BUNDLES AND PITH-EAYS 9 



other cases, as in the Oak, the cells of the pith have their walls 

 thickened, and turn from white to brown ; but even then its rela- 

 tively minute width makes it difficult to detect in a stem several 

 years of age. 



The procamUum strands extend, from the rudiments of leaves near 

 its apex, right through the stem into tiie root. They get their 

 name from a Latin word, cambio, to grow, being in a merely transi- 

 tory or embryonic condition. In Monocotyledons the whole of 

 their tissue passes into the condition of wood and bast ; so that 

 the bundle, as the strand in its permanent form is termed, being 

 incapable of any further growth in diameter, is said to be closed. 

 It is because it gives rise to a bundle (Greek, desmos, a bond) that 



B 



Fig. 5.— Three stages in the early development of an exogenous stem, m, pith ; 

 r, cortex ; h, primary xylem : fl", secondary xylem ; b, primary phloem ; B, secondary 

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



the procambium is termed desmogen. In those trees, however, with 

 which we are concerned, viz. Gymnosperms and Dicotyledons, 

 whilst the inner portion of each strand becomes wood or ocylem 

 (Qreek, xylon, wood) and the outer part bast or phloem (Greek, 

 phloios, bark), a band between these two parts remains embryonic. 

 This layer is called the cambium, or more precisely, for a reason 

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

 is, within the bundle. Such a bundle, poss6ssing 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 



