Corrélations de caractères chez les grains de pollen d'ombellifères

Abstract

In the Umbelliferae, some palynological features were chosen, in order to demonstrate many important correlations existing among these. In a study based on 200 genera (340 species), it has been possible to show that: 1. ( 1) The majority of very small grains ( P ⩽ 20 μ posses a subrhoomboidal or a subcircular shape (with, sometimes, a high symmetry: around 5); the ectoaperture is long; the exine very thin, and roughly the same thickness all round. 2. ( 2) Most small pollen grains (20 < P ⩽ 30 μ) have an oval shape (symmetry around 3, sometimes 4); the ectoaperture is long, and the exine thin being slightly thicker around the equator. 3. ( 3) The majority of average-sized pollen grains (30 < P ⩽ 40 μ) have a subrectangular shape (symmetry always about 3); the ectoaperture is average or short; the exine is rather thick, becoming thicker either at the poles, or in subpolar regions, or at the equator. 4. ( 4) Most of large pollen grains ( P > 40 μ) are constricted at the equator (symmetry around 3 and often bilateral); the ectoaperture is short or even reduced to an aperture which is superposed on the endoaperture; the exine becomes dilated at the equator (digital ectexine stratification, compound tectum). In addition to establishing the above correlations, the different pollen forms were also arranged in a phylogenetic series. The following range was suggested: subrhomboidal pollen grain (very primitive pollen) → subcircular pollen grain (primitive pollen) → oval pollen grain (moderately evolved pollen) → subrectangular pollen grain (evolbed pollen) → equatorially constricted pollen grain (highly evolved pollen). Data obtained from ontogenetical, morphological and anatomical studies suggest phylogenetic series which perfectly agree with those obtained on the basis of pollen morphology. The great importance of pollen-morphological studies for plant taxonomy and for phylogenetic research is emphasized. Also, palynology can be a very valuable tool in biogeography. As an example, recent studies by the author revealed that certain species, which are altitude endemics, have small, subrhomboidal pollen grains with primitive characteristics; this suggests that, within the family of the Umbelliferae, these species are in fact “living fossils” (a conclusion which is also supported by morphological, anatomical, ontogenetical, and cytological data).