Stoma blocked by tylosoids

Heide-Jørgensen H S. 1978. The xeromorphic leaves of Hakea suaveolens R.Br. I. Structure of photosynthetic tissue with intercellular pectic strands and tylosoids. - Bot. Tidsskr. 72: 87-103.

A structural analysis of the chlorenchyma of Hakea suaveolens is presented based on light microscopy as well as on transmission and scanning electron microscopy. A cuticle is found covering most palisade cells of the very airy photosynthetic tissue containing large amounts of tannins. Intercellular pectic strands of a filamentous ultrastructure are found distended between palisade cells mutually and between palisade cells and sclereids. The strands are regarded as a casual structure with no obvious function, developed as a mechanical consequence of intercellular space formation. As the leaves get older, some palisade cells redifferentiate into tylosoids which often divide into 2 to 5 cells (Fig. lower left). Finally, they block the stomatal pores making stomatal function impossible, which reduces transpiration without a significant reduction of photosynthesis. In spite of the large volume of intercellular space, it is argued that the chlorenchyma is truly xeromorphic.

According to Flora of Australia, Hakea suaveolens R. Br. is now named Hakea drupacea (C. F. Gaertn.) Roem. & Schult.