VASCULAR BUNDLES 311 



usually projects from the lower surface (sometimes from 

 the upper surface also) and contains several vascular 

 bundles. The outer tissue of the midrib below the 

 epidermis commonly consists of collenchyma (p. 279). 

 From the midrib large secondary veins branch off, and 

 these also are usually thicker than the general thickness 

 of the leaf, so that they project on the lower surface. 

 From the large secondary veins smaller ones branch, 

 and from these smaller ones still, and the branches join 

 again (anastomose), so that the whole substance of the 

 leaf is interpenetrated by a network of vascular bundles. 

 The smaller veins are embedded in the general thickness 

 of the leaf. Each of these smaller bundles is surrounded 

 by a sheath of living cells (bundle sheath) , which may or 

 may not contain chloroplasts (Fig. 50, sh.). The finest 

 branches of the bundle network are destitute of phloem 

 and each ends blindly in the mesophyll as a strand of 

 tracheids covered by the bundle sheath (Fig. 51, F). 



From these terminal tracheids the water coming from 

 the roots through the continuous xylem channels is drawn 

 by osmosis into the mesophyll cells, and the dissolved 

 salts also diffuse, though independently of the water 

 (cf. pp. 290, 296), into the same cells, which are constantly 

 using them up to form complex organic substances. 

 The sugars and ammo-compounds, formed as the result 

 of photosynthesis, diffuse out of the mesophyll cells into 

 the bundle sheath, and thence into the sieve tubes, along 

 which they pass into those of the larger veins, and so into 

 the midrib and out of the leaf. 



Fibres. The larger leaf bundles nearly always have 

 a strand of fibres, often crescentic in cross-section, on 

 the outer surface of the phloem, i.e. the surface turned 

 towards the lower face of the leaf, and there is sometimes 

 another similar strand on the upper surface of the xylem, 



