Xeromorphic Leaves of Hakea suaveolens R. Br. IV. Ontogeny, Structure and Function of the Sclereids

Abstract

Osteosclereids in the leaves of Hakea suaveolens are investigated from a developmental, structural and functional point of view. The sclereid initial cell is located outermost in a boundary parenchyma comprising 1-3 cell layers next to the palisade tissue. Boundary parenchyma cells, including sclereid initial cells, are the first cells to accumulate starch. Acid phosphatase was localised during development of sclereids and palisade cells. The outer ramifications of the sclereid form a pseudohypodermis, and the inner ramifications penetrate 1-3 cell layers inwards, making contact with specialised cells (tracheoids) of the vein endings and with fibres. The sclereids do not become lignified in the outer ramifications, and they remain alive as long as the leaf itself. Water pathways were localised by addition of the fluorochrome berberine sulfate to the transpiration stream. It is concluded that, besides giving mechanical strength, the sclereids are also important as vein extensions and that, like bundle sheath extensions, they conduct water to the epidermis and directly to both palisade layers. The percentage of the cross-sectional leaf area occupied by lignified water-conducting cells, including sclereids, is higher than in many other cylindrical, xeromorphic leaves. There are about 200 sclereids per square millimetre of palisade tissue in sun leaves, but the number falls in shade leaves and after fertilisation with nitrate and phosphate.