THE TISSUES OF THE STEM 



49 



cells will appear long and narrow, with square ends (Fig. 24). 

 Putting together the results of these three sections, the form of the 

 cell as a solid body would be flattened prismatic ; it is placed with 



FIG. 36. 



Diagrams of secondary thickening in stem of Dicotyledon, based on transverse 

 sections of the hypocotyl of Ricinus. A represents the stem before origin of inter - 

 fascicular cambium. B, same after it has been formed. C, after it has produced 

 internally a broad ring of secondary wood, and externally a narrower ring of secondary 

 phloem. R primary cortex. M=pith. = phloem. x = primary xylem. 6 = bast 

 fibres at periphery of phloem. ~/c=fascicular cambium. tc=interfascicular 

 cambium. fh = \vood of primary bundle. t/A=wood developed from inter - 

 fascicular cambium, ifp phloem developed from interfascicular cambium. By 

 the intercalation of the secondary tissues the primary bast, b, b, b, is removed some 

 distance from the primary wood x, x. In C the principal medullary rays extend the 

 whole distance through the ring, the secondary rays only part of that distance. 

 (After Sachs.) 



its pointed ends directed up and down, and its broader faces inwards 

 and outwards. The cells have very thin walls, and plentiful cyto- 

 plasm, with a large nucleus. In fact they show the characters of 

 embryonic tissue. The cells given off from the initial cell, after 

 further division pass over gradually to the mature state, forming 



