9.9: Seed Plants
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How old can a plant be?
This is obviously a seed plant. It is a Gingko tree, which is an unique species in that there are no close living relatives. Gingkoes can live for a very long time. Some specimens of this species are thought to be over 2,500 years old. The Ginkgo is also known as a living fossil, with fossils related to modern Ginkgo from the Permian period, dating back 270 million years.
Seed plants are called spermatophytes. The evolution of seeds by vascular plants was a very big deal. In fact, it was arguably as important as the evolution of vascular tissues. Seeds solved the problem of releasing offspring into a dry world. Once seeds evolved, vascular seed plants and their descendants diversified to fill terrestrial niches everywhere. Today, vascular seed plants dominate Earth.
Parts of a Seed
As shown in Figure below, a seed consists of at least three basic parts: the embryo, seed coat, and stored food.
- The embryo develops from a fertilized egg. While still inside the seed, the embryo forms its first leaf (cotyledon) and starts to develop a stem (hypocotyl) and root (radicle).
- The tough seed coat protects the embryo and keeps it from drying out until conditions are favorable for germination.
- The stored food in a seed is called endosperm. It nourishes the embryo until it can start making food on its own.
A typical plant seed, like this avocado seed, contains an embryo, seed coat, and endosperm. How does each part contribute to the successful development of the new plant?
Many seeds have additional structures that help them disperse. Some examples are shown in Figure below. Structures may help them travel in the wind or stick to animals. Dispersal of seeds away from parent plants helps reduce competition with the parents and increases the chance of offspring surviving.
Dandelion seeds have tiny “parachutes.” Maple seeds have “wings” that act like little gliders. Burdock seeds are covered with tiny hooks that cling to animal fur.
Classification of Seed Plants
The two major types of seed plants are the gymnosperms (seeds in cones) and angiosperms(seeds in ovaries of flowers). Figure below shows how the seeds of gymnosperms and angiosperms differ. Do you see the main difference between the two seeds? The angiosperm seed is surrounded by an ovary.
In gymnosperms, a seed develops on the scale of a cone. Only an angiosperm seed develops inside an ovary.
There are only about 1,000 living species of gymnosperms, whereas there are hundreds of thousands of living species of angiosperms. Living gymnosperms are typically classified in the divisions described in the Table below. Most modern gymnosperms are trees with woody trunks. The majority are conifers such as pine trees.
|Ginkgoes||There is only one living species (Ginkgo biloba); some living trees are more than 2000 years old; they originated in Asia but now are cultivated all over the world; they have been used for medicines for thousands of years.|
|Conifers||There are more than 700 living species; most are trees such as pines with needle-like leaves; they are often the dominant plants in their habitats; they are valuable to humans for paper and timber.|
|Cycads||There are about 300 living species; they are typically trees with stout trunks and fern-like leaves; they live only in tropical and subtropical climates; they have large, brightly-colored seed cones to attract animal pollinators.|
|Gnetae||There are fewer than 100 living species; most are woody vines with evergreen leaves; they live mainly in tropical climates; they are the least well known gymnosperms but the most similar to angiosperms.|
Evolution of Seed Plants
The earliest seed plants probably evolved close to 300 million years ago. They were similar to modern ginkgoes and reproduced with pollen and seeds in cones. Early seed plants quickly came to dominate forests during the Mesozoic Era, or Age of the Dinosaurs, about 250 to 65 million years ago.
As seed plants continued to evolve, Earth’s overall climate became drier, so early seed plants evolved adaptations to help them live with low levels of water. Some also evolved adaptations to cold. They had woody trunks and needle-like, evergreen leaves covered with a thick coating of waxy cuticle to reduce water loss. Some of the trees were huge, like today’s giant sequoia, a modern conifer (see Figure below).
The person standing at the foot of this giant sequoia shows just how enormous the tree is. Some early seed plants also grew very large.
Eventually, some gymnosperms started to evolve angiosperm-like traits. For example, cycad ancestors were the first plants to use insects as pollinators. They also used birds and monkeys to disperse their brightly colored seeds. Of modern gymnosperms, Gnetae probably share the most recent common ancestor with angiosperms. Among other similarities, Gnetae produce nectar, a sweet, sugary liquid that attracts insect pollinators. Most modern flowering plants also produce nectar.
- Most vascular plants are seed plants, or spermatophytes. They reproduce with seeds and pollen.
- Some modern seed plants are gymnosperms that produce seeds in cones.
- Identify the parts of a seed and the role of each part.
- Name and describe the divisions of gymnosperms.