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