The Structure and Reproduction of Gymnosperms
The Body Form and Reproductive Patterns of Cone-Bearing Plants
Aug 16, 2009
Gymnosperms are vascular seed plants that display very prominent and large bodies, mainly in evergreen tree form. During the ancient past gymnosperms dominated the landscape with their size and abundance. However, in modern times, gymnosperms are represented by less than one thousand species.
The Structure of Gymnosperm Types
Coniferophyta (Conifers) The basic body plan of conifer is that of an evergreen tree. Evergreen in the fact that most of these trees do not drop all their leaves (or in this case, needles) at once in the fall and grow them all back at once in the spring as do deciduous trees. Larches and bald cypress trees are an exception, however.
Conifers hold the record for the most massive and oldest living thing.
Cycadophyta (Cycads) The cycads range in size from short shrubs to moderate-sized long-lived trees and are native to tropical and subtropical regions. Zamia pumila, found in the sandy woods of Florida, is the only cycad native to the United States. Zamia has a stem that is most or entirely underground. As a consequence of its relative small stature, the reproductive cones of this cycad touch or lay flat on the ground.
The leaves of cycads characteristically occur in a cluster at the top of the stem; thus they resemble palms (and are often, but mistakenly, called “sago palms”). Cycads are highly toxic, with both neurotoxins (nerve poisons) and carcinogenic compounds (cancer-causing chemicals) present.
Ginkgophyta (Ginkgo) The Ginkgo biloba tree (or the maidenhair tree as it is sometimes known) is the only living species of this entire division. It is a moderate-sized tree with unique fan-shaped leaves that is often referred to as “a living fossil.” There are no known wild ginkgo trees and as a species it was thought extinct by Western botanists until it came to light that the plant was considered sacred and had been cultivated for centuries in Chinese temple gardens. The name Ginkgo is Chinese and means “silver apricot.”
Gnetophyta (Gnetophytes) The gnetales are a small assorted group that contains what are certainly the most unusual of the gymnosperms. They range in body form from shrubs to trees to climbing vines with large leathery leaves.
The desert shrub, Ephedra, is the only economically important member of this group. It produces the alkaloid ephedrine, which is useful in the treatment of bronchial asthma, sinusitis, hay fever, and the common cold.
Gymnosperm Reproduction Patterns
Since the reproductive process is essentially the same in all gymnosperms, the life cycle of a pine can be used as a model for the seed production process in other gymnosperm types.
The adult sporophyte forms male pollen cones and female egg cones. These cones may be on the same plant or on separate plants. Seed plants undergo sexual reproduction and do have gender. That pine tree in the park is a male or a female (dioecious) or possibly both at the same time (monoecious) depending on the species.
Within the male cone, structures called microsporangia produce microspore mother cells which give rise to microspores. These in turn mature into pollen grains (male gametes or sex cells).
In the female cone, structures called megasporangia produce megaspore mother cells which give rise to megaspores. These in turn mature into eggs (female gametes or sex cells). Female cones are woodier than male cones and female cones are the most visible and familiar to people.
This is due to the fact that female cones are larger than the male cones and they may hang on the tree for several years at least. The smaller, inconspicuous male cones usually drop off soon after shedding their pollen. The female cones of the sugar pine in California and Oregon are among the world’s largest measuring up to 45 cm (18 inches or 1.5 feet).
For the development of a seed, pollination must first occur. Pollination is the transferring of pollen grains from a male to a female plant reproductive structure. In gymnosperms, pollination occurs in the spring when wind-blown pollen grains fall on the open scales of the female cone. The pollen becomes trapped in a sticky fluid near the micropyle (opening leading into the ovule or egg chamber).
After pollination occurs the female cone closes and pollen grains are drawn into the ovule. The pollen grain then forms a tube that grows down into the ovule toward the egg. When the pollen tube touches the egg, sperm cells in the tube fuse with and fertilize the egg.
When the seeds resulting from this fertilization are mature, the scales of the female cone open and the seeds are dropped and dispersed. In most pines, this process takes over two years. However, cones of the jack pine will lay tightly closed on the forest floor for years until they are exposed to fire (and thus are known as “fire pines”).
While some gymnosperms are vines and shrubs with one species producing fleshy cones, the majority of gymnosperms are evergreen trees that produce seeds within hard cones.
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