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Comparative Leaf Structure of Six Species of Eusporangiate and Protoleptosporangiate Ferns

The structure of the mature leaves of the eusporangiate ferns Ophioglossum vulgatum L., Botrychium virginianum (L.) Sw., Angiopteris evecta Hoffm., and Marattia douglasii (Presl.) Baker and of the protoleptosporangiate ferns Osmunda regalis L. and Osmunda claytoniana L. was examined with light and electron microscopes. All leaf veins are collateral and are bound by a layer of tightly arranged mesophyll cells in the eusporangiate ferns and a cylinder of endodermal cells in the protoleptosporangiate ferns. In the smaller, distal veins the sieve elements commonly occur next to tracheids; in the larger, proximal veins the two types of conducting cells are separated by parenchymatic "xylem sheaths." The majority of vascular parenchyma cells are in contact with both sieve elements and tracheary elements. In some of the larger veins small groups of parenchyma cells are associated with only one or the other type of conducting cell. The parenchymatic cells of the leaf (epidermal, mesophyll, endodermal, and vascular parenchyma cells) are connected by plasmodesmata. The plasmodesmata on the endodermal-cell side of the endodermal-mesophyll cell walls of O. regalis and O. claytoniana are characterized by electron-dense structures reminiscent of the plasmodesmatal "sphincters" in mesophyll cells of the Zea mays leaf. Numerous pore-plasmodesma connections occur in the sieve element-parenchymatic cell walls. Contiguous sieve elements are united by sieve-area pores containing numerous electron-dense membranes. The relative frequency of cytoplasmic connections between various cell types indicates that the movement of photosynthates from mesophyll to the sites of phloem loading may follow different pathways in the eu- and protoleptosporangiate ferns.