STRUCTURE OF THE APERTURAL SPORODERM OF POLLEN GRAINS IN EUPHORBIA AND CHAMAESYCE (EUPHORBIACEAE)

The tricolporate pollen grains of 38 Mediterranean and Macaronesian sp ecies of Euphorbia L. and Chamaesyce S. F. GRAY have a special apertur al sporoderm not found in the other taxa of the Euphorbiaceae. At the apertural margo the ectexine is thinner because of shorter columellae and the thin, fragmented or even absent foot-layer. Ectexinous granule s, mixed with endexinous material, are present near the era. The endex ine is homogeneous and thickened under the colpi (at the end and at th e proximity of the end of colpus). Around the era, the endexine is gra nulate and lamellar with irregular cavities. The apertural intine pres ents a characteristic structure with thickenings running along both si des of the colpi. The arrangement and structure of these intinous thic kenings depend on the distance from the era. This special morphology o f the intine is present in all taxa studied here. The genus Euphorbia is considered to be the most evolved taxon of this family. The charact eristic apertural sporoderm may be an adaptative modification to diffe rent physiological conditions, so it may present an apertural mechanis m which is more adapted to harmomegathic changes and thus facilitate t he germination and the formation of the pollen tube.