December Acta 801. Neerl. 25(6), 1976, p. 385-400 Anatropy and ontogeny of the bitegmic ovule in Chrozophora A.H.L. Jussieu (Euphorbiaceae) J. Bor and R.N. Kapil Hugo de Vries-Laboratorium, Universiteit van Amsterdam and Department of Botany, University of Delhi, Delhi-110007, India SUMMARY A study of the initial ontogeny and curvature ofthe ovule in Chrozophoraobliqua and some other euphorbiaceous taxa explains why the outer integument develops asymmetrically in bitegmic anatropous ovules. The almost simultaneous occurrence of periclinal divisions in the dermatogenof the outer integumentaryprimordium and funicle and the synchronous of both funicle the and elongation (viz. raphe) outer integument (antiraphe) strongly sup- port the idea that this integument and the raphe are derivatives of the funicle. The primor- dium of the inner the other be rim-like integument, on hand, appears to an independent outgrowth of the nucellar base. Therefore, inner and outer integuments are not homologous appendages from an ontogenetic point of view and the concept of ‘congenital fusion’ must be discarded as an explanation. It has also been proposed that the dividing line of nucellus and funicle is situated between the attachment zones of the inner and the outer integument. the term “nucellus” is to be defined Accordingly, as that part of the ovule which is com- pletely surrounded by the inner integument. Both the are initiated divisions integuments by periclinal ofa subdermal initial and further divisions result in the vertical growth ofthe primordia. The outerintegument becomes multi- result of characteristic radial divisions in the subdermal layered as a layer, and a vascular bundle differentiates at its periphery. The large, curved nucellar beak is principally ofdermal origin. A hypostase differentiates below the embryo sac during prefertilization stages, whereas the nucellar takes after developmentof a podium place only fertilization. The carun- cle consists mainly of anaerenchymatous tissue. The diagnostic value ofa number of distinctive ovular features in Euphorbiaceaehas been discussed; occurrence of the general a nucellar beak and especially a thick, subdermally derived and vascularised inner integument in the taxa constituting the subfamily Crotonoi- deae s.s. seem indicate that this is a to homogeneous group. 1. INTRODUCTION The of recorded embryology Chrozophora was earlier by one of us in three representatives of this genus (Kapil 1956a, b); however, not much attention was given to the study of the ovular ontogeny. As the initial ontogenetic show and follow processes a striking similarity certain patterns, at least within the subfamilies Acalyphoideae, Crotonoideae and Euphorbioideae (cf. Web- 386 J. BOR and R. N. KAPIL STER 1975), it is likely that such ontogenetic features, when used as taxonomic 1 characters, will throw some light on phylogenetic relationships . have also re-evaluated the of related the Besides, we concept anatropy, as to initial development and delineation of funicle, nucellus, and integuments. The ovule is the found anatropous, bitegmic most common type among angios- but the of perms, origin anatropy has not been satisfactorily explained, as the of ovular based in which the have majority studies are on stages integuments already appeared (Strasburger 1872, 1879 and Warming’s extensive paper of 1878 the few This has resulted in number of are among exceptions). a tra- ditionally accepted, but controversial, morphological interpretations which are briefly summarized below (see also under “Discussion”). “ Although Mirbel (1829) was the first to introduce the term ovulum ana- Schleiden authoritative tropum”, (1837) gave an definitionof the curvature “ which has been accepted by many morphologists: Dieses Ende der Axe 2 2 [funicle] erleidet nun hdufig eine Kriimmung, so dass seine Spitze [nucellus] auf sich selbst zuriick gebogen wird... und mit dem gerade bleibenden Theil related the (raphe) verwdchst". Baillon(1876, pp. 167-8) development of the anatropous ovule to the form of the ovular primordium ("mamelon primitif") after its eitherbe somewhat curved. which, immediately inception, can erect or In the first instance the nucellus is inverted completely in relation to the funicle and the of the circular which the during, after, appearance swelling represents inner integument (“ovules reflechis" Ranunculaceae). In the anatropes , e.g., second Baillon group (e.g. Euphorbiaceae) suggests that the anatropy is caused by a unilateral development of the different ovular portions. Other authors (Strasburger 1872, 1879; Warming 1878; Goebel 1923; Netolitzky 1926) uphold the latter viewpoint, especially with regard to the outer integument. More and Bersier “ recently Bocquet emphasized that: The very early bending of the ovule beneath the chalaza is the basic phenomenon which is characteristic for anatropy" (Bocquet 1959; Bersier 1960; Bocquet& Bersier 1960). Integuments are supposed to originate at the base of the nucellarprimordium below the in the chalazal distal (Netolitzky 1926, p. 31) or nucellus, i.e., or funicular Strasburger region (see Schleiden 1837, p. 307; 1879, p. 35; Goebel 1923, p. 1734; Warming& Potter 1932, p. 242). The inner integument arises whereas as a ring-like swelling, the outher integument is usually only conspicu- ous on the side opposite the funicle (convex side of the ovule, l.s.), and... "is the side the suppressed on next to funicle..." (McLean & Ivimey-Cook 1956, p. 1391; see also Goebel 1923, pp. 1723, 1734). Or: "...fusion of the outer integu- ment and thefunicle is congenital andhistological evidence ofthis union is lacking" (Eames 1961, p. 259: see also Scheiden 1839, p. 35; Puri 1970 p. 3; Fahn 1974, 534 p. etc.). 1 In the not the of the seeds is different subfamily Phyllanthoideae only structure very but also the initial for (Corner 1976), ontogeneticprocesses displaya dissimilarity, instance, as regards the origin of the inner integument (which is dermal in Antidesma, Bischofia, Phyllanthus and Securinega). 2 Parentheses ours. 387 ANATROPY AND ONTOGENY OF THE BITEGMIC OVULE IN CHROZOPHORA described this remarkable Strasburger (1872, p. 416) phenomenon as: 2 "... er [outer integument] greift zu beiden Seiten um die Samenknospe und schliesst das Gewebe des Funiculus and later authors followed an an...”, many him. 2. MATERIALS AND METHODS Our observations are based on slides of Chrozophora obliqua and C. rottleri made earlier for a study of their life cycle (see Kapil 1956a,b). The illustrations necessitated better of (except fig. 5) are original, and were by a understanding the ontogeny of the euphorbiaceous ovule. 3. EARLY OVULE DEVELOPMENT Initial Ontogeny - The three ovular primordia arise on the apical flanks of the floral axis and consist of a numberofcharacteristic layers, referred to as derma- (1,), subdermatogen (1 ) and central core (1 ), respectively (fig. 1A, B). togen 2 3 They begin to curve immediately after inception due to a higher mitotic activity in the peripheral cell layers at the basal, abaxial side ofthe primordium (abaxial growth). As soon as the archesporial cell devides to form primary sporogenous and parietal cells, the apical dermal cells also begin to divide periclinally (fig. 1C). Simultaneous mitotic activity in subdermatogen and dermatogen is thus responsible for the prominent apical growth of the nucellus. The inner integument becomes visible as a circular swelling around the base of the nucellus, just above the curved ovular region (i.e., funicle:figs. ID, IE). the outer elevation at the side Concurrently, integument appears as an convex of the ovule. Both integuments are initiated by periclinal divisions of a sub- dermal the initial which push up dermatogen. The latter layer exhibits an increased anticlinalmitosis, mainly on the outer convex side, and leading to the vertical growth of the integumentary primordia. In the outer integument, especially, the apical dermal cells divide several times periclinally as well. In addition, a number of dermal cells of the inner funicular region, just next to the primordium of the inner integument, also starts dividing periclinally (fig. IE ; cf. Bor & Kapil 1975,fig. 3C). This suggests that not only the inner, but also the outer integument is initiated by a ring- of shaped meristem, spreading laterally from the convex side the ovule to the the inner side of funicle. However, as the meristematic activity is much more pronounced on the convex sideof theovule, the outer integument develops from the distal funicularregion as a crescent-shaped sheath. Anatropy - In order to comprehend the anatropous curvature of the ovule, successive developmental stages have been represented in serial order (fig. 2). The position of the megaspore has been considered to be inalterable, since its basal nucellar region, i.e., the zone of attachment of the inner integument 388 J. BOR and R. N. KAPIL 1 A-E - of ovule dermal initials of Fig. Development young (doi, outer integument; Up, of inner primordium integument; /,, / / central core: . nb, dermatogen; 2 subdermatogen; 3 of nucellar beak; ob, obturator; oip. primordium outer integument; pc, parietal cells; sdii. subdermal initial of inner integument; sdoi. subdermal initial of outer integument). A: Chrozophoraobliqua, I.s. ovular primordium. B, C: C. rottleri, same. D, E: C. obliqua, l.s. young ovule, initiation ofinteguments. Three zones are separated by a thicker line: dermatogen, subdermatogen and central core are distinguishablein figures A-D. ANATROPY AND ONTOGENY OF THE BITEGMIC