THE STEUCTURE :0P THE 'STEM 359 



arranged in a ring surrounding the central pith. They are 

 separated by conjunctive ground tissue, strands of which extend 

 from the pith to the circumferential pericycle. These strands 

 constitute the primary medullary rays. As they get older the 

 differentiation of the vascular bundles from the procambium 

 takes place, the protoxylem lying next the pith, and the proto- 

 phloem abutting on the pericycle. The bundles of such a stem 

 are collateral. When they are numerous, and consequently 

 close together, the protoxylems form a more or less evident 

 ring round the pith, and constitute the so-called ineduUary 

 sheath. 



The differentiation of the vascular elements does not usually 

 extend throughout the bundle, but a narrow layer of meristem 

 is left between the bast and wood, the camhium (fig. 742, c). 



This meiistem forms new wood and bast by repeated divisions 

 of a single layer of cells, the wood upon its interior, the bast 

 upon its exterior surface. Through the activity of the cambium, 

 therefore, the individual bundles grow in a radial direction. 

 The divisions of the cambium are chiefly tangential in du-ection, 

 but radial ones also take place, as the cambium becomes pressed 

 towards the exterior of the stem by the accumulation of the 

 wood it is continually forming. 



Soon after the differeniiation of the primary bundles is com- 

 plete, changes take place in certain cells of the medullary rays, 

 which become merismatic and form a band of cambium across 

 the rays. This always occurs between the cambiums of the con- 

 tiguous bmidles, so giving rise to a ring of cambium round the 

 stem at that pomt. The portion of the ring which is formed 

 in the rays, and which differs from the rest by being a secondary 

 meristeni; is known as the iiiter'fasciciilar cambium (fig. 742, 

 if.c). The interfascicular cambium not only forms new parenchy- 

 matous tissue in the ray, maintaining its existence, but part of 

 it forms also new wood and bast like the cambium of the 

 bmidle. AVe may get new vascular bundles intercalated in this 

 way between the original ones, or we may have the latter Qpn- 

 tinually increasing in breadth, their number remaining constant. 

 When new bundles are thus formed they can be distinguished in 

 the stem by not possessing any protoxylem, and by not being 

 continued outwards into the leaves as are the primary ones. 



New medullary rays or radial bands of parenchyma are 

 formed in the substance of the bundles by the cambium as the 

 mass of wood increases, which are known as secondary medullary 

 rays. They differ from the primary ones not only in this mode 



