EXINE INITIATION AND SUBSTRUCTURE IN POLLEN OF CAESALPINIA JAPONICA (LEGUMINOSAE: CAESALPINIOIDEAE)

Exine development in pollen of Caesalpinia japonica was studied using high resolution scanning electron microscopy, with attention to the initial developmental process of protectum formation and composition. The protectum is originated on the protuberant sites of the invaginated plasma membrane during the early tetrad stage. The present study shows that the initial protectum is composed of irregularly oriented fibrous threads. The fibrous threads accumulate and form a network on the plasma membrane. Granules 10–20 nm in diameter gradually aggregate within the network of fibrous threads during the tetrad stage. Subsequently the fibrous threads are almost masked by the granules. The developing protectum has a coarse texture within the callosic tetrad envelope. At the free microspore stage the granular protectum becomes homogeneous. The present study suggests that the protectum consists of an association of fibrous threads and granules. The fibrous threads may function as receptors and/or the skeleton of the developing exine.