EXINE FORMATION IN THE POLLINIUM OF DENDROBIUM

The position of the callose wall is related to the position of the pri mexine matrix that forms around the peripheral tetrads during microspo re development of the compound unit, the pollinium. We report a combin ed freeze-fracture and freeze-substitution study of the events associa ted with early exine development. Stage one of exine development is de position of protosporopollenin that is probably synthesised by the mic rospore and secreted to the primexine matrix where it is polymerised. Enzymes for the polymerisation of the protosporopollenin may be synthe sised by the microspores and then transported, via the endoplasmic ret iculum, to the plasma membrane. Stage two of exine development follows callose dissolution and deposition of tapetally derived sporopollenin . Hence exine form and exine deposition in Dendrobium appear to be the result of intimate cooperation between the microspore, the plasma mem brane, the callose and the tapetum.