Key words. Cellulose microfibril, cell wall texture, scanning electron microscopy, transmission electron microscopy. SummaryCellulose is the most abundant biopolymer on earth, and has qualities that make it suitable for biofuel. There are new tools for the visualisation of the cellulose synthase complexes in living cells, but those do not show their product, the cellulose microfibrils (CMFs). In this study we report the characteristics of cell wall textures, i.e. the architectures of the CMFs in the wall, of root hairs of Arabidopsis thaliana, Medicago truncatula and Vicia sativa and compare the different techniques we used to study them. Root hairs of these species have a random primary cell wall deposited at the root hair tip, which covers the outside of the growing and fully grown hair. The secondary wall starts between 10 (Arabidopsis) and 40 (Vicia) μm from the hair tip and the CMFs make a small angle, Z as well as S direction, with the long axis of the root hair. CMFs are 3-4 nm wide in thin sections, indicating that single cellulose synthase complexes make them. Thin sections after extraction of cell wall matrix, leaving only the CMFs, reveal the type of wall texture and the orientation and width of CMFs, but CMF density within a lamella cannot be quantified, and CMF length is always underestimated by this technique. Field emission scanning electron microscopy and surface preparations for transmission electron microscopy reveal the type of wall texture and the orientation of individual CMFs. Only when the orientation of CMFs in subsequent deposited lamellae is different, their density per lamella can be determined. It is impossible to measure CMF length with any of the EM techniques.

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The micropylar exudate of Gasteria verrucosa (Mill.) H. Duval was studied using light and electron microscopic techniques. Ovules may contain micropylar exudate before stigma receptivity. During successive phases ofstigma receptivity, the number of ovules with micropylar exudate and the amount of micropylar exudate per ovule increases. At the late phase of stigma receptivity, large amounts of micropylar exudate with a smooth to cauliflowerlike appearance were observed. Micropylar exudate is viscous and contains, among other components, proteins and carbohydrates. At all stages of the stigma investigated, ovules situated at the base of the ovary contain a larger quantity of micropylar exudate than those at the top. The appearance of micropylar exudate is related to the degree of development of the embryo sac and it originates primarily from the filiform apparatus. It is assumed that an uptake of water by the ovule initiates the outflow of micropylar exudate from the filiform apparatus into the micropyle. Both ovular pollen tube ingrowth and seed set mark the optimum pollination stage of the stigma, which for both events lies around the onset of stigma receptivity. When pollen tubes have reached the ovary, young micropylar exudate stimulates their growth rate. The presence of micropylar exudate seems to be a requirement for pollen tube penetration, and an interaction between the pollen tube and the micropylar exudate has been proposed. Possibly, the micropylar exudate serves as a nutritional source and, in an optimum condition, as an attractant for approaching pollen tubes. I

SUMMARY An ultrastructural study on the embryo sac and the micropylar part of the nucellus, at the time of stigma receptivity, was carried out to reveal possible changes due to pollination. At the time of stigma receptivity, the development of the egg cell, central cell, antipodals and the micropylar nucellar cells of Gasteria have been completed. Only the synergids and the filiform apparatus are still differentiating. Frequently, anomalies in the morphology of the unfertilized embryo sac are observed. Comparative observations before and 2·5 h after cross‐pollination did not show ultrastructural changes in the embryo sac and in the micropylar nucellar cells. The egg cell showed polarity. The nucleus and most of the cytoplasm lay in the micropylar half, while vacuoles filled most of the chalazal half. The two synergids did not differ from each other and a clear degeneration had not taken place. Based on the observed ultrastructural differences of the filiform apparatus and the synergid cytoplasm among the investigated ovules, and data of previous studies, a sequence of differentiation of the very large filiform apparatus was defined. The nucleus and most of the cytoplasm of the central cell were positioned near the degenerating antipodals. The ultrastructure of the micropylar nucellar cells indicates a secretory function of these cells and an involvement in the process of pollen tube acceptance.

Cell organelles changes during megasporogenesis and megagametogenesis in Gasteria verrucosa (Mill.) Haw.