Ultrastructure of the mature pollen of Michelia figo (Lour.) Spreng. (Magnoliaceae).

The structural organization of the Michelia figo mature pollen was investigated. The pollen wall consisted of an outer exine and an inner intine, the former being coated by a thin polysaccharide pellicle. The intine comprised three structurally distinct layers that were equally thick throughout the pollen surface. The generative cell (GC) was closely associated with the vegetative cell (VC) nucleus and its periplasm was found to maintain communication with the sporoderm through a complex plasmalemmic cord. In the freeze-fixed pollen a fluffy coat was detected on the cytoplasmic face of the VC plasmalemma bordering the GC. The plastids were present in only the VC and usually contained abundant small starch grains. In a few pollen grains, however, little or no starch existed and in this case one or more electron dense inclusions appeared in the plastids. Microbodies were found in both the VC and GC. In the VC they presumably have a glyoxysomal function as indicated by the numerous lipid droplets in the cytoplasm and the spatial relationship of microbodies with lipid droplets and/or mitochondria. In the GC the function of the microbodies is unclear once this cell had no abundant lipid reserves and the microbodies did not show any preferential relationship with other organelles. The most conspicuous feature of the VC cytoplasm was the high amount of storage vacuoles which displayed a striking different appearance after one and the other of the fixation techniques used. In contrast to the chemically fixed pollen they were quite polymorphic in the freeze-fixed pollen, and appeared uniformly filled with fibrillar material. Enzymatic digestion with protease has revealed most of this material to be proteinaceous in nature. The existence of phytin reserves is, however, also probable. These protein storage vacuoles closely resemble those in storage tissues of seeds and fruits.