667 
Embryology of Onagraceae (Myrtales); 
characteristics, variation and relationships 
Hiroshi Tobe and Peter H. Raven 
Abstract 
Tobe, H.’ and Raven, P.H? CDepartment of Natural Environmental Sciences, Faculty of Integrated Human 
Studies, Kyoto University, Kyoto 606, Japan; ^Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri 
63166, LI.S.A.) 1996. Embryology of Onagraceae (Myrtales): characteristics, variation and relationships. 
Telopea 6(4): 667-688. Here we report features of the embryology of 14 genera of Onagraceae, as 
a companion study to our earlier report on Ludwigia alone. We found that the 4-nucleate Oenothera 
type embryo sac that sharply distinguishes Onagraceae from all other Myrtales is common to all 
genera. Shared features of the nuceUus and seed coat structure, however, indicate that Onagraceae 
more closely resemble Lythraccae than other Myrtalean families. Differences between onagraceous 
genera are found in 1) the mode of anther wall formation (the Basic or the Monocotyledonous 
type), 2) the number of cells in the ovule archesporium (one-celled or multi-celled), 3) the nature 
of early development of the inner integument (retarded or not retarded), and 4) the thickness of 
the parietal tissue in the nucellus (thin or thick). Based on comparisons in these and other 
embryological characteristics, we have concluded that: 1) Ludwigia (Jussiaeeae) differs sharply 
from the rest of the family in having a one-celled archesporium in its nucellus; 2) Hauya (Hauyeae) 
and eight of the genera of Onagreae (except Gayophytum) closely resemble one another but differ 
from the other genera of the family in their markedly thick parietal tissue in the nucellus; 
3) Gayophytum, unlike other Onagreae, resembles Epilobium (now including Boisduvalia) in having 
retarded early development of its inner integument and in having thin parietal tissue; 4) Clarkia 
heterandra (formerly segregated as the monotypic genus Heterogaura) differs from other species of 
Clarkia and from other Onagreae (except Gayophytum) in its nucellar histology. 
Introduction 
Onagraceae are a well-defined plant family, comprising seven tribes, 16 genera and 
about 650 species (Raven 1979, 1988; Hoch et al. 1993). The family belongs to the 
order Myrtales (Dahlgren & Thorne 1984; Johnson & Briggs 1984; Chase et al. 1993), 
but is quite isolated, marked as monophyletic by at least five autapomorphies (for 
review, see Raven 1988). Leaf, wood and floral anatomy have been studied 
extensively; chromosome numbers are known for most taxa and chromosome 
morphology for all major groups; and breeding systems and pollinators, flavonoids 
and palynology have been investigated for much of the family (see Raven 1988; 
Hoch et al. 1993). Recent molecular analyses of relationships in the family, 
summarized in Conti, Fischbach & Sytsma (1993), while not entirely consistent with 
one another, nevertheless have provided phylogenetic models within which to 
examine comparative data from other sources. 
Regarding the embryology of the family, about 100 publications are available from a 
bibUography compiled by Davis (1966: for publications until 1965) and Nagendran & 
Dinesh (1989: for articles published between 1965-1985). Most of the works published 
from the 19th century to the middle of the 20th century had described micro- and 
megasporogenesis and megagametogenesis (i.e., embryo sac formation) using light 
microscopy. In these works, the distinctive pattern of megasporo- and megagametogenesis 
named the 'Oenothera' type, which was reported by Geerts (1908) in Oenothera glazioviana 
('O. lamarckiana') for the first time, was confirmed in 12 of the 16 genera. More recently, 
many studies have used fluorescence or transmission electron microscopy to investigate 
