) 

 becomes the Cambium of that bundle. 



Completion of the Bundles. — At the same time the cells lying in contact 

 with the outer surfaces and with the sides of each xylem-bundle similarly 

 become a cambium for that bundle (y), and sometimes produce second- 

 ary xylem, upon their inner faces, in contact with the primary xylem 

 there, and secondary phloem upon their outer faces. By these processes 

 each bundle which undergoes them, previously consisting of one kind 

 of tissue, therefore an incomplete bundle, comes to consist of both kinds 

 of tissue and becomes a complete bundle. 



The Cambium-circle. — Connecting the cambium arcs of the adjacent 

 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 cylinder of cambium 

 {x, y, z), though a somewhat irregular and wavy cylinder, standing 

 between the zone formed within by the primary and secondary xylem- 

 bundles and their intervening portions of the medullary rays, and the 

 outer primary and secondary phloem-bundles (when the latter develop) 

 with their intervening portions of the medullary rays. Although this 

 cambium forms a cylinder, as stated, it is usually referred to as the 

 " Cambium-ring," or " Cambium-circle," because it presents this appear- 

 ance in transverse section. 



Continuous Multiplication of the Structures. — Provision is now made 

 for the growth of all portions of the stele. Additional complete fibro- 

 vascular bundles are now developed in the medullary ray spaces between 

 the others, fed by a portion of cambium in a similar manner. 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 ray tissue penetrating 

 it nearly to the center, and outside of the cambium-cylinder a hollow 



