Flower Whorls and Femaleness

Flower Structure (review)

The flower is basically a strobilus. Here is a generic flower:

The carpel is basically a megasporophyll. It is folded around the ovules to form a hollow chamber, the locule, with walls, the ovary. Since pollen no longer has access to the micropyle of the ovules, it must be deposited on the stigma. The stigma is sticky to hold and promote pollen tube germination. The pollen tube must grow through the style to reach the locule (chamber in the ovary). This growth is fueled by digestion of the female tissues, and by special transfer cells that line the tubular style in many species. Thus, the microgametophyte is dependent upon the female sporophyte tissues for nutrition and logistics. Ultimately the pollen tube finds the micropyle of an ovule, probably by chemotropism. The chemo-attractant may be some combination of Calcium ions, Boron ions, and growth hormones (perhaps auxins).

Meanwhile, in the ovary, the female development occurs. The megasporophyll has the ovule attached to its upper (inner) surface. The site of connection is called the placenta. Xylem and phloem connections cross from the megasporophyll (carpel wall) into the ovule by means of a stalk called the funiculus. Thus the megagametophyte stage will be supported nutritionally by the sporophyte. The table has been turned!

The ovule contains the megasporangium (nucellus) which, in turn, contains one megasporocyte. This diploid cell undergoes meiosis to produce the four haploid megaspores. Typically three of these degenerate, leaving one functional megaspore. The megaspore is not shed and remains in the megasporangium. From here the development varies among the species; what is described next is the "common" pathway--one of several possible developmental sequences found in angiosperms. The megaspore undergoes mitosis to produce 8 haploid nuclei in a free-nuclear megagametophyte. Cytokinesis partitions these eight nuclei into seven cells...the mature megagametophyte.

At the chalazal end (opposite the foot=funiculus) of the megagametophyte are three antipodal cells. These are all that remain of the once-dominant gametophyte thallus! There is the central cell with two nuclei (one from each pole, the so-called polar nuclei). This cell will join with a sperm to make the polyploid endosperm (an autapomorphy of angiosperms). At the micropylar end of the ovule are three cells: the egg cell and two synergids. The egg cell will be joined by a sperm cell to form the zygote. The synergids will enzymatically digest the end of the arriving pollen tube to assist in sperm release. It is possible that the synergids are all that remain of the archegonium sterile jacket cells.

This pathway for ovule development is outlined as follows:

Color Key
Male nuclei are shown in cyan, the endosperm is shown in yellow.

It is important to note that the details of the embryo sac (the megagametophyte) vary among species of angiosperms. The "Polygonum" type described above applies to the majority of angiosperm species but there are many other patterns. One would study these in a graduate course on angiosperm anatomy.

Syngamy in angiosperms is double, another autapomorphy. One sperm joins with the egg to form the diploid zygote, the other sperm joins with the two polar nuclei in the central cell to form the polyploid endosperm.

The endosperm develops rapidly. Its polyploid genetic status apparently gives it an amazing ability to sequester nutrients from the maternal sporophyte. Sugars are polymerized into starch, amino acids into protein, and acetyl-CoA is maneuvered into lipids. These nutrients will be used later by the developing embryo. Just after karyogamy, the newly formed endosperm cell repeated divides mitotically to form a free-nuclear endosperm cell. This cell can be unbelievably large...the vacuole and liquid cytoplasm of this cell form what we know of as "coconut milk" in the seed of that species of palm for example. If you have had a piña colada, you have drunk liquid endosperm. Later, cytokinesis divides the endosperm into a cellular mass. Indeed the "coconut meat" is solid endosperm that we grate and put on baked goods for example. Solid endosperm of other species (corn, wheat, rye, barley, etc.) is typically ground into flour, mixed into a dough, and then baked to form bread, crackers, pie crust, cake, bagels, etc.

While the endosperm garners nutrients from the maternal sporophyte, the zygote formed by the union of egg and sperm develops. This development involves mitosis and cytokinesis and later morphogenesis. The zygote divides to make a chain of cells called the suspensor. The bulbous cell at the micropylar end of the suspensor is usually connected by a uniseriate filament of cells to a tiny cell at the chalazal end. (Obviously more examples of unequal cytokinesis!) This tiny terminal cell will become the embryo proper. It is pushed into the liquid endosperm region by growth of the suspensor. Is it possible that the suspensor is the evolutionary relic of the haustorial foot in young sporophytes? The suspensor is genetically identical to the embryo proper.

The terminal cell divides mitotically to form a globular embryo...a spherical cluster of a few tiny cells. Polarity is established and the end distal to the suspensor develops two lobes (at least in dicots!) which become leaf primordia and are called cotyledons when mature. The proximal portion of the embryo attached to the suspensor differentiates into a root apex. This stage of development is called the heart stage. The embryo then elongates into what is referred to as the torpedo stage...obviously a vocabulary term from war years in the 20th century. Torpedo means fusiform at one end and with two fin-like projections at the other. This embryo may elongate enough to need to bend within the confines of the ovule, and its development may use up all available endosperm as well. The nutrients in that case are transferred to the embryo...usually stored as reserves in the cotyledons. Thus mature seeds may lack endosperm.

It is important to note that as soon as the included embryo becomes dormant that the name ovule is changed to seed. The seed consists of a seed coat (the ovule integument), storage tissue (endosperm or cotyledon depending on species), and a dormant embryo. This change forces another. The carpel is renamed fruit. As you recall the gymnosperms were noted for their naked seeds. The megasporophyll had a seed on its exposed surface. In angiosperms, because the megasporophyll encloses the seed in a locule, the surrounding ovary wall tissue is the fruit. To disperse seeds then, the carpel must open in some way to shed the seeds within, or it must rely upon an animal to open the carpel to disperse and release the seeds. There will be more on that later. For now, suffice it to say that the fruit is thus an autapomorphic feature of angiosperms. Because this feature is shared by all angiosperms then it is synapomorphic among them.