Young sporangiophore of Syncephalastrum
Rhizopus has regular bona fide sporangia (asexual spores produced inside a spherical to ellipsoid container). When sporangia are very small they are called sporangioles. In Syncephalastrum sporangioles are narrowly cylindrical and radiate out from a large apical swelling (vesicle) at the tip of the stalk (sporangiophore). Because of their cylindrical shape and the serial development of the sporaniospores the sporangioles in Syncephalastrum are called merosporangia (but sporangioles is still correct!). At maturity a row of sporaniospores is produced inside each merosporangium (click here). Thamnidium also produces sporangioles but they are different in shape fromthose in Syncephalstrum. If you are interested check out Thamnidium elegans in the Index!