Abstract
The microsporogenous tissue of at least some species of all 17 genera of Onagraceae is divided by transverse septa composed only of tapetum or of both tapetum and parenchyma. The presence of these septa apparently constitutes a distinctive characteristic of the family. In Calylophus sect. Salpingia, Gaura, Clarkia, Heterogaura, and two unrelated species of Ludwigia, the septa are transverse and mostly parenchymatous. In Hauya and Calylophus sect. Calylophus, similar transverse septa together with vertical ones divide the sporogenous tissue into many small spheroidal or ellipsoidal packets. The condition in which occasional tapetal septa divide the sporogenous tissue is clearly the ancestral one in Onagraceae. Polysporangiate anthers divided by parenchymatous septa have evolved independently at least twice each in the branches leading to Ludwigia and to all other members of the family, for a total of no less than four times. Based on this feature and the unique pattern of histogenesis of their integuments, Hauya (despite its retention of many plesiomorphic features), Calylophus, and Gaura may well be directly related to one another.
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