Botany Photo of the Day

Today's photo of the day features a photograph taken by me, with a write-up from Connor (Connor neglected to mention the sweet fragrance of this as-yet-unidentified species, so I'll throw that in). Connor writes:

Species of Illicium can be found in both the New and Old World. Native species can be found in both southeast Asia and southeastern North America. The Illiciaceae are contained within the Austrobaileyales, one of the basal angiosperm lineages. The sister group to the Austrobaileyales includes the monocots, eumagnoliids, and eudicots.

In The four-celled female gametophyte of Illicium (Illiciaceae; Austrobaileyales): implications for understanding the origin and early evolution of monocots, eumagnoliids, and eudicots, Williams and Friedman trace the evolution of the female gametophyte through various angiosperm lineages.

The most common female gametophyte, or embryo-sac, in angiosperms is the Polygonum-type. It consists of two synergid cells which attract the growing pollen tubes, and an egg at the micropylar end. At the chalazal end three antipodal cells are present which are thought to function in embryo-sac nutrition. At the centre of the Polygonum-type embryo-sac are two polar nuclei which can fuse to become a binucleate central cell and the triploid (3n) endoserpm upon fertilization.

By examining the four-celled gametophyte found in Illicium, Williams and Friedman hypothesized that this type (also found in another basal angiosperm lineage, Nympheales) is an ancestral feature in the angiosperms. The Polygonum-type and other seven-celled / eight-nucleate embryo-sacs are the product of several modifications to this ancestral type which included the addition of the three antipodal cells and the binucleate central cell. This embryo-sac is a derived character of the monocots, eumagnoliids, and eudicots. Interestingly, the authors note that a seven-celled / eight-nucleate female gametophyte is also found in the most basal angiosperm clade, Amborellaceae. However it does not function in the same way as the similar embryo-sacs found in the higher angiosperms and is thought to be a separate derivation of the seven-celled/ eight-nucleate female gametophyte.

In Developmental evolution of the sexual process in ancient flowering plant lineages (PDF), the same authors provide a thorough description of each embryo-sac development mentioned above along with diagrams.