Lilies

Female gametophyte development in lilies takes a different course. When the megaspore mother cell divides to produce four megaspores, all of the megaspores remain functional and reside within the embryo sac. Three megaspores then migrate to the upper end, where they fuse to produce a 3n nucleus. The remaining megaspore rests at the lower end of the sac. The remaining two nuclei undergo two mitotic divisions. The first of the divisions increases the number of nuclei in the embryo sac to four, and the second division increases the number to eight. Four of the nuclei are triploid, and four are haploid. These nuclei then position themselves as described earlier: three at the upper end (the antipodal nuclei), three at the lower end (an egg cell and two synergids), and two in the center (the polar nuclei). Of the two polar nuclei, one is triploid and one is haploid. Double fertilization as earlier described then occurs. One sperm unites with the egg to produce the zygote, a one-celled sporophyte, and the other sperm unites with the polar nuclei to produce a 5n endosperm nucleus.

Endosperm Nucleus

The Embryo Plantlet has Consumed the Endosperm

Endosperm Nuclei

Embryo Plantlet

Cotyledons

Cell Membranes Form Around the Endosperm Nuclei

Nucellus

Embryo Plantlet

Nucellus

Cotyledons o

Meiosis Produces Four Megaspores o

Megaspore Mother Cell

Megaspore Division

Triploid Nuclei

Haploid Nuclei

Nuclear Eight Nuclei Two Polar

Division are Produced Nuclei

Triploid Nucleus o

Two Nuclei Formed o

Megaspore Mother Cell

Triploid Nucleus o

Two Nuclei Formed

Triploid Nuclei

Haploid Nuclei

Nuclear Eight Nuclei Two Polar

Division are Produced Nuclei o

Meiosis Produces Four Megaspores

Megaspores Migrate

Figure 28-7 Female gametophyte development in a lily, (a) Megaspore mother cell undergoes meiosis to produce four megaspores, (b). All four megaspores remain functional, (c) Three migrate to the upper end of the embryo sac, one to the lower end. (d) The three at the upper end fuse, forming a triploid nucleus. There are then two nuclei, (e) They divide twice, producing eight nuclei, four of which are triploid, and four haploid. (f) One of each cluster of four nuclei migrates toward the center of the embryo sac to form polar nuclei (g).

Was this article helpful?

0 0

Post a comment