Up to this point all of the terrestrial plant life cycles discussed have had a major feature in common: homospory. Here, the sporophyte produces a single type of sporangium, which in turn produces a single type of spore mother cell. Each SMC undergoes meiosis, resulting in 4 haploid spores. All spores thus produced are capable of growing into mature, multicellular gametophytes. These gametophytes may produce both antheridia and archegonia.An alternative scheme is seen in:

(1) all seed plants
(2) a few ferns
(3) two extant genera of Lycophyta (Selaginella and Isoetes)

In these plants, a sporophyte produces two types of sporangia: micro- and megasporangia. The development and products of these sporangia are similar to those of homosporous plants, but with some important differences. Microsporangia contain microspore mother cells, each of which undergoes meiosis, forming microspores. Microspores grow into microgametophytes that possess antheridia. Megasporangia contain megaspore mother cells, each of which undergoes meiosis, forming megaspores. Megaspores grow into megagametophytes with archegonia.

(A) Homospory vs. Heterospory

The principal difference between homospory and heterospory is the separation of sexes at different points in the life cycle. Nevertheless, the evolutionary implications of this difference are profound. For example, seed plants, which have female gametes in ovules and male gametes in pollen grains, must show some differentiation of male and female gametophytes. Furthermore, an earlier (in the life cycle) separation of the sexes can be regarded as a division of labor, a common theme of evolutionary advancement. It may also be that heterospory promotes outcrossing among plants, since microspores may drift farther from the sporophyte than megaspores.

Examine the living specimens of Selaginella in lab. At the distal portions of the branches you can see strobili (sing. = strobilus) that contain two types of sporangia (Rust, fig. 30a; V & C fig. 6.36, 6.38). (The Lycophytes are of additional evolutionary significance in that they are the "most primitive" extant land plant that bear sporangia in protective structures). If both types of sporangia are present, they will be distinguished by their relative size (which is which? micro- and mega-).

Examine longitudinal sections of Selaginella strobili from prepared slides (see left and below). It will be obvious which structures are megasporangia and which are microsporangia. Can you discern any structures inside the sporangia? What are these structures? What is their immediate destiny?

In addition to being a) the most "primitive" extant heterosporous genus, and b) a member of the most "primitive" strobilate group, Selaginella shows another important evolutionary step. When megaspores germinate on the ground, the resulting megagametophyte does not leave the megaspore; rather, it is retained throughout its development within the megasporangial wall. This endosporic development is considered to be a significant evolutionary trend toward further protection (from what?) of the megagametophyte. This trend is carried much farther in seed plants, where megagametophytes are retained inside ovules.

Examine a prepared slide of a section of a Selaginella megagametophyte, noting the megaspore wall as well as the megagametophyte itself. Are archegonia present?

Examine the array of heterosporous ferns on display, acquainting yourself with the diversity of heterosporous non-seed plants.

Questions

1. What is a probable reason why bryopsids have never had representatives of any great height?

2. How might self-fertilization be prevented in moss plants which produce both antheridia and archegonia in the same tip?

3. Contrast the method of dispersal of gametes and spores in both mosses and ferns.

4. Is there an advantage of the gametangia in ferns being on the lower surface of the gametophyte?

5. Is there be any advantage to the gametangia being more exposed, as in the bryophytes?

6. Is there any advantage in having the archegonia and antheridia mature at different times on the same fern gametophyte? Explain.

7. What evolutionary advances do ferns have over the bryophytes? In terms of gamete movement and spore production and dispersal only?