Epidermal cells of Posidonia australis Hook. f. leaf blades have no cell wall ingrowth but the porous cuticle may facilitate nutrient absorption. Numerous plasmodesmata are present between epidermis and mesophyll cells suggesting that photosynthates can be transported by a symplastic route. Lignified vascular bundle sheath cells may act to reduce apoplastic exchange of solutes and water between the mesophyll and vascular tissues. Each vascular bundle contains at least one sieve tube and a xylem element. Cell walls of xylem elements are usually strongly hydrolysed and have little secondary thickening. Mechanical support, combined with flexibility in the leaf blade, is due to the presence of cellulose and hemicellulose and to the absence of lignin in the thickened walls of the epidermal and fibre cells. In the leaf sheath, only fibre cells adjacent to the abaxial epidermis or scattered amongst the parenchyma tissues have lignified cell walls and middle lamellae. These sheath fibre cells appear to persist over long periods. The vascular bundles are continuous between the leaf blade and the leaf sheath but the fibre strands are discontinuous causing a zone of weakness at which the senescing leaves break. © 1978.
