THE GREEN LEAF 63 



" veins " which are left, and constitute the 

 delicate network of " skeleton leaves " on 

 macerating leaves in water. The vascular 

 bundles are rather complicated in structure, 

 and they represent the most highly specialised 

 tissues of the plant body. A vascular 

 bundle consists of wood (xylem) and bast 

 (phloem), and a thin band of tissue known 

 as cambium often lies between them. They 

 anastomose freely in the oak leaf, and in 

 the stalk they are collected into a few large 

 vascular strands which join the vascular 

 tissues of the stem. Similarly the root pos- 

 sesses vascular strands, and these are like- 

 wise joined with those of the stem or trunk, 

 and thus there is a general continuity of the 

 vascular tissue throughout the plant. The 

 water enters the root from the soil, passes up 

 the trunk, and flows thence into the leaves, 

 travelling through certain specialised cell 

 elements of the wood. In the leaf it is dis- 

 tributed to various kinds of cells, and especially 

 to those containing the bulk of the chloro- 

 phyll (P, in Fig. 9) in which photosynthesis 

 is especially active. The greater part of it 

 evaporates from the cells into the large air 

 spaces which are present in the leaf substance, 

 and the contained air is thus saturated with 

 aqueous vapour. 



The cuticle, which forms the outermost 

 membrane of the epidermis, would prevent 

 the exit of any water, either as liquid or 

 vapour, if it were perfectly continuous, In the 



