POLLEN WALL ULTRASTRUCTURE AND ONTOGENY IN ZIZIPHUS-LOTUS L (RHAMNACEAE)

The Ziziphus lotus L. (Rhamnaceae) pollen grain is tricolporate, angulaperturate and striato-rugulate. Its exine is characterized by a thick tectum, a thin infratectal layer that appears granular, and a thick nexine. At the aperture there is an endexinic zone with numerous lamellae whose arrangement confers a shredded appearance to the inner face of the apertural endexine. The intine is very thick and includes a zwischenkorper full of trabecula. The ontogenic study of this pollen grain type allows a better understanding of its ultrastructure at the mature stage. Our major findings are as follows: (1) The infratectal layer is columellate in the beginning and seems to become granular later because of the thickening of the tectum and the nexine allowing only the middle of the columella to be seen. (2) The aperture is entirely built up during the tetrad stage. First, oncus and procolumellae are set up. In the mid-tetrad stage endexinic lamella bound the oncus laterally and at the bottom. These lamellae thicken at the free microspore stage, becoming numerous and coalescing around and under the oncus, separating it from the endexinic zone. (3) The thick apertural intine with its zwischenkorper pushes out the discontinuous and weak exine, thus eliminating the oncus.