Evolutionary development of the seed
Seeds consist of an embryo already packaged within the seed, and nutritive tissue surrounded by a protective coat.
1. A seed contains a well-developed multicellular young plant with embryonic root, stem, and leaves already formed, whereas a plant spore is a single cell.
2. The parent plant protects and supports the young plant in the seed during its development; spores do not receive such attention.
3. A seed contains a food supply. After germination, in which the young plant begins to grow and establish itself as an independent plant, it is nourished by food stored in the seed until it becomes self-sufficient. Because a spore is a single cell, minimal food reserves exist to sustain the plant that develops from a germinating spore.
4. The seed is protected by a multicellular coat. Seeds (and spores) can survive for extended periods of time at reduced rates of metabolism and then germinate when conditions become favorable.
5. Seeds and seed plants have been intimately connected with the development of human civilization. From prehistoric times, early humans collected and used seeds for food. The food supply stored in seeds contains proteins, oils, carbohydrates, and vitamins that are nourishing for humans as well as for germinating plants.
6. Because seeds are easy to store, provided they are kept dry, humans could collect seeds during times of plenty, and save them for times of need. Few other foods can be stored as conveniently or for as long as seeds.
7. The elimination of the necessity of water to transport the sperm grom the microgametophyte to the megagametophyte for fertilization to occur.
8. This development made it possible for seed plants to complete their life cycles in relatively dry environments, compared to those of non-seed-producing vascular plants.
Seed production is an adaptation of great significance for the survival and dispersal of plants. In fact, this was part of the competititve advantage that allowed the gymnosperms to supercede the other vascular plants as the dominant type of vegetation on land. Only the later evolution of flower and fruit allowed another group of seed plants (the angiosperms) to displace the gymnosperms from their preeminent position.
Gymnosperms are seed-bearing plants that lack the combination of specialized features that characterize the flowering plants. The name gymnosperm, means naked seed. (e.g. gymnasium); that is, the seeds are not enclosed within fruits.
Gymnosperms, then, are all fruitless seed plants. And they are made up of a heterogeneous group of plants characterized by the production of naked seeds. Estimates form fossil records indicate that gymnosperms must have evolved approximately 300 million years ago from non-seed producing ancestors of the extinct division of Progymnospermophyta, which were fern-like in appearance.
1. They lack the folded, marginally-sealed carpels that characterize the flowering plants.
2. The pollen-receptive structures are the ovules rather than the stigmatic portion of the carpels.
3. Most gymnosperms lack vessels in their xylem (unlike flowering plants which have both vessels and tracheids), except for the gnetophytes, which have vessels.
Considering the relatively small number of living gymnosperms (about 720 species in 65 genera), they are remarkably diverse in their reproductive structures and leaf types.
Gymnosperms, like angiosperms (the flowering plants), differ from seedless plants (like mosses and ferns) in not requiring water for sperm to swim in to reach the egg. This means that the movement of pollen (male gamete) to ovule (female gamete) in seed plants relies on airborne transport, not water transport. Consequently, most gymnosperms produce huge amounts of pollen.
1. In gymnosperms, pollen is found (located) in stamen-like structures called strobili (various types of cones). (strobilus = singular).
2. The pollen grains of Pinus and several other genera have bladder-like wings.
3. Each male of a pine tree cone annually releases an estimated 1-2 million pollen grains.
4. Pollination in gymnosperms involves a pollination droplet that protrudes from the micropyle when pollen grains are being shed. The droplet provides a large, sticky surface that catches the normally wind-borne pollen grains of gymnosperms so that the ovule is more likely to be fertilized. After pollination the droplet evaporates and contracts, carrying the pollen grains into the pollen chamber and into contact with the ovule.
The most dramatic differences between gymnosperms and other plants involve pollen and seeds and the organs that bear them. These features differ significantly from those of comparable organs of flowering plants.
Strobilus (cone) – the reproductive structure in gymnosperms. In conifers, this consists of an ovoid, cylindrical, or spherical cluster of sporophylls (cone scales) arranged around a central axis. The place where seeds of gymnosperms are produced; essentially, an evolutionarily modified branch.
There are 4 major divisions of plants within the gymnosperms:
1. Ginkgophyta (Ginkgo: maidenhair tree),
2. Cycadophyta (Cycads),
3. Gnetophyta (Gnetophytes), and
4. Pinophyta or Coniferophyta (the conifers).
1. Ginkgo biloba, or maidenhair fern because of the resemblance of its fan-shaped leaves to those of maidenhair ferns, is the only living representative of the division. It is the oldest known genus (and species) of living trees. Fossil ginkgo leaves and wood have been discovered that date back 200 million years, and are nearly identical to those of the modern-day ginkgo.
2. It is exclusively dioecious and deciduous.
3. It has distinctive fan-shaped leaves with dichotomous venation; it is deciduous. Both attributes resemble angiosperms. However, the fleshy seed coat of Ginkgo can easily be mistaken for a fruit.
4. The seeds of Ginkgo include a massive integument (outer coating of an ovule; later becoming testa of a seed) that consists of a fleshy outer layer, a hard, stony middle layer, and an inner layer that is dry and papery.
Mature seeds have
the size and appearance of small plums, but the fleshy integument has a
nauseating odor (like a vomitorium) and irritates the
skin of some people. (Nevertheless, pickled Ginkgo seeds are a delicacy in some
b. Paired ovule, not in cones, produced at the tips of short branches on the female trees.
5. They are a popular cultivated tree, but it is apparently extinct in nature.
a. All living Ginkgo trees are descendants of plants
that were grown in temple gardens of
b. Most of the genetic diversity of Ginkgo has probably been lost in cultivation, because most nurseries propagate only cuttings of microsporangiate trees (male trees) to avoid the stinky and messy seeds.
c. Introduced to the American continent and
d. Part of its popularity has to do with its putative resistance to many types of air pollution common around industrial sites.
B. Cycads (from the Greek kykas, “a palm”). Cycadophyta
1. Very important plant group during the Triassic period, which occurred aprox. 250 million years ago, and is sometimes referred to as “the age of the cycads”.
2. In the few surviving cycads, there are about 10 genera and 100 spp., distributed primarily in the tropical and subtropical regions of the world.
There only 2 species of cycads native to the
3. All spp. of cycads are dioecious, producing microsporangial (male) cones and ovulate (female) cones at the terminal end of the trunk (which is unbranched).
4. Cycads have palm-like leaves that bear no resemblance to leaves of other living gymnosperms.
a. Under favorable conditions, cycads usually produce one crown of leaves each year.
b. The roots of some cycads may grow at the surface of the soil and contain nitrogen-fixing cyanobacteria.
c. Very slow growing; in one case, a plant reached the height of 6’ only after 1,000 years of growth.
5. Seeds of cycads are like those of Ginkgo, except the inner layer is soft instead of papery.
6. Whereas Ginkgo has rather loose microstrobili and single-seeded stalks, and deciduous simple leaves, cycads have large strobili and persistent palm-like leaves.
C. Conifers (Coniferophyta or Pinophyta)
1. The informal name of the group, conifers, signifies plants that bear cones, even though other members of the gymnosperms also include cone-bearing species.
Members of the
genus Pinus, considered typical for the Pinophyta,
fare the most abundant trees in the
a. The genus Pinus, by far the largest genus within the conifers, consists of about 100 spp.
b. Pines actually have 2 kinds of leaves. The more obvious leaf is the pine needle, a simple leaf, which is produced singly or clustered in definite numbers, called fascicles, of generally 2 to 5 needles
c. A fascicle always forms a cylinder or collar when the leaves are held together. (They are actually short shoots that are surrounded at their base by small, nonphotosynthetic, scalelike leaves that usually fall off after 1 year of growth).
d. Needle-bearing fascicles are shed a few at a time, usually every 2-5 years, so that any pine tree, while appearing evergreen, has a complete change of leaves every 5 years or so. (Bristlecone pines are an exception: their needles last an average of 25-30 years). All genera of Pinophyta are evergreen except the larches, bald cypress (Taxodium distichum), and the dawn redwood (Metasequoia glyptostroboides) which are deciduous.
e. Pines produce 2 kinds of cones (strobili).
1. Male cones, usually 1 cm or less in length, are smaller than female cones, and generally occur in dense clusters on the ends of the lower branches each spring.
2. The more familiar female cones, which are on the tree year-round, are usually located on the upper branches on the pine tree, and bear seeds after sexual reproduction has occurred. Female cones vary considerably in size among pine species: about 2” in bristlecone pine to over 2’ in sugar pine.
f. So, pines are monoecious; male and female cones are produced separately, but on the same tree.
g. Unlike other gymnosperms, in which pollination, fertilization, and seed maturation occur within the same year, the pines have an extended reproductive cycle of 14-20 months. (The period elapsed between the appearance of the ovulate cone and the maturation of pine seeds in those cones. Fertilization takes place about 1 year after pollination, and seed maturation takes several additional months). Some seeds are shed as soon as they mature, whereas others remain within the female cones for weeks, months, or even years before being shed.
h. Other members of Pinophyta in the Northern Hemisphere also have narrow leaves that have a small point at the tip, but they are not held together in a fascicle like those in Pinus. These include yews (Taxus), fir (Abies), larches (Larix), redwoods (Sequoia sempervirens), and spruces (Picea).
5. Leaves of Cypresses (Cupresses) and juniper (Juniperus) are scalelike at maturity.
6. Diversity within Pinophyta is reflected in the wide variety of reproductive structures and variations of the reproductive cycle.
a. Pines, firs, Douglas-firs, spruces, etc bear cones (stobili) of different sizes and shapes.
b. Seed cones may be absent entirely, as in the yews, which bears seeds surrounded by a fleshy, cuplike covering that is an outgrowth from the base of the seed. The seed covering is red and attracts birds, which eat them and disperse the seeds.
c. Junipers have stobili, fleshy cones, with a berry-like appearance. The fleshy scales are fused together and completely envelop the seed. Cones at maturity may be orange, red, brown, purple, or black, depending on the species. Birds eat the cones and swallow the seeds, which are then dispersed in their droppings.
Pinaceae is monoecious; Cupressaceae is monoecious or dioecious.
7. Vegetative features of Pinaceae are important in their ecological success.
a. Most species have a strongly dominant central axis and grow with a characteristic spire-like habit, which helps to maximize photosynthesis when the solar angel is low, and enables the tree to shed snow.
b. The stems, leaves, and cones have resin ducts that contain substances that seal damaged surfaces and are somewhat insect-resistant.
c. The needle-like leaves have sunken stomata and a thick epidermis to retard water loss.
d. Members of Pinaceae have small, ephemeral pollen cones and small to large woody ovulate (seed) cones.
i. A pollen cone consists of a central axis and numerous small, flat, microsporophylls, each with two microsporangia on its lower surface. Scales of pollen cones remain small and soft. They are usually produced in clusters near the tips of lower branches (ovulate cones are produced either singly or in pairs on the younger branches). Within the same species the mature pollen cones are always small, commonly less that 1” long, whereas the ovulate cones are large, measuring up to 2’ long in the sugar pine. The pollen cone develops during early spring.
ii. Ovulate cones are more complicated. The cone has a woody axis to which are attached spirally arranged cone scales (ovuliferous scales), each of which is subtended by a scale-like bract. Two ovules are attached to the upper surface of each scale. These mature in most members of the family as winged seeds that break free from the cone scale at the time the woody cone scales are dry and separate.
iii. The transfer of pollen from the pollen cone to the ovulate cone is called pollination. After the release of pollen grains, the pollen cone dries up and falls off. A pine tree produces a prodigious amount of pollen, most of which never lands on the female cones, but instead settles on any stationary object as a fine yellow powder.
iv. Ovulate cones usually remain on the tree for about 2 years.
v. Pine seeds are usually dispersed during autumn when the scales of the ovulate cones separate.
vi. Cones of some pines so not open so gracefully or easily: these cones often require a fire to cause them to open and release their seeds (serotiny). The cones of some pines explode like popcorn when heated.
e. Economically the Pinaceae is the most important gymnosperm family. Many species are important lumber trees. In much of the world the Pinaceae is the principal source of wood pulp for paper (white spruce, Picea glauca). Some are economically valuable as sources of resins (sticky, aromatic substance in resin canals of conifers), turpentine (volatile liquid used in solvents as paint thinner and varnishes), rosin (used in the manufacture of floor waxes and paper coating; on bows of stringed instruments, and baseball pitchers use it for better grip on the ball(, amber (hard, yellowish to brown translucent substance, is fossilized resin, used chiefly in jewelry),and other extracts.
i. Douglas fir and Hemlock/Douglas fir mix used in construction. Strong wood, relatively free of knots as a result of rapid growth with less branching than other conifers.
ii. Redwoods (Sequoia sempervirens) are prized for their wood, which contains substances that inhibit the growth of fungi and bacteria.
iii. Spruce wood is important to the music industry. Tracheids of spruces have spiral thickenings on the inner walls, which apparently give the wood a resonance that makes it ideal for use as soundboards in violins and guitars.
1. This group includes some of the most distinctive, and bizarre, seed plants.
2. There are 3 clearly defined, morphologically dissimilar genera of these gymnosperms: Ephedra (40 spp.), Gnetum (30 spp.), and Welwitschia (1 sp).
3. One feature they all share, in addition to being gymnosperms, is the presence of vessel elements in their wood, a cell type absent in other gymnosperms.
4. Ephedra (Mormon tea, ma huang, joint fir – the first two names referring to its use as a stimulating or medicinal tea. Ma huang is an Asian species, E. sinica, which contains chemicals that are similar to those of human neurotransmitters, like ephedrine, a drug used for the relief of allergic symptoms because of its ability to constrict blood vessels. It is also a heart stimulant and hass been sold over-the-counter in weight-c0ntrol medications and herbal energy-boosters. (In 1996 at least 15 deaths were attributed to chronic use or overdose of products containing Ephedra, as were the two athletes who recently died). Joint fir refers to the leafless appearance of Ephedra stems, which resemble the jointed stems of Equisetum, horsetail.)
a. Ephedra is not actually leafless; its leaves are small and lose their photosynthetic capability as they mature. Most photosynthesis in Ephedra occurs in green stems.
restricted to desert or semi-desert habitats, the arid southwest in the
5. Ephedra are either monoecious or dioecious.
6. Most spp of Gnetum are tropical and dioecious, either climbing vines of trees, all with broad leaves similar to those of woody dicots.
7. Welwitschia mirabilis is the sole living representative of its genus and looks more like something out of a science fiction movie than a real plant.
species is confined to the
d. Most of its moisture is derived from fog that rolls in from the ocean at night.
e. The woody stem is concave and bark-encrusted, maybe as much as 1.5 meters in diameter, and connected to a very large taproot. Much of the stem is underground.
f. Mature plants have a pair of strap-shaped leaves (up to 9’ long), which persist throughout the life of the plant. Each leaf has a meristem at its base, which constantly replaces tissue that is lost at its drier, aging tip.
g. As the plant ages, the leaves periodically split lengthwise giving the plant the appearance of having many leaves.
h. Welwitschia is dioecious, producing male and female strobili in different plants, originating from the axils (angle formed between upper side of petiole and stem) of leaves.
i. Growth of these plants is unusually slow die to the dry environment.
Gnetophytes are the most distinctive of gymnosperms because of their similarities with angiosperms.
a. They have flower-like compound strobili.
b. They have vessels in secondary xylem.
c. They have double fertilization. (However, this double fertilization does not result in the formation of endosperm. Instead, the diploid cell from fertilization by the second sperm disintegrates).