Ovule diversity is expressed in several respects. The main aspects of diversity are ovule size, degree of ovule curvature, nucellus thickness, integument number and thickness, formation of the micropyle, funiculus length, degree of vascularization of the ovule and diverse histological differentiation (e.g. hypostase, postament and endothecium).
Ovules are around 0·5 mm long in many angiosperm clades at the time of fertilization. In small-ovuled clades they are approx. 0·15 mm long. Large ovules may reach >2 mm. Diversity of ovule size may be extensive even at the level of orders.
Ovules can be straight or curved in various ways. Straight, uncurved ovules (orthotropous, atropous; Fig. A) are radially symmetric (or disymmetric). In curved ovules the nucellus is either straight (anatropous ovule, Fig. B) or it is also involved in the curvature (campylotropous ovule, Fig. C). Ovules that are only slightly curved are hemitropous (hemianatropous). The three terms ‘orthotrope’, ‘anatrope’ and ‘campulitrope’ (in French) for the different types of expression of curvature were used by Mirbel (1829)
(see also Wagenitz, 2003
). Curved ovules are either monosymmetric or, when they twist, in addition may be asymmetric. The latter is the case in pendant ovules on a lateral placenta. In extant angiosperms, anatropous ovules are most probably ancestral (Doyle, 2008
; Endress and Doyle, 2009
). Some other types, in addition to the three most commonly distinguished types (orthotropous, anatropous and campylotropous), have been described (e.g. Bocquet, 1959
; Bouman and Boesewinkel, 1991
; Taylor, 1991
; Batygina 2002
), but will not be treated here, as their systematic significance is unexplored.
The nucellus is diverse in thickness and length. van Tieghem (1898)
coined the terms ‘plantes crassinucellées’ (plants with crassinucellar ovules) (Fig. 10A, B) and ‘plantes tenuinucellées’ (plants with tenuinucellar ovules) (Fig. 10C–F) to distinguish between thick and thin nucelli. This distinction between crassinucellar and tenuinucellar has long been used in the embryological and morphological literature. In a review by Warming (1913)
‘ovules eusporangiates’ and ‘ovules leptosporangiates’ (in French) were distinguished, corresponding to crassinucellar and tenuinucellar. A more detailed classification was attempted by Shamrov (1997
), containing developmental aspects but without a phylogenetic framework. A phylogenetic framework for a progressively more refined classification was used by Endress (2003
) and Endress and Matthews (2006)
(see ‘Nucellus structure in angiosperm ovules’).
Integuments are diverse in number and of differential thickness (Fig. 10G–K). The number can be reduced from the basic two to one or (exceptionally) none. van Tieghem (1898)
considered integument number in his ovule classification as ‘plantes bitegminées’ (plants with bitegmic ovules) and ‘plantes unitegminées’ (plants with unitegmic ovules), and also used this distinction in his angiosperm classification (van Tieghem, 1901
). Shamrov (2000
) dealt with integument diversity from a developmental point of view. Endress and Matthews (2006)
and Endress (2010
) found new correlations in integument thickness with macrosystematics from a phylogenetic point of view. Further, integuments can be lobed or unlobed, and the outer integument can be annular or semi-annular (review of basal angiosperms in Endress and Igersheim, 2000a
The micropyle may be formed by one or both integuments. In some cases there is no micropyle at the time of ovule maturity, and adjacent parts (the funiculus or obturator) may be in contact with the rim of the integuments or directly with the nucellus.
Ovules are borne on the placentae of the carpels. They may have stalks (funiculi) of different length or may be sessile. When they are sessile they may have a narrow or an extensive attachment area.
Most ovules have a vascular bundle extending from the placenta through the funiculus and raphe to the chalaza. In a number of clades, vascular bundles also reach into one of the integuments. This is mostly in combination with large seeds. At the other extreme, there are ovules with only an undifferentiated procambial strand in the funiculus and raphe or even without any procambial strand at all. Such ovules are small and also otherwise reduced.