Anatropy and ontogeny of the bitegmic ovule in Chrozophora A.H.L. Jussieu (Euphorbiaceae)

A study of the initial ontogeny and curvature of the ovule in Chrozophora obliqua 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 dermatogen of the outer integumentary primordium and funicle and the synchronous elongation of both funicle (viz. the raphe) and outer integument (antiraphe) strongly support the idea that this integument and the raphe are derivatives of the funicle. The primordium of the inner integument, on the other hand, appears to be an independent rim-like 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. Accordingly, the term “nucellus” is to be defined as that part of the ovule which is completely surrounded by the inner integument. Both the integuments are initiated by periclinal divisions of a subdermal initial and further divisions result in the vertical growth of the primordia. The outer integument becomes multilayered as a result of characteristic radial divisions in the subdermal layer, and a vascular bundle differentiates at its periphery. The large, curved nucellar beak is principally of dermal origin. A hypostase differentiates below the embryo sac during prefertilization stages, whereas the development of a nucellar podium takes place only after fertilization. The caruncle consists mainly of an aerenchymatous tissue. The diagnostic value of a number of distinctive ovular features in Euphorbiaceae has been discussed; the general occurrence of a nucellar beak and especially a thick, subdermally derived and vascularised inner integument in the taxa constituting the subfamily Crotonoideae s.s. seem to indicate that this is a homogeneous group.