Ultramicroscopic examination of mature massulae of Habenaria arinaria (Orchidaceae).

The mature massula of H. arinaria was examined by means of transmission electron microscopy, with the aim to understand the nature of cohesion between grains, the accumulation of pollen storage reserves, and the behavior of the nucleus of the vegetative cell in this composite type of pollen. The massula was a union of a large number of polygonal pollen grains that were tightly linked together. The exine within the massula were highly simplified, consisting of a single layer of nexine-2, lacking tectum, bacula, and nexine-1, while all the four layers comprised the exine on the massula surface. The two layers of nexine-2 of adjacent grains fused into a seamless whole. Undoubtedly the fusion of the nexine-2 was the mechanism by which the grains of the massula were linked together. No starch grains, lipid bodies, or storage proteins were present in the mature massula, and so the composite pollen of this species belonged to a novel type with regard to storage reserves. The vegetative nucleus was not lobed and revealed a huge amount of highly condensed chromatin, indicating a quiescent status. The condensed status of the vegetative nuclei in this composite type of pollen system is in striking contrast to the highly decondensed status reported in the free type of pollen grains.