Comparative Leaf Structure of Six Species of Heterosporous Ferns

Abstract

The structure of the mature leaves of the heterosporous ferns Marsilea macropoda Engelm. ex A. Br., Regnellidium diphyllum Lindm., Pilularia americana A. Br., Salvinia natans L., Salvinia auriculata Aubl., and Azolla caroliniana Willd. was examined by light and electron microscopy. Nearly all of the lamina veins are embedded entirely in mesophyll tissue. All veins are concentric (amphicribral), and most are delimited by a tightly arranged cylinder of endodermal cells. Sieve elements usually occur next to tracheary elements with hydrolyzed primary walls. The majority of vascular parenchyma cells are in contact with both sieve elements and tracheary elements. The veins of M. macropoda, R. diphyllum, and A. caroliniana contain transfer cells. Whereas the vascular parenchyma cells are united by plasmodesmata, the sieve elements are connected to each other by sieve pores and to parenchymatic cells by pore-plasmodesma connections. The relative frequency of cytoplasmic connections between various cell types indicates that the transport of photosynthates from mesophyll to the site of phloem loading may follow different pathways in the five genera.