BUNDLES AND PITH-RAYS. 



11 



we shall see presently, the fascicular cambium, the cambium, that 

 is, within the bundle. Such a bundle, possessing such a cambium- 

 layer, is termed an open one. 



Between the bundles, connecting the pith in the centre with 

 the cortex on the outside of the ring of bundles, are parts of the 

 original or ground-tissue of the stem, which are known as primary 

 medullary rays or pith-rays (Fig. 5). In Dicotyledons they are 

 often broad and conspicuous ; but in Gymnosperms they are so 



FIG. 5. Three stages in the early development of an exogenous stem. 

 m, pith ; r, cortex ; h, primary xylem ; //, secondary xylem ; 6, primary 

 phloem ; B, secondary phloem ; c, cambium ; ms, pith-ray. 



narrow as not to be visible to the naked eye. From the cambium- 

 layer in one bundle to those in the bundles on either side of it 

 the formation of cambium extends, across the primary pith-rays, 

 so that instead of mere strips of cambium running longitudinally 

 down the stem between the xylem and phloem of each bundle, 

 there is now a cylindrical sheath of cambium extending from the 

 embryonic tissue of its terminal bud downwards over the whole 

 stem. In transverse section this sheath appears as a ring, and is 

 accordingly sometimes called the cambium-ring. Those parts of 

 it that extend between the bundles are termed inter-fascicular 

 cambium, in contradistinction to the precisely similar tissue within 



