Some structures in Azolla megaspores, and an anomalous form

Abstract

On the assumption that the “float” apparatus of Azolla arose by the “capture” and retention of separate floats developed as massulae round abortive megaspores, a model is proposed, accounting hypothetically for the separate origin of three-floated, nine-floated, fifteen-floated and twenty-four-floated species in different types of cytological behaviour during origin of the functional megaspore. It is considered irrelevant to ask whether nine-floated arose from three-floated or vice versa. Each type arose de novo and possibly more than once, out of the initial cytological situation which is constant for all species. An abnormal form of Azolla imbricata is described, which in terms of the model appears to represent a reversion to a Salvinia-like ancestral form, by loss of linked genes responsible for the Azolla “syndrome”. Anatomical and morphological studies on five modern and two Paleocene species are used to illustrate adaptation of perine as float-retaining mechanisms. A new structure, the manicula, produced from the perispore, is described in A. teschiana and A. velus.