Role of the Tapetum in the Development of Tradescantia Pollen

Abstract

WE feel obliged to comment on Godwin's¹ criticism of our recent article² on exine formation and the role of the tapetum in Tradescantia bracteata, and we take his points in order. We reiterate our claim that the bulk of the exine develops at an early stage in our plant. At the time of dissolution of the tetrad the ektexine appears to be fully developed, the baculae have attained maximum height and both apertural and non-apertural regions are differentiated (Fig. 1). Furthermore, the endexine is thicker at this stage than at maturity by which time it has stretched and may have become discontinuous. Our observation of the early development of exine within the tetrad should not be difficult to reconcile with observations made by other workers on other plants^3–7. We can see no a priori reason why it should be assumed that the timing of exine development is identical in all plants. Godwin seems to accept the view that the callose walls of the tetrad are impermeable to exine precursors, and states⁸, in referring to “primexine” within the callose wall: “There seems no reason to suppose that even at this stage the exine elements are not made of sporopollenin although this implies that the precursors are derived at this stage from the microspore and not from the tapetum”. We are describing just such a system but with the difference that the bulk of the exine is produced in this way and not just the primexine.