A route for the passage of substances through the developing pteridophyte exine

Abstract

The developing exine ofLycopodium gnidioides is traversed from the outer to the inner surface by a series of anastomosing channels filled or lined with fibrillar glycoprotein. When living sporangia are incubated in colloidal iron, particles of ferric iron can be detected in the exine channels, the intine and the spore cytoplasm, and some iron is retained by the surface coatings on the spore. Although there is some diffuse iron staining of the exine between the channels, the main concentration of particles is associated with these structures. This, together with the fact that the proximal lasurae of the spore are closed during development, is taken as evidence that the iron has passed from the locular fluid to the surface of the protoplast principally by way of the exine channels. Results obtained from fixation in a glutaraldehyde-lanthanum nitrate mixture support this interpretation. While the exine channels are in existence, therefore, the spore protoplast is in open communication with the locular environment. The study provides no evidence to suggest that the iron which entered the spore cytoplasm did so by endocytosis. It is possible that iron altered the permeability of the plasma membrane by damaging its structure; entry of iron to the cytoplasm being effected through the damaged membrane.