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Development of nonlignified fibers in leaves of Gnetum gnemon (Gnetales)¹

²Harvard Forest, Harvard University, Petersham, Massachusetts 01366 USA and The Kampong of the National Tropical Botanical Gardens, 4013 Douglas Road, Coconut Grove, Florida 33133 USA; ³Fairchild Tropical Botanic Garden, 11935 Old Cutler Road, Coral Gables (Miami), Florida 33156 USA and Department of Biological Sciences, Florida International University, Miami, Florida 33199 USA

ABSTRACT

Leaves of Gnetum gnemon have an extensive anastomosing network of thick-walled cellulosic fibers that permeate mesophyll tissues. Brochidodromus venation is precise with major veins originating by uniseriate plate meristems. In mesophyll differentiation, laticifers appear before fibers and more or less parallel to major veins. Fiber initials appear later, mostly within the subhypodermal mesophyll cell layers, but otherwise adjacent to the leaf margin or the major veins. Fibers are early binucleate and sometimes become four-nucleate. Fiber initials extend by symplastic but mainly intrusive apical growth, become irregular, little branched and interpenetrate other mesophyll layers. They make frequent contact with other fibers forming the anatomosing system, but remain thin-walled until leaf expansion is complete. Sclereids are little developed, thus fibers become the main mechanical system of the mature leaf. Once expansion is complete, maturation of fibers involves rapid formation of a cellulosic but unlignified secondary wall that is non-lamellate and almost occludes the cell lumen. These fibers are contrasted with the gelatinous (tension) fibers developed eccentrically in stems of Gnetum. Apart from their mechanical function, fibers may also have a hydraulic function in maintaining a highly hydrated internal leaf atmosphere.

Key Words: brochidodromous venation • fibers • Gnetales • Gnetum • intrusive growth • leaf development • mesophyll differentiation

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