Gymnosperms and Vascular Plants Help (page 2)
Introduction to Gymnosperms and Vascular Plants
The great majority of vascular plants (other than ferns and their fern-like relatives) reproduce by means of seeds. One important group are the gymnosperms ( JIM -nuh- sperms ) or vascular plants with “naked” ( gymno -) “seeds” ( sperms ). By naked, it is meant that the seeds of the gymnosperms are exposed, and not enclosed within fruits.
As the Ancient World’s climate became cooler and drier, many of the vast fern swamps of the Carboniferous Period died and dried up. Arising in their place were the conifers ( KAHN -uh- fers ) or “cone-bearers.” The conifers, as their name suggests, produce cones that hold their naked seeds. Most of the conifers are evergreen trees or shrubs that bear cones. These conifers are truly “ever-green,” because they keep their green, needle-like leaves all year. The thin leaves are an adaptation to cold, dry conditions in that their needle shape gives them a small surface area exposed to the air, thus reducing evaporation or drying of the conifer plant.
The conifers include pine trees, fir trees, spruce, larch, hemlock, and yew bushes. Today, much of the still-natural land of the Northern Hemisphere is covered by beautiful coniferous (kuh- NIF -er-us) forests, graced with tall spruce, fir, and pine trees.
The Life Cycle Of A Pine Tree
Since pine trees and other conifers are called gymnosperms, it is important to examine their typical life cycle, wherein their “naked seeds” play an important role. Each of the major steps are described below. (Consult Figure 9.4 for a summary.)
Step 1 A healthy mature sporophyte (pine tree)
An entire pine tree, itself, is technically called a mature sporophyte ( SPOR -uh- fight ). It is literally a “spore” ( sporo -) or seed “plant” ( phyte ) that has matured. The same individual pine tree has two different sexes of cones hanging from its branches – small male cones, and not far away, big female cones. The small male cones are called pollen ( PAHL -un) cones , while the much larger female cones are termed the seed cones . (It is the large female seed cones that we usually notice on the tree branches.)
Both types of cones produce spores for reproduction. Hence, when the male fertilizes the female, an interaction between their spores eventually results in a seed, which grows into a mature pine tree or sporophyte.
Step 2 Meiosis and spore production in both male and female cones
Pine cones produce spores by the process called meiosis (my- OH -sis). Meiosis means “a condition of lessening,” wherein one cell divides into two daughter cells, each containing only half the number of chromosomes found within the original parent cell.
The small, male pollen cones consist of many loose, oval scales fastened together. Within each scale, meiosis produces tiny microspores ( MY -kroh-spores). The microspores, each containing half the number of chromosomes of the parent tree, soon develop into gametes (sex cells). The male gametes become pollen granules – a “fine flour” ( pollen ) of tiny, yellow-colored grains.
The large, female seed cones consist of many tough, pear-shaped scales arranged into a spiral shape. The scales are woody and tilted outward at an angle. Each of these female scales holds two ovules ( OH -vyools) or “little eggs.” Each of these ovules (little eggs) produces a megaspore ( MEH -gah-spore) – a “large” ( mega -) spore.
Step 3 Pollination (followed by fertilization) of female seed cones
When the male cones have matured, their scales pop open and release a cloud of pollen (millions of microscopic, yellowish grains). During pollination ( pahl-uh - NAY -shun), a pollen grain lands on a female cone and enters one of its ovules. After pollination, meiosis is finally triggered within the ovule, and the megaspores eventually develop into female gametes, the egg cells. Fertilization finally occurs when the pollen grain from the male grows out a tiny tube and releases a sperm cell into an egg cell.
Step 4 Zygote, embryo, and seed formation
The result of fertilization is a zygote. The zygote undergoes repeated mitosis, adds many cells, and becomes an embryo. The original ovule within the female cone eventually develops into a pine seed, which contains the embryo. The seed provides continuing nourishment for the developing pine tree embryo, and it is covered by a tough seed coat. But the seed is still “naked” in the sense that it is not buried within a fruit. Finally, the seed falls to the ground, germinates (sprouts) under favorable conditions, and grows into a pine tree.
Other Types Of Gymnosperms
Although conifers are the most common and widespread type of gymnosperms, there are several other varieties of seedless vascular plants. These are the cycads ( SIGH -kads), ginkgos ( GING -kohs), and gnetophytes ( KNEE -toh- fights ). A cycad is literally “a kind of palm.” Cycads are not really palm trees, at all. Rather, they are a group of palm-like tropical plants with very long, fern-shaped leaves. About 100 species of cycads still survive. The group grew heavily during the Mesozoic Era, when they shared the mostly warm, still-tropical planet with the dinosaurs. Cycad trees bear cones but, unlike pines, some individual trees hold only male cones, while other trees hold female cones only.
The word ginkgo comes from the Chinese for “silver apricot.” The ginkgo genus is very ancient, but now there is only one surviving species. Like cycads, ginkgos are separated into either male trees or female trees. The male trees have cones, while the female ginkgo tree, as its name suggests, produces soft, fleshy seeds that look like apricots. However, these apricot-like seeds smell like rancid butter! The ginkgo tree has delicate, fan-shaped leaves that give it the nickname “maidenhair tree.” Paleontologists and botanists have found fossils of ginkgos that are at least 200 million years old, yet are almost identical to the modern plant. Hence, the ginkgo (silver apricot, maidenhair tree) has sometimes been called a “living fossil.” Ginkgo trees are not found in the wild. They have been grown domestically in China since antiquity, and now grace yards and streets in many cities worldwide as a popular ornamental tree.
Gnetophytes are a group of vines, shrubs, and small trees growing only in the tropics or in desert regions. Gnetophytes, like all gymnosperms, reproduce by means of spores and cones. One of the strangest gnetophytes is the Welwitschia (well- WIT -she-uh) plant. It is named after Dr. Friedrich Welwitsch (1806–1872), who first brought the odd specimen to Great Britain. The Welwitschia is only found in the deserts of Southwest Africa, where it appears as just two giant, strap-like leaves growing from the ground! Most of the huge, carrot-shaped stem is hidden below the ground surface, and it can grow to over 1 meter (3 feet) in diameter.
As a group, the gnetophytes are often treated as an evolutionary link between the conifers and the true flowering plants. The reason is that many types of gnetophytes bear cones (like conifers), but at the same time, the cones somewhat resemble flowers.
Today on Education.com
- Coats and Car Seats: A Lethal Combination?
- Kindergarten Sight Words List
- Child Development Theories
- Signs Your Child Might Have Asperger's Syndrome
- 10 Fun Activities for Children with Autism
- Social Cognitive Theory
- Why is Play Important? Social and Emotional Development, Physical Development, Creative Development
- GED Math Practice Test 1
- The Homework Debate
- Problems With Standardized Testing