POLLEN-TUBE PENETRATION AND FERTILIZATION IN LILIUM-LONGIFLORUM (LILIACEAE)

The ultrastructure of the embryo sac, nucellus, and parts of the micro pyle of Lilium longiflorum were studied both before and after pollen t ube penetration to examine the interactions between ovule and pollen t ube, using transmission electron microscopy and light microscopy. Befo re pollen tube penetration the egg cell and two synergids are similar. No filiform apparatus was detected and no synergid degeneration occur s prior to pollen tube penetration. The polar nuclei do not fuse until fertilization. No differences in embryo sac ultrastructure were detec ted between pollinated ovules unpenetrated by pollen tubes and unpolli nated flowers of a comparable age. Shortly after the discharge of the pollen tube two enucleated cytoplasmic bodies with different ribosome densities were observed in the degenerated cytoplasm. These structures border both on the central cell and the egg cell as well as each othe r and are interpreted as remains of sperm cytoplasm after transmission of sperm nuclei. In the central cell both the sperm nucleus and the p olar nuclei are associated with endoplasmic reticulum (ER). ER is thou ght to be a transport mechanism to achieve contact between the haploid polar nuclei and the sperm nucleus. In the egg cell sperm nucleus ali gnment is not visibly achieved by ER. The persistent cells of the egg apparatus and the central cell appear to become more metabolically act ive after pollen tube penetration. Pollen tube penetration already occ urs despite the absence of a filiform apparatus and a low level of dif ferences between the cells of the egg apparatus.