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The Pollen Morphology of the Sapotaceae

Madeline M. Harley
Kew Bulletin
Vol. 46, No. 3 (1991), pp. 379-491
Published by: Springer on behalf of Royal Botanic Gardens, Kew
DOI: 10.2307/4110538
Stable URL: http://www.jstor.org/stable/4110538
Page Count: 113
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The Pollen Morphology of the Sapotaceae
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Abstract

The pollen morphology of 398 species from all but five of the 53 genera of the Sapotaceae defined by Pennington (1991) in his recent generic review, has been studied using light, scanning and transmission electron microscopy. Within the family pollen diversity is sufficient for twelve pollen types to be described, eleven of which have been further subdivided. A key to the pollen types and further keys to the numerous subtypes that occur within Pollen Types I - VIII are provided. In two Chrysophyllum species, C. marginatum subsp. marginatum and C. inornatum, the structure of the pollen wall is anomalous within the Sapotaceae. The infratectum is comprised of long, slender columellae whereas in all other species examined it is narrow, granular or comprised of reduced columellae. In the other anomalous species, Englerophytum stelechantha, the apertures are more or less porate and represent an extreme of the colporate condition observed in other species in the family. The pollen types are considered in relation to Pennington's revision of the family and the contribution that pollen morphology has made towards resolving taxonomic uncertainties. A broad tribal correlation is observed between Mimusopeae with Pollen Types I & III; Isonandreae with Pollen Type II; Sideroxyleae with Pollen Type VI; Chrysophylleae with Pollen Type VII and Omphalocarpeae with Pollen Type VIII. There are examples of genera in which only one pollen type or subtype occurs, although not exclusively: Burckella, Pollen Type I; Sarcosperma, Pollen Type IV and Omphalocarpum, Pollen Type VIII. A few pollen subtypes are restricted to only one genus: Delpydora subtype VIIC; Sarcaulus, subtype XIA; Chromolucuma, subtype XIC and Diploön, subtype XIIA. Pollen Type IX occurs only in Pouteria, sect. Oxythece. Aperture configuration and the appearance of the tectum from scanning electron microscopy have been found to be particularly useful in defining differences between pollen types. The wall stratification, comprising a very thick tectum and foot layer intersected by a narrow infratectum, is unusual. Both the functional rationale of this type of pollen wall and the occurrence of a granular infratectum in both primitive and more highly evolved angiosperm families are considered. A variable number of (usually 3-4 or 4-5) regularly spaced apertures between pollen grains within single samples is apparently normal within the Sapotaceae, possible reasons for this are discussed. Two major evolutionary trends are postulated, culminating either in 3-4-colporate, prolate grains with continuous equatorial endexinous thickening and differentiated ornamentation, or in 4-5-colporate, subprolate grains with discontinuous equatorial endexinous thickening and undifferentiated ornamentation. Pollination in the Sapotaceae is not well documented; possible vectors include insects, bats and wind. Available data is summarized and discussed. A number of fossil records for the Sapotaceae from the upper Cretaceous onwards are reviewed and a summary diagram provided. Pollen characteristics from selected, mainly Tertiary, records for the family from many parts of the world are compared with the pollen types described in the present paper. Some previously described affinities of fossil pollen with recent sapotaceous pollen types are considered doubtful, in particular an early record from the upper Cretaceous. An appendix provides, from published data and personal observations, the main characteristics of pollen in four families allied or associated with the Sapotaceae: Ebenaceae, Myrsinaceae, Styracaceae and Symplocaceae. Characteristics of pollen in two other families: Burseraceae and Meliaceae less closely associated with Sapotaceae but with pollen that often shows similar characteristics, are also summarized. Features by which the various grains can be distinguished from those of Sapotaceae are highlighted. Two tables provide data for; 1. Correlation of pollen types with Pennington's systematic arrangement and 2. A list of all species examined, arranged systematically and providing full pollen data in condensed form.

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