Robert D. Warmbrodt

STUDIES ON THE ROOT OF ZEA MAYS L.-STRUCTURE OF THE ADVENTITIOUS ROOTS WITH RESPECT TO PHLOEM UNLOADING

The adventitious roots of Zea mays L. were studied by transmission electron microscopy to determine whether a symplastic pathway of phloem unloading in this tissue is feasible. In addition to cytological aspects of the various cell types of the root, the structure, distribution, and frequency of plasmodesmata between cell types were determined. The protoplasts of contiguous cells of the root are joined by various numbers of cytoplasmic connections. Except for the connections associated with secondary and tertiary state endodermal cells, the cytoplasmic connections are similar to those between similar cell types in other species, i.e., plasmodesmata between parenchymatic elements, and pore-plasmodesmata between sieve-tube members and parenchyma cells (companion, vascular parenchyma, or pericycle cells in the Zea root). The plasmodesmata of secondary and tertiary state endodermal cells are conspicuously constricted in the region of the suberin lamella. Also, the plasmodesmata between cortical and endodermal cells and between endodermal and pericycle cells are characterized on the endodermal cell side of the wall by an electron-dense area surrounding their neck regions. The highest frequency of cytoplasmic connections occurs between sieve-tube members and their associated companion cells. Results indicate the structure of the Zea root is compatible with a symplastic pathway of phloem unloading and transport to cells of the cortex and pith.

Comparative Leaf Structure of Six Species of Heterosporous Ferns

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.