THE PHYSIOLOGY OF PLANTS 



Fig. 12.— Diagrammatic representation of the 

 Gradual Formation of the Woody CriiNDER of 

 A Dicotyledon. 



A, transverse section through the apex of a shoot; the 

 several vascular bundles are arranged in a ring in 

 the ground tissue, dividing it into central pith (M) and 

 an external cortex (R) ; x, wood or xylem ; p, bast or 

 phloem. 



B, transverse section of a somewhat older shoot. The 

 lighter portion between the bundles represents the 

 medullary rays. In each bundle there lies between the 

 wood and the bast a layer of cambium, fc {fascicular 

 cambium). This cambium is continued into the medul- 

 lary ray, as interfascicular cambium (ic). Thus the 

 cambium ring is formed. 



C, transverse section through a mature shoot. The wood 

 of the bundle (fh) and the interfascicular wood (ifh) 

 form together the wood cylinder ; ic and fc form the 

 cambiimi ring. Outside this are the bast areas {p and 

 ifp), the soft bast of the bundles being protected by 

 the hard bast groups, b (after Sachs). 



medullary to the sur- 

 rounding elements, the 

 various zones of the 

 stem become homo- 

 geneous, and we can 

 distinguish with the 

 naked eye, an even 

 woody cylinder and a 

 correspondingly homo- 

 geneous ring or belt of 

 bast. 



But each bundle 

 possesses, as we have 

 seen above, a layer of 

 cells lying between 

 the bast (Fig. 12, e,2), 

 and h, h, h) and the 

 wood (Fig. 12, C, X, 

 ifh), which layer is 

 able to, and does 

 actually, increase the 

 number of its cells 

 (Fig. 12,0, fc). This 

 layerof cells represent- 

 ingsome of the original 

 procambium cells, 

 which have remained 

 active, and continue 

 to multiply, we term 

 cambium. The layers 

 of cambium in each 

 of the several bundles 

 are also all equidistant 

 from the centre of the 

 axis. They do not, 

 however, touch each 

 other laterally, as a 

 medullary ray runs 

 between the bundles. 



