78 PRACTICAL BOTANY 



inferior surfaces of the leaf) are studded with projecting multi- 

 cellular hairs. 



3. That the arrangement of the tissues in the large nerve re- 

 sembles that in the petiole, though less complicated. Thus it 

 often has but .one large central bundle, with smaller lateral ones. 

 The position of the xylem and phloem relatively to the whole 

 leaf corresponds to that in the petiole, i.e. xylem towards the 

 upper surface, phloem towards the lower 



4. Smaller veins, with correspondingly reduced vascular 

 bundles, are found scattered through the thinner part of the 

 section. 



Next examine the thinner part of the section with a high 

 power, and, starting the study of them from the upper surface, 

 note successively the following tissues : 



1. Upper layer of epidermis, continuous with that covering 

 the nerve : it is a single layer of cells, covered externally by 

 cuticle. The epidermis bears numerous multicellular hairs, 

 already noted in connection with apical bud. Stomata occur 

 in considerable numbers. Beneath this layer lie 



2. Thin-walled, oblong cells, with copious protoplasm, and 

 chlorophyll- grains ; these cells are arranged with the longer 

 axis perpendicular to the outer surface, and form two layers ; this 

 tissue, from the form and arrangement of the cells, is called the 

 palisade-parenchyma ; below it is 



3. A mass of parenchymatous cells of irregular form, with 

 large intercellular spaces ; in general characters they resemble 

 (2) : this is the spongy parenchyma. (2) and (3) are together 

 included under the general term mesophylL Carefully examine 

 the chlorophyll-grains in this tissue : their form, mode of 

 multiplication by dividing, and their position in the protoplasm. 



4. Embedded between (2) and (3) are numerous smaller 

 vascular bundles (nerves) of various size, often reduced to a 

 single pitted or spiral tracheid, surrounded by a colourless 

 sheath of parenchyma similar to those in the petiole. The 

 course of the bundles is diverse, since they form the reticulate 

 system of veins : they may thus be seen in the sections to have 

 been cut transversely, obliquely, or longitudinally, 



5. A second layer of epidermis bounds the section on the 



