|Reproductive adaptations in Conifers (Pinus)
As an example of the gymnosperm system of reproduction, you will examine the genus Pinus, the pine. You should be aware, however, that pines are among the more advanced of all conifers and that, in most aspects, the conifers are considered an advanced lineage of gymnosperms. The conifers are the most numerous gymnosperms today. Familiar examples are pine, spruce, fir, juniper, cedar, cypress, and redwood. The sporophytes are trees or shrubs. This particular characteristic is not really an advancement over many of the lower or primitive vascular plants because they too evolved many tree-forms that dominated the Carboniferous forests. The great difference in success is related more to other changes that you will investigate in the next two labs. The most significant of these are:
1) a reduction in the gametophyte stage and its retention in/on the sporophyte
Gymnosperms (in fact all seed plants) are heterosporous and produce separate male and female gametophytes. You will begin by examining the sporangia that produce separate microspores and megaspores, then follow the development of the gametophytes, and finally examine the process by which male gametes are carried to female gametes.
|(A) The Male or Staminate Strobilus
(Rust, fig. 35; 36 a-c; C & V fig. 6.81)
Observe fresh or frozen male cones from the sporophyte. Note that this structure is composed of a number of spirally arranged sporangia-bearing scales arranged on a central axis. The scales are modified leaves (i.e. homologous to leaves) that bear sporangia (microsporophylls). Carefully remove one of these units and locate the sporangium. There are two on each scale. On which side of the sporophyll are they attached? These sporangia are microsporangia and produce microspores. In pines and other higher plants the spores are not released. Instead, they remain in the microsporangium and develop there into the microgametophyte.
Examine prepared slides of staminate cones and also make wet-mounts of the fresh microsporophylls after teasing the sporangia apart. Locate the sporangia on the undersurface (abaxial surface) of the microsporophyll. The sporangia are filled with pollen. Pollen grains consist of a resistant wall with a few-celled microgametophyte inside. Pollen in pines also has two air bladders on the sides of the microgametophyte. What might be the purpose of these bladders? Examine the stages in the development of the microgametophyte in the following figure which shows the steps starting at the microspore mother cell (microsporocyte; pollen mother cell) and ending with the mature male gametophyte with sperm.
The microspore develops into the microgametophyte by dividing into two prothallial cells and one antheridial cell. The prothallial cells are representative of the thallus that occurred in the ancestors of the lower vascular plants. The prothallial cells tend to degenerate in pine micro gametophytes. The antheridial cell is the sole remnant of an antheridium. The antheridial cell divides to form a generative cell and a tube cell. In this two-cell stage, the pollen is shed. If the pollen grain lands on the appropriate place on a female cone, it germinates and a pollen tube forms. The generative cell divides three times, eventually ending up with two non-flagellated sperm. (Note that other extant gymnosperms, including cycads and Ginkgo have ciliated sperm that remain within the pollen tube).
Transport of the male gametophyte in the pollen grain to the micropyle in the ovule is called pollination (more about which in Lab 13). Obviously, pollination is unique to pollen-producing groups, the only extant members of which are the gymnosperms and angiosperms. One consequence of pollination is that the sperm cells are never exposed to the external environment. Pollen is wind-dispersed in gymnosperms, and is produced in such large quantities that it often covers all outside surfaces during springtime.
|(1) The Ovule (Rust, fig. 37)
Ovules are complex structures that contain the megasporangium (also called the nucellus) and megaspore mother cells. After meiosis, megaspores are produced, only one of which survives to grow into a multicellular megagametophyte (the other three degenerate in situ). Megagametophytes contain archegonia, which in turn contain the female gametes (= egg cells). The entire ovule is surrounded by a vascularized protective layer called the integument or seed coat. Thus, by the time fertilization occurs, there is a series of "layers" constituting the seed.
|(a) Megaspore mother cell
Study a prepared slide labeled "megaspore mother cell" and answer the following:
|(b) Mature gametophyte with archegonia
Study the prepared slide showing a mature gametophyte with archegonia and answer the following:
|(B) The Female or Ovuliferous Strobilus
(Rust, fig. 35a; 36 d-e; C & V fig. 6.79-6.80)
First examine preserved young (first-spring) cones. When alive, these cones are relatively soft and green or reddish. Observe the demonstration material showing several stages of development of the female cone. Also set up are a variety of cones showing the diversity of pines native to California.