An Evolutionary Survey of Plants II:
The Seed Plants

Plants in the divisions Cycadophyta, Ginkgophyta, Gnetophyta, Coniferophyta, and Anthophyta have in common the ability to produce seeds. The divisions Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta (examples are Araucaria and Sequoiadendron) contain the plants commonly referred to as gymnosperms; the division Anthophyta contains the plants referred to as angiosperms which are the flowering plants. The gymnosperms and angiosperms, while they both produce seeds, differ with respect to the location of seed development, some of the processes leading to seed formation, and some of the tissues present in the vegetative plant body.

Megasporangia megagametophytes. and egg formation

In seed-producing plants the mature sporophyte produces its modified megasporangium (nucellus) enclosed in one or more layers of protective tissue (integuments) and attached to the sporophyte by a stalk (funiculus). This entire structure is called an ovule.

Meiosis of certain cells (megaspore mother cells) of the nucellus leads to the production of haploid megaspores enclosed within the ovule. These spores are never dispersed from the ovule but rather remain enclosed in its protective integuments and attached to the parent sporophyte. Within each ovule a single megagamatophyte develops from one or more of the megaspores. Each megagametophyte eventually produces one or more egg cells; in gymnosperms these are contained in archegonia, in angiosperms they are not.The eggs remain within the ovule until their fusion with sperm to produce diploid zygotes.

Microsporangia, microgametophytes. and sperm formation

The sporophytes of seed-producing plants must also develop microsporangia. Sporogenous tissue within each microsporangium matures into microspore mother cells which eventually undergo meiosis to produce haploid microspores. The development of microgametophytes from these microspores usually begins while the microspores are still held within the microsporangium attached to the adult sporophyte plant. The microgametophytes are very small (consisting of only a few cells) and develop enclosed within the microspore wall, which simultaneously becomes thicker.

A pollen grain is a partially formed microgametophyte, enclosed in a modified microspore wall, at the time it is released from the microsporangium. To continue the process of reproduction, pollen grains must be transferred to the vicinity of a mature, egg-containing, ovule. This process is called pollination and occurs by a variety of mechanisms - the pollen grains can be carried by wind, or insects, or birds, etc.

Following pollination, the pollen grain germinates by producing an elongate protrusion called a pollen tube which grows into the ovule and extends toward the egg cell. At the same time, microgametophyte development continues, resulting in the formation of sperm which travel through the pollen tube to reach an egg.

Fertilization and seed production

When a sperm fuses with an egg cell within an ovule, a diploid zygote is produced. Each zygote undergoes mitosis leading to the formation of an embryo (young sporophyte). At this point, the integuments harden to form a protective coat around the embryo and a seed is formed. The seed consists of the hardened integuments, which are diploid tissues of the parent sporophyte and are now called a seed coat; some remaining fragments of the nucellus and the haploid megagametophyte; the diploid embryo which is an offspring sporophyte; and possibly some nutritive tissues produced along with the embryo.

Mature seeds are detached from the parent sporophyte and dispersed. Under the appropriate environmental conditions they will eventually germinate to produce a new adult sporophyte.

Gymnosperms versus Angiosperms

The life history of pine, 2 (a gymnosperm) can be compared to the life history of lily, 2 (a angiosperm).
A model used for angiosperm development is Arabidopsis. This can be viewed here and here.

A major difference between gymnosperms and angiosperms is the placement of the microsporangia and ovules on the parent sporophyte. In gymnosperms they are borne on microsporophylls and megasporophylls. The sporophylls often occur in clusters called strobili, or cones. Clusters of microsporophylls form microstrobili (staminate cones); clusters of megasporophylls form megastrobili (ovulate cones).

In angiosperms, microsporangia and ovules are borne within flowers. Flower structure varies among species, but often consists of the following components:

1. a peduncle or flower stalk

2. a receptacle, to which other floral parts are attached

3. one or more pistils; each consisting of a pollen-receptive stigma, a style, and an ovary within which ovules are found

4. stamens; each consisting of a filament, and a lobed anther - each lobe of the anther contains a microsporangium (pollen sac)

5. colorful petals, that may serve as attractants for pollinators

6. small, leaf-like sepals

The pistils and stamens may, in fact, be modifications of microsporophylls and megasporophylls.

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