The Female gametophyte in two Kenyan genera of podostemaceae

Abstract

Critical observations regarding the female gametophyte ontogeny of Podostemaceae have been made (Battaglia, 1987) warranting investigations to be clarified. Hence the present study in the two genera, Tristicha and Inversodicraea, growing naturally in Kenya.

The plant materials collected were fixed in Formalin-Acetic-alchohol (FAA) and preserved in 70 ethnol. Fixed plant specimens were identified under a binocular stereomicroscope. They were, T.trifaria, 1. bifurcata and 1. Keniensis.

Individual flower buds of all stages were dehydrated in the ethanol-xylol series and imbedded in paraffin wax of 52 melting points. Serial microtome sections were cut at 7-12um thicknesses. Sections were stained in Heidehnain's iron alum-hematoxylin and counterstained with erythrosin or fast green in clove oil (Johansen, 1940). DPX mountant was used to prepare permantent slides.

Photographs showing the habit and habitat of the specxies were taken in the field, using Minolta Camera attached with zoom and macro lens. Camera attached with zoom and macro lens. Camera lucida drawings of the female gametophytee stages were drawn at table level using a 'Leitz' monocular microscope, at different magnifications.

The gynoecium is tricarpellary in T. trifaria but bicarpellary in the two investigated species of Inversodicraea. The former taxon has a trilocular ovary while in the latter taxa it is bilocular. The ovary possesses several anatropous, tenuinucellate and bitegmic ovules on the axile placenta; the outer integument organizes the micropyle. The micropylar dyad cell degenerates regularly and doesn't participate in subsequent stages of ontogeny. This process of female gametophyte development, corroborate the concept of selective callose deposition in cells walls and shift in nutrient supply during megasporogenesis. The primary chalazal nucleus degenerates, without further division. This is the 'Strike' phenomenon. The rationale of classifying the female gametophyte ontogenies on the functional aspect of the establishment of polarity at the primary two-nucleate stage alone is not suffice; other determinant factors are significant. The micropylar megaspore nucleus undergoes two successive mitotic divisions and four nuclei are produced. The organized female gametophyte consists of two pear-shaped synergids, a large central egg cell and a polar cell. The polar cell degenerates before the pollen tube enters the female synergids. The female gametophyte development is of the monosporic category, Apinagia type-form B. The ontogeny does not reveal the presence of antipodal cells. Syngamy occurs. The second male gamete is never discharged but remains in the pollen tube. Previous reports of double fertilization are not confirmed. Tension force between the fast growing integument relative to the nucellar layer a role in pseudo-embryo sac formation. The cell walls of nucellar cells fail to resist the acropetal net tension force of the inner integument, disorganize, break and release 'naked' protoplasts in the cavity formed. The role of lytic enzymes is pointed out. The organized cavity is filled with multinucleate protoplasmic mass and the entire structure formed is the pseudo-embryo sac. The term 'Nucellar Plasmodium' for pseudo-embryo sac, considering the organization and ontogeny, is acceptable. The nucellar plasmodium is formed before fertilization in I. bifurcata and I. keniensis but organizes during post-fertilization period in T. trifaria.