When studying woods we must consider the width of the medullary rays, the structure and cell contents of the medullary ray cells; the structure, color, and cell contents of the wood parenchyma; also the wood fibres. CHAPTER VI LEAVES Leaves collectively constitute the greatest manufacturing plant in the world. Most of the food, clothing, and medicine used by man is formed as a result of the work of the leaf. The cell contents, structure, and arrangement of the different cells of the leaf differ in a marked degree from the cell contents, structure, and arrangement of the cells in the other organs of the plant. This accounts for the presence of the large amount of chlorophyll in the leaf, the presence of stomata, and the peculiar arrangement of the cells. It should be ascertained if the stomata are above, even with, or below the epidermis; the nature of the epidermal cells, and, when present, the nature of the hypodermal cells; the number of layers of palisade parenchyma and whether it is present on both surfaces of the leaf, and the nature of the outgrowths from the epidermal cells. KLIP BUCHU The cross-section of klip buchu (Plate 108) has the following structure: =Epidermis.= The epidermal cells of klip buchu are modified to form papillæ, the walls are yellowish white, and the papillate portion of the cell is nearly solid. =Hypodermis.= The hypodermal cells are never intact because the mucilage contained in the cells swells when placed in water and breaks the thin side walls. =Upper Palisade Parenchyma.= The palisade parenchyma is two layers in thickness. The cells of the outer layer are greatly elongated and are packed with chlorophyll. The inner layer of palisade cells is more irregular, and the cells are much shorter than the cells of the outer palisade layer. [Illustration: PLATE 108 CROSS-SECTION OF KLIP BUCHU JUST OVER THE VEIN _A._ Papillate upper epidermis. _B._ Hypodermal cells with broken side walls, due to expansion of mucilage contents. _C._ Palisade cells, showing two cells filled with chlorophyll. _D._ Palisade like mesophyll. _E._ Endodermis. _F._ Vascular strand of vein. _G._ Conducting cells with spirally thickened walls. _H._ Characteristic leaf mesophyll. _I._ Short, thick palisade cells on the under side of leaf, just under the vein. _J._ Under hypodermal cells. _K._ Papillate under epidermis.] =Spongy Parenchyma.= The spongy parenchyma cells are branched; therefore, large intercellular spaces occur between the cells. =Under Palisade Parenchyma.= The palisade cells of the under epidermis are short and broad, and they contain fewer chlorophyll grains than the upper palisade cells of the upper epidermis. These cells occur only under the veins. =Under Hypodermis.= The under hypodermal cells are shorter and broader than the upper hypodermal cells.