Time relationships of sporopollenin synthesis associated with tapetum and microspores in Lilium.

The development of the sporopollenin orbicules (Ubisch bodies) on the tapetal cells of Lilium begins while the spores are still enclosed in the meiotic tetrads. Spherosome-like structures, the pro-orbicular bodies, accumulate in the vicinity of the plasmalemma early in the tetrad period, and are extruded into the space within the degenerating inner walls of the tapetal cells. There they acquire a coating of sporopollenin, the accretion continuing until after the release of the spores from the tetrads. Some orbicules remain attached to the plasmalemma by stalks. Synthesis of a material of the general class of sporopollenin begins in the primexine of the young spore in the mid-tetrad period, again outside of the cell membrane, but within the callose tetrad wall.A general scheme for sporopollenin formation in the anther is given. According to this, (a) precursors are synthesised both in the young spores and in the tapetum, and released into the extracellular space; and (b) polymerisation occurs on initiating sites outside of the cell membranes, these sites being the surface of the proorbicular bodies and of the special lamellae concerned in exine growth.Synthesis of sporopollenin in the anther is virtually complete before the main synthesis of the pigmented pollen coat substances (Pollenkitt) begins in the tapetum. It is therefore improbable that the carotenoids produced in the final phase of metabolic activity in the tapetum can be sporopollenin precursors.