Light and electron microscopy of Circinella umbellata (Mucorales)

Abstract

The spinose sporangium of Circinella umbellata which dehisces by means of irregular fissures produces, by cytoplasmic cleavage, sporangiospores with a surface ornamentation of interrupted low ridges. The wall of the sporangiospore comprises an outer electron-dense and an inner, thicker electron-lucent layer. The two wall layers are also discernible in replicas of cross-fractured sporangiospores. In freeze-fracture preparations the outer surface of the plasmalemma appears pitted. During the spherical phase of growth of germinating sporangiospores, the surface ridges become less distinct through expansion of the spore. Germ-tubes emerge through breaks in the sporangiospore wall. The two-layered wall of the germ-tube is deposited internal to that of the sporangiospore wall. The zygosporangium, formed between apposed suspensors, is covered by a smooth, thin primary wall layer which partially disintegrates during enlargement of the zygospore to expose the irregular surface of the secondary wall layer of the zygosporangium.