CHAPTER V. 



THE OAK GENERAL STRUCTURE OF A TREE-TRUNK 

 HISTOLOGY OF XYLEM. 



THE seed of the oak (Quercus sessilis and pedunculata) 

 contains an embryo with two large fleshy cotyledons ; these 

 do not serve as assimilating organs, but supply food to the 

 plumule, which springs up above ground and developes 

 into the stem of the young tree. 



In its younger stages the plumule bears hardly any 

 obvious resemblance to the woody trunk of the older tree. 

 It is herbaceous rather than tree-like ; its structure is 

 that of an annual plant, such for instance as the sunflower. 

 It has a considerable mass of central pith, a ring of 

 scattered vascular bundles, and a cortex covered by 

 epidermis. Compare this structure with that of an oak 

 trunk : here the epidermis has disappeared, the pith is 

 visible only as a relatively small speck in the centre of 

 the section, while the concentrically marked wood, which 

 makes up the bulk of the trunk, does not much resemble 

 the scattered bundles of the seedling. The problem is to 

 understand how the structure of the tree has developed 

 from the herbaceous structure of the seedling. 



D. E. B, 5 



