Microsporogenesis and Development of Seed in Lobelia cardinalis

Abstract

1. Each theca of the anther of Lobelia cardinalis contains many microspore mother cells. The tapetal cells are first uninucleate, later becoming binucleate. 2. Cell plates are formed on the cytoplasmic strands midway between the nuclei of the tetrad. 3. The generative cell divides to form two male gamete cells before the pollen grain leaves the anther. 4. Each carpel of the two-loculed pistil contains many ovules, which are anatropous at maturity. 5. Prior to the formation of the single integument, a single archesporial cell is found in the hypodermal layer of the ovule and functions as the megaspore mother cell. 6. Seven pairs of chromosomes are present at meiosis. 7. A linear row of four megaspores is formed, and the larger spore toward the chalazal end of the ovule is the functional megaspore which develops into an eight-nucleate seven-celled megagametophyte; the other megaspores disintegrate. The cells of the nucellus toward the micropylar end of the ovule disintegrate upon division of the nucleus of the megaspore. 8. A vascular strand is differentiated early in the development of the ovule and extends part way into the base of the funiculus. Conductive tissue extends from the vascular strand to the nutritive tissue at the chalazal end of the megagametophyte. 9. Double fertilization occurs. At metaphase in the first division of the endosperm nucleus the chromosomes of the endosperm nucleus and those of the male gamete nucleus move independently toward the mid-plane of the spindle. A four-celled endosperm is formed prior to division of the zygote nucleus. 10. The synergids and the antipodals function as chalazal and micropylar haustoria, respectively. The synergids become greatly enlarged and digest the cells of the integument about the micropyle. The nucellar cells at the chalazal end of the megagametophyte are digested by the antipodals, which later digest the cells of the adjoining nutritive tissue.