Pollen wall stratification and pollination

Qualities of the stratified pollen walls were evaluated for their possible role in pollination (pollination modes, and pollen tube formation). The importance of studying pollen grains in their respective natural state is noted. Examples of pollen morpholo

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Abstract. Qualit ies of the stratified pollen walls were evaluated for their possible role in pollination (pollination modes, and pollen tube formation) . The importance of studying pollen grains in their respective natural state is noted. Examples of pollen morphological features specific to pollination vectors are rare and difficult to demonstrate. However, some complex, but significant correlations are reported . Key words: Angiosperms, pollen,

pollen wall, pollination, exine, intine, aperture, ornamentation.

General comments on the topic The question of whether and to what extent the features of the (angiosperm) pollen wall reflect adaptations to the respective pollination mode is not a new one, but it remains unsettled. In a previous conference on pollen form and function (Blackmore and Ferguson 1986) mostly problems of correlations between the ea sily accessible surface of pollen grains and the respective pollination mode were addressed: e.g . the contribution by Thanikaimoni (1986) in this volume on apertural adaptations to the modes of pollination. More facts and ideas were presented in a later pollen conference in 1990 (Blackmore and Barnes 1991). At that time, details of pollen wall stratification and its possible relation to polli-

nation were barely debated . Meanwhile, new data provided more information on relations between sporoderm features and pollination. To my knowledge, the present review is the first to address the features of the sporoderm (i.e. qualities of the stratified pollen wall) and their possible relation to pollination, whereas several previous original papers and reviews have addressed the possible correlation of surface features with the various pollination modes. Concerning approaches to polyad function , for reproductive effort and success and for polyad function expressed through pollen wall morphogenesis, the reader is referred to the classical review by Knox and McConchie (1986). It should be stated that we are far from a complete understanding of all correlations of pollen surface features with pollination vectors. Similarly, the role of the various sporoderm strata in this context is poorly understood . Some of the major reasons deserve a special note: 1. Muller (1979 , p . 628) hits the point: "The large variability of angiosperm exine morphology probably reflects compromises between the different functional aspects, often with one function dominating", and additionally (p . 595) : " Ba sically, any exine structure is a compromise between the protective, the harmomegathic, and the reservoir function".

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M. Hesse: Pollen wall stratification and pollination

2. In many taxa (even at the family level), too little is presently known about pollination vectors (such as in many Araceae genera without any trap-like features , e.g. Pothos, Gymnostachys) . Significant correlations between pollen vectors and the manifold, probably relevant pollen- janther structures are rare . Some of the numerous sceptic statements are collected here. Ferguson and Harley (1993, p. 239 ff) : "Base