Gymnosperms

Background

As land-dwelling plants developed further past ferns, several modifications were developed for water conservation. These included:

1. Gametophytes became even more reduced in size, often staying within the parent sporophyte plant.
2. Swimming sperm (which requires water for dispersal) was replaced by dispersal of the whole male gametophyte (pollen) by wind or insects. Pollen (poll, pollen = fine flour) is the male gametophyte, NOT sperm! However, pollen contains sperm or at least sperm nuclei. pollination is the transfer of male gametophytes to the female plant. This still must be followed by fertilization of the egg by a sperm nucleus which is produced by the male gametophyte.
3. Seeds came into being. In general, a seed consists of an embryo sporophyte which has grown from the one-celled zygote, specially packaged “food” to aid in the embryo’s early growth and development, and a seed coat (often derived from structures of the parent sporophyte plant) which is modified for protection and/or dispersal of the seed.
   Two categories of seed-bearing plants are recognized: the Gymnosperms (gymno = naked; sperma = seed), in which the seeds develop on the surface of the reproductive structures (thus also called the “naked-seed plants”) such as the cones in pine, and the Angiosperms (angio = vessel, receptacle, container), in which seeds develop within a specialized structure, called an ovary, on the adult sporophyte (also called the “flowering plants”).

Types of Gymnosperms

• Cycads have leaves that look somewhat like palms, but eventually the plant gets a big cone in the center for reproduction.
  
• Ginkgo trees are a very primitive type of tree that were first found high in the mountains of China, all but extinct. Now, thanks to horticultural intervention, they can be found in cultivation world-wide in temperate zone climates. Ginkgoes are dioecious (di = two; oecious, eco = house), that is, they come in male and female plants. While the males make fairly inconspicuous reproductive structures, the females make “fruits,” which when ripe, fall to the ground and smell like you just stepped in something a dog left behind. Because of this, female ginkgo trees are not very popular with home owners, and many nurseries sell “artificially” propagated male trees. Ginkgo leaves are fan-shaped, thick and leathery with parallel veins, and turn bright yellow in autumn.
• Members of Division (=Phylum) Coniferophyta (coni, konos = cone; fer = bear, carry; phyta = plant) are usually called the “conifers”. These include things like pines, firs, spruces, yews, junipers, cedars, cypress, and redwoods. Many conifers are evergreen, that is they retain their leaves in winter. The leaves are needle-shaped to conserve moisture, an adaptation for life in drier, more windy areas than ferns can tolerate. Some kinds can be very old and/or large: in California, a number of redwoods are over 100 m tall, there is a sequoia with a trunk diameter of 26 m, and there is a bristlecone pine which is over 4600 years old.
  

Pine is the most commonly-studied example of a gymnosperm.

In pine as in other vascular plants, the sporophyte is the dominant generation. It consists of an underground network of roots supporting an upright stem, the trunk, which bears whorls of branches. The needles (leaves) typically occur in clusters of two or more. The sporophyte bears two types of reproductive structures referred to as male and female cones. Each cone is considered to be a modified branch with a number of modified leaves, called scales or sporophylls. Each sporophyll bears a structure called a sporangium in which the spores are produced. Unlike the ferns, in gymnosperms the spores as well as the gametes come in two sizes: separate microspores and megaspores (micro = small; mega = large, great) are produced in separate sporangia and develop into male and female gametophytes, respectively.

The “male” cones typically are found in clusters at the tips of lower, side branches, and usually take several years to develop. In these cones, the modified leaves are called microsporophylls. Each microsporophyll bears a microsporangium in which the microspores are produced. Still inside the microsporangium, each microspore divides and “grows” to form a four-celled (four nuclei, anyway) male gametophyte, also known as pollen which contains two sperm nuclei. A grain of pine pollen also has two large air sacs to make it buoyant in the wind, and these give the pollen a “Mickey Mouse hat” appearance.

Pollination is the transfer of the whole male gametophyte to the female plant. In pine, the pollen is blown by wind. Note that this is NOT the equivalent of fertilization — that must still occur later!

The female cones typically form higher up in the tree, and also usually take several years to develop. Their modified leaves/scales are called megasporophylls and produce megaspores. Each scale (megasporophyll) has two areas (megasporangia or ovules) where megaspores can develop. Each ovule has a micropyle (pyle = gate, orifice), a small hole in the near end of the ovule wall so the sperm can enter.

When a pollen grain lodges between the “stem” of the female cone and a megasporophyll/ovule, we say that pollination has occurred. Then, to accomplish fertilization, a tube from the pollen (called a pollen tube) grows into the micropyle, then into the female structures. This stimulates production of the actual megaspore by the female. The megaspore develops/“grows” into a female gametophyte and (like other female gametophytes) produces a few archegonia, each with an egg in it. The pollen forms two sperm nuclei which travel up the pollen tube toward the archegonia. Eventually one of these sperm nuclei fertilizes one of the eggs in one of the archegonia in each of the ovules. The other sperm nucleus and all other eggs and archegonia in that female gametophyte disintegrate so one zygote per ovule is left.

From the zygote, cells divide and grow into an embryo sporophyte (2n). This forms in the middle of a female gametophyte (1n) which serves as its first food. Typically, parts of the old megasporophyll from the parent pinecone (2n) form a protective seed coat and the “wing” for dispersal.

When the seed is mature or ripe, it separates from the female cone and is carried by the wind to a new location. When the seed germinates, the embryo continues to grow into a new pine tree. Initially, it uses the nutrients provided by the female gametophyte until its first leaves are above ground and large enough to do photosynthesis.