394 MANUAL OF BOTANY 



walls show curious projections of cellulose into the interior. 

 They contain the chloroplasts. In the centre is a well-marked 

 endodermis enclosing a many-layered pericycle and usually two 

 vascular bundles. These are exceptional in containing a layer 

 of cambium between the wood and the bast. The pericycle is 

 also peculiar : it contains a number of tracheida which abut 

 upon the xylem, and serve to convey water from the latter to 

 the mesophyl] of the leaf. They are spoken of as transfusion 

 tissue. The phloem is in contact with a number of cells con- 

 taining a somewhat dense protoplasm. Besides these a band 

 of sclerenchymatous fibres is found at the back of the phloem. 

 This transfusion tissue is a special mechanism which is needed 

 in these leaves, as the ultimate ramifications of the vascular 

 bundles are not in apposition with the cells containing the 

 chloroplasts. 



The mesophyll of the leaves of the Conifers contains also a 

 variable number of resin-ducts of similar structure to those of 

 the stem. 



The vascular bundles of the leaf-blade, as we have seen, are 

 the continuations of those of the axis of the phyllopodium. If 

 the axisiamonostelic or polystelic, as the ramifications proceed, 

 the bundles ultimately become isolated, and the blade is there- 

 fore astelic or schizostelie. Outside the steles or t)ie separate 

 bundles, traceable for a varying distance, is often a band of 

 sclerenchyma or coUenchyma, extending in the main ribs to the 

 epidermis of the lower surface. As the bimdles are traced 

 further and further from the axis they get thinner and thimier, 

 gradually losing the distinctive characters of their woody and 

 bast elements. The woody part persists furthest, and ultimately 

 consists only of a few traoheids. They end generally in plexuses, 

 or with free terminations among the mesophyll parenchyma. In 

 some cases apeculiar structure, known as a loater-gland, lies in 

 the mesophyll between the epidermis and the end of the bundle. 

 These bodies consist of a mass of small cells with thin walls, 

 usually covered by a sheath which is continuous with the endo- 

 dermis of the bundle. The tracheids of the bundle terminate 

 abruptly at the lower end of the gland. One or more water 

 stomata pierce the epidermis over the latter. Sometimes a 

 water-gland is found above a mass of tracheids formed by 

 the fusion of several bundles. 



In certain leaves, especially the bud scales of the Conifers 

 and those of the Horsechestnut, there is usually a layer of cork 



