P.J. Robbertse

Effect of boron on directionality of pollen tube growth in Petunia and Agapanthus.

The effect of boron on pollen tube growth was tested using Petunia Juss. styles and the semivivo technique, while Agapanthus L’Herit. pollen was used for in vitro germination experiments. Petunia pollen tubes protruded only from the cut ends of styles incubated in media containing boron. When styles were incised between the cut end and the pollen tube front and either the cut end or the incision was exposed to a boron-containing medium while the other wounded area was exposed to a boron-free medium, the direction of pollen tube growth was changed. Pollen tubes protruded only from those wounds exposed to a boron-containing medium. When a boron gradient was created on agar strips and Agapanthus pollen was germinated in vitro alternatively on either the end containing a low or a high boron concentration, pollen tubes consistantly grew towards the higher boron concentrations. This is the first demonstration of a possible chemotropic response of pollen tubes to boron.

A re-evaluation of the structure of the mango ovule in comparison with those of a few other Anacardiaceae species

The mango ovule has been described by some authors as unitegmic and by others as bitegmic, but in the latter case with integuments fusing quite early. The results of this investigation suggest that it is a pachychalazal ovule in that the greater part of the ovular envelope consists of chalazal tissue and that only the micropyle is formed by one massive integument. The pachychalazal ovule could possibly be a characteristic of the tribe Anacardieae. Bitegmic ovules in which the outer integument is continuous with two labellate outgrowths of the massive funicle (‘raphe-chalazal ovules’) are apparently characteristic of the tribes Spondieae and Rhoideae. All previously investigated species as well as Sclerocarya birrea (A. Rich.) Hochst. subsp. caffra (Sond.) Kokwaro and Lannea discolor (Sond.) Engl. were found to be chalazogamous.

The influence of temperature and boron on pollen germination in Mangifera indica L.

Fruit set in the mango (Mangifera indica L.) cultivar ‘Haden’ is exceptionally low. In an effort to determine the reason for this phenomenon, the influences of temperature and application of boron on pollen germination and the effectiveness of cross- versus self-pollination were investigated. It was found that the highest percentage of successfully pollinated stigmas (stigmas with germinating pollen grains) was obtained when: (i) pollination occurred at 25°C and 30CC, (ii) stigmas and pollen were treated with boron and (iii) ‘Haden’ stigmas were pollinated with ‘Zill’ pollen rather than being self-pollinated.

A preliminary study on the germination of Sclerocarya birrea subsp. caffra

Seeds of Sclerocarya birrea (A. Rich.) Hochst. subsp. caffra (Sond.) Kokwaro, commonly known as marula, occur in locules within a hard lignified endocarp. The endocarp appears to restrict germination mainly by offering mechanical resistance. It may also restrict the leaching of germination inhibitors and possibly serve as a barrier to oxygen diffusion. The endocarp does not restrict water uptake. High germination counts were only obtained by removing opercula. Leaching slightly improved germination of seeds in intact endocarps. The optimum germination temperature of opercula-removed seeds was between 27° and 37°C. Storage improved rate of germination.

The structure of the seed of Mangifera indica L. and notes on seed characters of the tribe Mangifereae (Anacardiaceae)

The anatropous, unitegmic and pachychalazal mango ovule develops into the campylotropous, pachychalazal seed. The undifferentiated seed coat is of dual origin, developing from the integument as well as from the pachychalaza. The term ‘peritesta’ is suggested for the peripheral, band-like integumentary part of the seed coat. The major saddle-shaped chalazal part is associated with a tanniniferous hypostase. The discussion includes the anatomy and histochemistry of the seed. The pachychalazal seed with undifferentiated seed coat probably characterizes the tribe Mangifereae. After consideration inter alia of seed characters, it is concluded that, according to generally accepted criteria, the Anacardiaceae is phylogenetically one of the more advanced forest families.

The genus Eugenia (Myrtaceae) in southern Africa: Structure and taxonomic value of wood

The anatomy of 60 samples of mature bark representing 12 southern African species of Eugenia s.str. was studied. Bark surface patterns are correlated with internal structure and the taxonomic significance of the bark features is explored. Two bark categories are distinguished. Bark type X [E. capensis (Eckl. & Zeyh.) Sond., E. natalitia Sond., E. simii Duemmer and E. umtamvunensis Van Wyk] is characterized principally by a smooth or weakly dippled-scaly surface, abundant dilatation tissue (pseudocortex) in the inner bark, phellem with one type of phelloid cell and a well-defined phelloderm. Bark anatomy was found to be useful to separate species. Bark type Y [E. erythrophylla Strey, E. verdoorniae Van Wyk, E. woodii Duemmer, E. zeyheri (Harv.) Harv., E. zuluensis Duemmer and three undescribed species] is characterized mainly by a flaky surface, little dilatation growth in the inner bark, phellem with two types of phelloids in the phellem and the lack of a distinct phelloderm. With the exception of E. zuluensis these species cannot be distinguished on the basis of bark features. Bark characters confirm a previously proposed distinction between two supraspecific groups among native species of Eugenia. For diagnostic purposes bark structure is more useful than wood. It is suggested that some of the differences between the two main bark types can be interpreted in terms of the relative rates of formation of xylem and phloem at the vascular cambium.

Effect of temperature on the growth of Lachenalia cv. Ronina during the bulb preparation phase

Lachenalia cultivars have excellent characteristics as flowering pot plants. The effect of three temperature regimes on the growth of small bulbs of Lachenalia cv. Ronina to flowering size (± 7 g) was studied. Potted bulbs were grown in three temperature controlled cabinets with temperature regimes chosen to represent a cold, a moderate and a warm winter. Plants were destructively harvested to investigate environmental effects on plant morphology and additionally flowering, anthesis and flower senescence were monitored. Under all three temperature regimes bulb growth followed a typical sigmoidal curve. Root growth does not support optimal bulb size under the different temperature regimes. Leaf growth in plants under the moderate temperature regime was more vigorous and foliage had a healthier appearance than in the low and high temperature regime treatments. Inflorescence emergence was earlier under the low and moderate temperature regime. At the end of the growing season, bulbs grown in the high temperature treatment formed secondary inflorescences. The best temperature regimes for Lachenalia bulb production were the low and moderate temperature regime which represented the cool and moderate winter climate in South Africa.

Intra-ovarian trichomes in Bequaertiodendron magalismontanum: location, origin, structure and possible function in the reproductive process

Bifurcate, unicellular epidermal trichomes are characteristic of the Sapotaceae, but have only been reported on external surfaces of aerial plant organs. In Bequaertiodendron magalismontanum (Sond.) Heine & J.H. Hemsl. such trichomes were found inside the pistils of mature female and morphologically hermaphroditic flowers and inside young fruits. It is suggested that the intra-ovarian trichomes serve as bridges between stylar and ovarian transmitting tissue to facilitate the passing of highly selected pollen tubes across the interlocular compital space. In origin, structure and proposed function these thick-walled, intra-ovarian trichomes conform to characteristics typically assigned to obturators.

Aspects relating to seed production in Gloriosa superba L.

Open- and hand pollination, ovule morphology, seed structure and germination of Gloriosa superba L. are discussed as possible factors influencing reproduction. The protogynous flowers in the frondose raceme are psychophilous and ornithophilous with a stigma receptivity period of at least four days. Development of the femaie gametophyte conforms to the Polygonum type, The inner integument acts as an endothelium due to the absence of the nucellus on the lateral sides of the embryo sac, where the nucellus has degenerated. A well-developed funicular obturator directs pollen tubes to a nucellar epistase where a partial self-incompatibility reaction appears to occur. A hypostase on the chalazal side contains transfer cells connecting the vascular tissue with the embryo sac. Seeds are tegmic, surrounded by a sarcotesta showing adaptations for endozoochory. Seed production was slightly better after cross-pollination, and also when 4B hour-old pollen was used, especially when pollination was done in the second flower phase. When the three factors were combined and applied to the proximal flowers in the inflorescence, seed production was significantly improved.

Pappea capensis: monoecious, dioecious or androdioecious?

Pappea capensis used to be regarded a dioecious plant. In this short communication, evidence is provided which shows that Pappea capensis is monoecious with a distinct male phase followed by a distinct female phase.