SUNFLOWER LEAF 127 



multicellular hairs, already noted in connection with 

 the apical bud. Stomata occur in considerable 

 numbers. Beneath this layer lie 



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

 and chlorophyll-grains they 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. Embedded between (2) and (3) are 



4. Numerous smaller vascular bundles (nerves) of 

 various size, often reduced to a single pitted or spiral 

 tracheide, surrounded by a colourless sheath of paren- 

 chyma similar to those in the petiole. The course of 

 these 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 lower side ; it has the same characters as the 

 upper layer, but stomata are more frequent. Note the 

 two small guard-cells of each stoma, and below them 

 the large air-chamber. 



Hairs as before seen on the upper surface. Note the 

 mucilaginous walls of these hairs. 



The structure of the leaf of the Sunflower is an ordinary type 

 for herbaceous plants : compare with it, by means of similar sec- 

 tions, that of the leaf of the Wallflower (Cheiranthus Cheiri) : the 



