Ovular development and morphology of the outer integument of Magnolia grandiflora (Magnoliaceae)

Abstract

Ovular development of Magnolia grandiflora was examined to determine the morphology of the outer integument. At the time the ovular primordium begins incurving, the outer integument is initiated subdermally, and later the inner integument arises from the dermal layer. Whereas the inner integument is annular, the outer integument is formed as a semiannular rim interrupted on the concave side of the funiculus. Later the outer integument becomes a hood‐shaped envelope. The obturator is formed as a transversely elongate outgrowth on the concave side of the funiculus and intervenes in the gap of the outer integument. During further development the inner integument produces several distal lobes, and the outer integument becomes bilobed. The exostome is a transverse slit with a middle notch, formed by the outer integument and the obturator. Presumed wide occurrence of the hood‐shaped outer integument in primitive families suggests that it is a primitive state. The outer integument is compared with the ovuliferous sporophylls of the glossopterid and caytonialean seed ferns.

Developmental morphology of ovules and seeds of Nymphaeales

“…of Portugal (Friis, Pedersen, and Crane, 1999) have the micropyle-hilum complex, suggesting that ovules with the outer integument hood-shaped already existed in early Cretaceous. Therefore it seems possible that the hood-shaped outer integument is primitive in the Nymphaeales, as has also been argued for other basal angiosperms (Matsui, Imaichi, and Kato, 1993;Umeda, Imaichi, and Kato, 1994;Imaichi, Kato, and Okada, 1995).…”

“…It has been suggested that the outer integuments are cup shaped and asymmetrical (Bouman, 1984). However, recent developmental and morphological studies have shown that in many basal angiosperms the outer integument is sharply asymmetric and hood shaped, that is, it is lacking on the concave (funicular) side of the ovule (Matsui, Imaichi, and Kato, 1993;Umeda, Imaichi, and Kato, 1994;Imaichi, Kato, and Okada, 1995;Igersheim, 1997, 1999;Endress, 1997, 1998; see also Johri, Ambegaokar, and Srivastava, 1992).The origin of bitegmy has been variously interpreted in contrast to the agreement on the origin of the first (inner) integument by fusion of telomes (Herr, 1995). Crane (1985Crane ( , 1986 and Doyle and Donoghue (1986, 1987) postulated that the outer integument is derived from the leafy lamina of a Caytonialike ancestor, while the carpel enclosing ovules is derived by lateral expansion of its leaf axis.…”

“…It has been suggested that the outer integuments are cup shaped and asymmetrical (Bouman, 1984). However, recent developmental and morphological studies have shown that in many basal angiosperms the outer integument is sharply asymmetric and hood shaped, that is, it is lacking on the concave (funicular) side of the ovule (Matsui, Imaichi, and Kato, 1993;Umeda, Imaichi, and Kato, 1994;Imaichi, Kato, and Okada, 1995;Igersheim, 1997, 1999;Endress, 1997, 1998; see also Johri, Ambegaokar, and Srivastava, 1992).…”

“…Crane (1985Crane ( , 1986 and Doyle and Donoghue (1986, 1987) postulated that the outer integument is derived from the leafy lamina of a Caytonialike ancestor, while the carpel enclosing ovules is derived by lateral expansion of its leaf axis. It is also hypothesized that the bitegmic ovules have been derived from unitegmic and orthotropous ovules of glossopterid-like gymnosperms by the enclosure of the unitegmic ovules by the ovuliferous leaf to form the outer integument (Stebbins, 1974;Dahlgren, 1983;Kato, 1990;Stewart and Rothwell, 1993;Umeda, Imaichi, and Kato, 1994;Imaichi, Kato, and Okada, 1995;Doyle, 1996). Kato (1991) also speculates that the anatropy is an extreme modification of the hyponastic curvature of leaves (ovuliferous leaf).…”

Seed coat thickness in the evolution of angiosperms

“…Lobed iis have been considered remnants of ancestral integuments or a sign of atavism as paleozoic ovules displayed a lobbed integument [83]. Nevertheless, it has been argued that lobation might have evolved as a way to restrict the circumference of the integument and allow the formation of a micropyle, as an alternative mechanism to distal integument thickening [38].…”