STRUCTURAL ASPECTS OF IN-VITRO POLLEN-TUBE GROWTH AND MICROPYLAR PENETRATION IN GASTERIA-VERRUCOSA (MILL) H DUVAL AND LILIUM-LONGIFLORUM THUNB

In vitro penetration of the micropyle of freshly isolated Gasteria ver rucosa ovules by pollen tube was monitored on agar medium. 40-60% of t he micropyles were penetrated, comparable with in vivo penetration per centages. When germinated on agar, Gasteria pollen tube elongation las ts for up to 8 h while plasma streaming continues for about 20-24 h. T he generative cell divides between 7 and 20 h after germination, and a fter 20 h the pollen tube arrives at one of the synergids. The sperm c ells arrive after 22 h. The whole process takes more time in vitro tha n in vivo. In fast growing pollen tubes, a pulsed telescope-like growt h pattern of tube elongation is observed. The formation of pollen tube wall material precedes tube elongation and probably prevents regular enlargement of the pollen tube tip-zone. Rapid stretching of the new p ollen tube wall material follows, probably due to gradually increased osmotic pressure and the use of lateral wall material below the tip. T he stretching ceases when the supplies of plasma membrane and excretab le wall material are exhausted. Multiple pollen tube penetration of th e micropyle occurs in vitro as it does in vivo. Most pollen tube growt h ceases within the micropyle but, if it continues, the pollen tubes c url. Inside the micropyle the pollen tube shows haustorial growth. At the ultrastructural level, the wall thickening of in vitro pollen tube s is quite similar to that in vivo. Before transfer of pollen tube cyt oplasm a small tube penetrates one of the synergids. Sperm nuclei with condensed chromatin are observed in the vivo prometaphase nuclei are found in the most chalazal part of a synergid, against the egg cell nu cleus and nucleus of the central cell at a later stage. Using media fo r Lilium ovule culture, Gasteria ovules were kept alive for at least 6 weeks. Swelling of the ovule depends on pollen tube penetration. The conditions for fertilization to occur after in vitro ovular pollinatio n seem to be present.