Vascular Seed Plants

The mobile sexual reproductive part of seed plants is the multi-cellular seed. Seeds are an evolutionary success story. Plants with seeds have a greater chance of reproductive success than seedless plants. Inside the tough, protective outer coat of a seed is an embryo and a nutrient supply. When conditions are too hot or too cold, or too wet or too dry, the seed remains inactive. When conditions favor growth, the seed sprouts, or germinates—that is, the embryo begins to grow into a young plant, called a seedling.

There are two main groups of seed-bearing vascular plants, gymnosperms and angiosperms. The four phyla of gymnosperms produce naked seeds, which means the seeds are not enclosed and protected in fruits. Most gymnosperms are evergreen and bear their seeds in cones. A cone is a reproductive structure composed of hard scales. The seeds lie open on the surface of the scales. The one phylum of angiosperms produces seeds that are enclosed and protected in fruits. Angiosperms are commonly referred to as flowering plants. Cones serve some of the same functions for gym-nosperms that flowers serve for angiosperms.

figure 28-12

Phylum Cycadophyta

Cycads (SIE-KADZ), such as the one shown in Figure 28-12, are gym-nosperms of the phylum Cycadophyta (sie-kad-AHF-uh-tuh). Although cycads flourished during the age of the dinosaurs, only about 100 species survive today. Most are native to the Tropics and grow slowly. Some cycads live for almost a thousand years. Many are endangered because of habitat loss, overcollection, and their slow growth. Most cycads have fernlike, leathery leaves at the top of a short, thick trunk. Cycad plants are either male or female, and they bear large cones. Cycads are mostly used as ornamental plants.

Phylum Ginkgophyta

Like cycads, ginkgoes (GING-kohz) flourished during the time of the dinosaurs. The only species existing today is Ginkgo biloba, which is native to China. It is called a living fossil because it closely resembles fossil ginkgoes that are 125 million years old. The ginkgo tree has fan-shaped leaves that fall from the tree at the end of each growing season—an unusual characteristic for a gymnosperm. Trees that lose their leaves at the end of the growing season, like the ginkgo, are called deciduous. Most gymnosperms are evergreens and retain their leaves year-round.

Ginkgoes are tolerant of air pollution, making them good plants for urban settings. Ginkgo seeds are considered a delicacy in China and Japan. Notice the plum-shaped, fleshy seeds on the ginkgo shown in Figure 28-13. They are often mistakenly called berries or fruits.

Phylum Coniferophyta

The conifers (KAHN-uh-fuhrz), which are gymnosperms of the phylum Coniferophyta (kahn-uh-fuhr-AHF-uh-tuh), include pine, cedar, redwood, fir, spruce, juniper, cypress, and bald cypress trees. They are important sources of wood, paper, turpentine, ornamental plants, and Christmas trees. Juniper seeds can be used to flavor food. Amber is yellow or brownish yellow fossilized resin that once flowed from ancient conifers. Prehistoric insects are often preserved in amber.

The cycad is a gymnosperm that looks like a palm or fern. Cycads can sometimes grow to 18 m (60 ft) in height.

Quick Lab

Examining Ferns

Materials disposable gloves, lab apron, potted fern, hand lens, water

Procedure

1. Put on your disposable gloves and lab apron.

2. Choose a frond of the fern, and examine its underside for the structures that contain spores.

3. Wash the soil from the underground structures. Examine the fern's horizontal stems and rhizome.

Analysis How do ferns differ from nonvascular plants? What enables ferns to surpass nonvascular plants in height and size? From what part of the fern do the fronds grow?

figure 28-13

The ginkgo, Ginkgo biloba, has large seeds and unusual fan-shaped leaves. This gymnosperm tree can reach heights of 24 m (80 ft).

figure 28-13

The ginkgo, Ginkgo biloba, has large seeds and unusual fan-shaped leaves. This gymnosperm tree can reach heights of 24 m (80 ft).

(a) Fir needles and cones

(b) Pine needles and cones figure 28-14

(c) Yew needles and seeds

(a) Fir needles and cones

(b) Pine needles and cones

(c) Yew needles and seeds figure 28-14

The needles and cones of conifers come in many shapes and sizes. (a) The fir tree displays its female cones. Its needle-shaped leaves grow evenly all around the branch. (b) The pine tree shows its small male and larger female cones. Some pines reach heights of 60 m (200 ft). (c) The seed of the yew tree is surrounded by a red covering that looks like a berry. Its leaves are flat, pointed needles that are dark green on top and pale green underneath.

Conifers are woody plants, and most have needle or scalelike leaves, as shown in Figure 28-14. A conifer usually bears both male and female cones. Small male cones typically grow in clusters. Male cones release clouds of dustlike pollen, and then the cones fall from the branches. The pollen falls or blows into the larger female cones, where the egg cells are attached to the scales of the cone. After pollination, the female cone closes up tightly. This protects the developing seeds, which mature after one or two years. The mature seeds are released when the female cone opens.

Redwoods and giant sequoia trees provide a majestic forest setting along the West Coast of the United States. These conifers are the Earth's tallest and most massive living organisms. The tallest living coastal redwood, Sequoia sempervirens, is about 110 m (360 ft) tall, the height of a 30-story building. The most massive tree is a giant sequoia, Sequoiadendron giganteum, estimated to weigh 5,600 mega-grams (6,200 tons).

Phylum Gnetophyta

Gnetophytes (NEE-tuh-FIETS), an odd group of cone-bearing gym-nosperms, have vascular systems that more closely resemble those of angiosperms. As Figure 28-15 shows, Ephedra (ih-FED-ruh) is a genus of desert shrubs with jointed stems that look like horsetails. It is the source of the drug ephedrine, which can be used as a decongestant.

figure 28-15

Ephedra viridis, called Mormon tea, grows on the rim of the Grand Canyon. This highly branched shrub has small, scalelike leaves. It is the source of the drug ephedrine and can be brewed to make a tea.

Figure 28-16 shows the unique Welwitschia mirabilis plant. The plant's stem is only a few centimeters tall but can grow to 1 m (3.3 ft) in diameter. Two leaves elongate from their base on the stem and then become tattered and split lengthwise by the wind. A mature leaf may be nearly 1 m (3.3 ft) wide and 3 m (10 ft) long. Welwitschia grows in the Namib Desert of southwestern Africa. The Namib Desert lies near the Atlantic Ocean, so a thick night fog often rolls in over the desert. Welwitschia apparently gets most of its water from the dew that condenses from the fog.

Phylum Anthophyta

Anthophyta (an-THAHF-uh-tuh), the largest phylum of plants, includes over 240,000 species of flowering plants. Angiosperms, or the flowering plants, are seed plants characterized by the presence of a flower and fruit. Botanists define a fruit as a ripened ovary that surrounds the seeds of angiosperms. The ovary is the female part of the flower that encloses the egg(s).

Angiosperms grow in many forms and occupy diverse habitats. Some are herbaceous plants with showy flowers, such as violets and impatiens. Others, such as rose bushes, are shrubs. Some angiosperms are vines, such as grape plants. Oak, aspen, and birch trees are all flowering plants that have woody stems, although you may never have noticed their small flowers. Grasses are also angiosperms, but you must look closely to see their small, highly modified flowers. The world's largest flower, which can grow to 1 m (3.3 ft) in diameter, is shown in Figure 28-17.

figure 28-16

Welwitschia mirabilis has a short, wide stem and twisting leaves. The female plants of this unusual gymnosperm bear large seed cones that are reddish in color.

figure 28-16

Welwitschia mirabilis has a short, wide stem and twisting leaves. The female plants of this unusual gymnosperm bear large seed cones that are reddish in color.

Was this article helpful?

0 0
Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook


Post a comment