Exine and the Role of the Tapetum in Pollen Development

Abstract

CYTOCHEMICAL investigations and examination by electron microscopy of developing anthers in Tradescantia bracteata reveal that exine proper develops at a time when the microspores are separated from the tapetum by the callose walls of the tetrad. The bulk of exine is produced at an early stage, and at the time of release of microspores from the tetrads it has attained a pattern very similar to that seen at maturity. We have found no evidence to support the view that sporopollenin is produced by mitochondria¹, nor have we observed Ubisch bodies or sporopollenin deposits in the tapetum of this plant at any stage, as have been reported in a number of other species^1–3. Six days after break-up of the tetrads, when the deposition of intine has begun, channels 20–40 mµ in diameter can be seen traversing the endexine from the pollen surface to the intine. Between the intine and endexine, tubular structures believed to be sites of sporopollenin polymerization can be seen in a plane perpendicular to the long axis of the pollen grain. In tangential sections through the exine itself it often appears split into parallel bands of fibrils again perpendicular to the long axis of the pollen grain (Fig. 1a and b).