THE STRUCTURE OF THE SHOOT 67 



FIG. 26. Diagram of sections of stem of dicotyledon at three ages. 

 A, young condition, showing commencement of differentiation of 

 the plerome and its vascular strands: a, strand; b, limits of the 

 plerome; c, periblem; m.r., medullary ray; pi, pith. B, a little 

 older stage: p, bast; x, wood; c, cambium; i.e., interfascicular cam- 

 bium: (one of the strands has been shaded). C, older stage, after the 

 commencement of secondary thickening: px, protoxylem or first- 

 formed wood; x, secondary wood; ph, secondary bast. (After Sachs.) 



