The fine structure of the exine in relation to the stickiness of angiosperm pollen

Abstract

The variable stickiness of entomophilous (mostly sticky) and anemophilous (always powdery) pollen always depends on the particular distribution and consistency of the pollenkitt on the pollen surface and is related to the fine structure of the exine. Two different “strategies” in exine evolution following the trend from entomophily towards anemophily are significant: 1. (1) Pollen grains without a tectum: the sticky pollen results mainly from the deposition of a great amount of homogeneous, mostly electron-dense pollenkitt between the columellae — so the sporoderm becomes incrusted with pollenkitt. In contrast powdery pollen results mainly from the deposition of a tiny amount of mostly heterogeneous, usually electron-transparent pollenkitt between the columellae, while a considerable amount of pollenkitt remains on the loculus wall. 2. (2) Pollen grains with a tectum: the sticky pollen results mainly from the attachment of a moderate amount of usually electron-dense, homogeneous pollenkitt on the tectum surface, while pollenkitt also enters into the exine cavities. In contrast powdery pollen results mainly from the deposition of an always heterogeneous, usually electron-transparent pollenkitt as a thin film covering parts of the tectum surface, while a great deal of an inhomogeneous, but mostly electron-dense pollenkitt fills up the exine cavities. In both cases the characters of “amphiphilous” pollen hold the balance between the extremes with respect to pollen stickiness, to the homogeneity and electron-density of pollenkitt and the mode of its distribution on the pollen surface.