THE AZOLLA‐ANABAENA AZOLLAE RELATIONSHIP

S ummary Two morphologically distinct classes of epidermal trichomes, termed simple and branched hairs, are shown to occur in Azolla caroliniana Willd. leaf cavities. The two hair types have significantly different frequencies and patterns of distribution during the development and maturation of the leaf cavities. There are only two multicellular branched hairs per cavity. These are consistently found in the same locations along the path of the foliar trace. The number of simple hairs increases rapidly during leaf development. There are about 20 simple hairs in each mature cavity and these are randomly distributed over most of the cavity wall. Clearing techniques confirmed these distributional patterns in Azolla caroliniana Willd., Azolla filiculoides Lam. and Azolla mexicana Presl. The distributional pattern was not altered by the absence of the endophytic Anabaena in Azolla caroliniana Willd. These findings are discussed in regard to the possible function of the two hair types in the Azolla–Anabaena symbiosis.