CHAPTER VIII. 



THE TREE ITS SHOOT-SYSTEM. 



WHEN we cut into an old branch or stem of the oak 

 (Fig. 26) it is at once obvious that considerable changes 

 have been produced since it was a twig or young shoot- 

 axis, such as exists in the young plant. Of these changes 

 the two following are the most conspicuous. The pith, 

 instead of being surrounded by a cylinder of small vas- 

 cular cords, the diameter of which hardly exceeds its 

 own, as was the case in the one-year-old shoot-axis (Fig. 

 9), is now a mere speck in the middle of a huge mass of 

 wood many hundreds of times as broad as itself, and the 

 cambium cylinder which was developed, as we saw, in 

 the primary vascular bundles, is now a large (though 

 still thin) layer encircling this huge wood mass. Again, 

 in place of a delicate epidermis surrounding a soft, green, 

 cellular cortex, as we had in the young stem, there is 

 here a hard, brown, rugged bark, splitting off in thick 

 ridges on the outside. 



The two chief series of change may be inferred from 

 comparing the two conditions, and taking into consider- 

 ation all we have learned so far. The pith is the same 



