132 PRACTICAL BOTANY 



c. The cellulose-layer, which abuts on the cavities 

 of the cells : this stains blue with chlor-zinc-iodine, 



i. These several layers are not easily distinguished in prepara- 

 tions simply mounted in glycerine, but may be readily distin- 

 guished in sections treated with a weak solution of fuchsin: 

 a and b stain much more readily than c. 



ii. Treat sections with concentrated sulphuric acid, a retains 

 a sharp contour ; the rest of the wall swells, and loses distinctness 

 of outline. 



iii. Boil some sections for a long time with strong potash, a 

 and the cuticular granules of b will be dissolved, while c and the 

 cellulose matrix of b will remain. 



2. Hypoderma, a strengthening tissue, which is 

 immediately below the epidermis, and consists of a 

 single layer of cells : at the midrib it may widen into 

 two layers : the walls are pitted, and stain bluish with 

 chlor-zinc-iodine. 



3. The palisade-parenchyma, composed of thin- 

 walled, oblong, closely-packed cells, with their longer 

 axes perpendicular to the surface of the leaf; the cells 

 are somewhat irregularly arranged in three layers ; 

 observe nuclei and chlorophyll-grains: here and 

 there are cells (idioblasts) with but little protoplasm, 

 in which is inclosed a large crystal. Passing towards the 

 lower surface of the leaf, this tissue merges gradually 

 into 



4. The spongy parenchyma, the cells of which 

 resemble those of (3) in general characters ; but their 

 shape is various, and large intercellular spaces occur. 

 Idioblasts with crystals are scattered here and there. 

 Embedded between (3) and (4) are 



5. Vascular bundles of various size ; the direction 

 in which these run is not uniform, as is naturally the 



