Tobe & Raven, Embryology of Onagraceae (Myrtales) 
681 
Some comments seem in order regarding the embryological features of Clarkia helcrandra, 
which until recently has been segregated as the genus Heterogaura, but is now 
unambiguously assigned to Clarkia as a monotypic section closely related to C. dudleyam 
and the other species of section Peripetasma (Lewis & Raven 1992). Its relationships 
were first revealed by evidence from restriction enzyme analysis of chloroplast DNA 
(Sytsma & Gottlieb 1986a, b) and subsequent analysis of nuclear rDNA (Sytsma & 
Smith 1988). Embryologically, however, C. heterandra differs from all other species of 
Clarkia in the histology of its nucellus. All other species of Clarkia and of the entire tribe 
Onagreae (except for Gayophytiim), as well as Hauya, have markedly thick parietal 
tissue, so that the underlying tetrad of megaspores is deeply buried and positioned 
nearly at or a little above the bottom of the nucellus. In contrast, in C. heterandra the 
parietal tissue is thin at the megaspore tetrad stage, and the tetrad of megaspores is 
positioned at the middle of the nucellus. Later both the parietal cells above and the 
nucellar cells below the megaspores divide rapidly to form a massive nucellus. This 
marked difference in nucellar histology clearly distinguishes C. heterandra from other 
species of Clarkia and other genera of Onagreae. Since the macromolecular evidence of 
its close relationships with and probably derivation from Clarkia section Peripetasma is 
unequivocal, however, we conclude that the unusual embryological features of 
C. heterandra, like its distinctive morphological and anatomical characteristics, were 
derived within its evolutionary line after its separation from other species of Clarkia. 
