Plant Kingdom Project

Introduction

A plant is defined as a multicellular organism with cell walls that are made of cellulose, and it develops from multicellular embryos. Plants are very interesting organisms and make up an entire kingdom in the Eukarya Domain: Kingdom Plantae. The first land plants evolved from an organism similar to multicellular green algae. These early plants adapted to dry environments, causing several groups to evolve. However, all plants need sunlight, water, minerals, gas exchange, and a transport system in order to survive. Plants have evolved many adaptations in order to meet these select needs in the dry environment on land. They are highly important to life because they are the base of all land food chains and they provide shade, shelter, and oxygen for land animals. They are autophototrophs meaning they make their own food and get energy from the sunlight. Plants use green pigments called chlorophyll a and b to make their own food.

Plant life cycles have two phases called alternation of generations. The first phase is the diploid phase; this phase is known as the sporophyte, or spore-producing plant. The sporophyte forms haploid spores which then grow into haploid gametophytes. This phase alternates with the second phase, the haploid phase, which is also known as the gametophyte, or the gamete-producing plant. In this phase the plant forms gametes, the male and female reproductive cells. During fertilization the male and female gametes fuse together and produce a new sporophyte.

Today plants are split into four basic groups: Bryophytes, Pterophytes, Gymnosperms, and Angiosperms. Each group is slightly similar to another, and each group is determined based upon adaptations such as water-conducted tissue, seeds, and flowers.

Bryophytes are the earliest land plants and have been around for at least 400 million years. Compared to other plants they are very simplistic, and have the least adaptations to land. Their life cycles depend on water to reproduce. Therefore, they usually only grow in wet habitats. If they are in really wet, they can even grow as epiphytes which are plants that grow on other plants. They do not have vascular tissue or other specialized tissues to conduct water and nutrients. They also do not have true leaves, stems, or roots. Instead of roots they have rhizoids, which just anchor the bryophytes to the ground; this is why they grow where the soil quality is poor. Some examples of Bryophytes include mosses (bryophyte), hornworts (Marchantiophyte), and liverworts (Anthocerotophyte). The most commonly found bryophytes are mosses.

In the life cycles of bryophytes, the gametophyte is the dominant stage. It is the only plant with a dominant gametophyte. The sporophyte is also parasitic on the gametophyte; it grows from the embryo which it being held in the female sex organ. However bryophytes can produce sexually or asexually. They have two structures that produce reproductive cells. One structure is called the antheridia which produces sperm cells. The other structure, the archegonia, produces the egg cells. Water is essential to reproducing. During reproduction the sperm cells swim trough the water to fertilize the eggs. After the sperm cells swim through the water to get to the egg cell, the egg is fertilized and is a diploid zygote. This zygote then grows in to a sporophyte which will produce spores. The spores are carried off by will and water and land in another moist habitat. There it will grow into the haploid gametophyte plant. An example of this gametophyte plant would be what we know as moss.

Like most land plants, bryophytes have multicellular sex organs and the gametes are protected by a coating of other cells. Also like other land plants, the zygote is formed in the female sex organ and develops into an embryo. Byrophytes are very important to the environment. They absorb water, which allows them to provide humidity over dry periods, or prevent flooding. Bryophytes also have many important adaptive features to different types of climate, habitat, and surfaces. There are types of bryophytes that have adapted to on growing on rocks, acidic surfaces, or tree bark. There are also types of bryophytes that can thrive in polluted areas, while some can only thrive in unpolluted areas. Some bryophytes even have adaptive features that allow them to grow in areas with less water, as opposed to more delicate moss that grows in very wet areas.

Pterophyte, also known as seedless vascular plants, are really the first true land plants. As opposed to the bryophytes, they have new adaptations. Some adaptations they have are true roots, leaves, and stems. The roots help the plant absorb water and minerals from the ground. The leaves allow the plant to make food through photosynthesis. The stem connects the leaves and roots and is the support system of the plant. However, the most important adaptation they have is vascular tissue. Vascular tissues are special tissues that carry water and nutrients throughout a plant. There are two types of vascular tissues. The two types are phloem, which carries nutrients and food from place to place within the plants, and xylem, which moves water to all parts of the plant from the roots.

In the life cycle of pterophyte, the diploid sporophyte is the dominant stage. Just like the bryophytes, the sporophyte makes haploid spores. In this type of plants, the spores are in structures called sporangia. The sporangia are usually in clusters called sori. They are located on the underside of the fronds which are large leaves with many divisions on it. When the spores are ready, or ripe, they burst from the sporangia. They are then carried by wind and water which is similar to the bryophyte spores. These spores will land, and grow into haploid gametophytes in the right conditions. Pterophytes also have the reproductive organs antheridia and archegonia; they are located on the underside of the gametophyte. Once the organs are mature, the flagellated sperm from the antheridia will swim to the egg in the archegonia to fertilize the eggs. Since the sperm swim to the eggs, they need water to fertilize as well as the bryophytes.

Some examples of seedless vascular plants are ferns, club mosses, and horsetails. Out of the three the ferns are the most common. Ferns have underground creeping stems called rhizomes as well as strong roots. Ferns have fronds which are coiled in bud forming to make “fiddleheads”.

Gymnosperms are oldest surviving plants two types of seed plants, the other being angiosperms. Gymnosperms include conifers, gnetophytes, cycads, and ginkgoes. One of the biggest adaptations of the gymnosperms is they reproduce by producing seeds instead of spores. Gymnosperms are cone-bearing plants meaning they produce seeds directly on the surface of their cones. The seeds that are made by the Gymnosperms are protected by a seed coat. Parts of their stems, when they become old, turn woody which is another distinction between seedless vascular plants.

Gymnosperms also have an alteration of generations and produce a sporophyte and gametophyte. The sporophyte is the dominant stage of the life cycle. The gametophytes are actually made up of only a few cells and grow and mature within the cones. One of the great adaptations that the gymnosperms has is it doesn’t need water to reproduce. It reproduces using seeds not spores, and seeds don’t need water during reproduction. Due to this great adaptation, gymnosperms can live in almost any habitat.

The adaptations that let seed plants reproduce without water are the production of cones, the transfer of sperm by pollination, and then the protection of embryos in the seed.

To reproduce, or produce a seed, the first step is pollination. During this process, the male gametophyte, which fits in to a tiny pollen grain is carried by wind, water, and animals to the female gametophyte, the cone. Once the male and female gametophytes join together, the female egg is fertilized. After fertilization, the zygote grows into a tiny plant called the embryo. When conditions are sufficient, the embryo grows by using a stored food supply inside the seed. The seed coat surrounds the embryo and keeps it from drying out. The purpose of the seed is to protect the zygote of seed plants, and allow it to grow when conditions are right so it has the best chance of surviving. Seeds can be very tolerant of different types of environments such as extreme heat, cold, or drought.

Angiosperms are the most adapted and advanced of all land plants, and the most common. In angiosperms, the sporophyte is the dominant stage, and the gametophyte grows within the plant itself. There are two types of angiosperms: monocots and dicots. Instead of cones, they have reproductive organs called flowers. Angiosperms also use pollination as a way to reproduce. However, unlike gymnosperms, the flowers attract animals which then carry the pollen to other flowers. This is much more efficient then the wind pollination seen in most gymnosperms. Another adaptive feature the angiosperms have is the flowers contain ovaries. These ovaries are a special tissue that covers and protects the seeds. This structure is the part of the flower that in some Angiosperms develops into a fruit.

The life span of an angiosperm can be put in one of three groups. The first group of angiosperms is annuals. Annuals complete their life cycle after one growing season. The next group is biennials. These complete their life cycle after two years, but they produce seeds and die in the second season. The third group is perennials. Angiosperms in this category live through many years. Some even die in the winter and grow back in spring. Some have woody stems; these are what we know to be trees.

Another adapted feature that angiosperms have is a tube that receives the pollen called the stigma and style. This is called the style. In angiosperms there are both male and female reproductive organs. The male organs are the anther and the filament. The anther holds the pollen grains. Animals pass the pollen from flower to flower. Once pollen from one flower makes it to another flower, it goes to the female reproductive organs which include the stigma, style, ovary and ovule. The pollen grain goes in the stigma and down the style. It goes into the ovary where it then meets the ovule and fertilizes it. This develops into the seeds and they either are released when the time is right, or the ovary matures into a fruit.

Monocot is the more simplistic of the two types of angiosperms. The two are split and named for the number of seed leaves it has. Mono- means one or only, thus monocots only have on seed leaf, or cotyledon. A cotyledon is the first leaf that appears on the embryo of a plant. Besides seed leaves there are many other differences between monocots and dicots. Monocots have a unique arrangement of veins in the leaves. The veins run parallel down the leaf in straight or slightly curved line. The petals and other floral parts in a monocot usually are in multiples of three. Another feature is their stems have vascular tissues arranged in a complex array of bundles. Monocots also have a more fibrous root system. The roots don’t go as deep and they are more spread out widthwise. Examples of monocots include lilies and irises.

Dicots, also called eudicots (true dicot), is the more complex of the two types of angiosperms. As opposed to the monocot, the dicot has two cotyledons because di- means two. The veins in the leaves of a dicot are not parallel, but instead they branch out from the center bottom of the leaf where the stem is located. Another difference in the dicot is its vascular tissue is arranged in a more organized, ring structure. Dicots also have floral parts and petals in certain multiples. However their floral parts are in multiples of four or five. Dicots even have a different root structure than monocots. Their roots usually have a taproot present, which is where there is a central root that goes farther into the soil and has other roots branching off from it. Examples of dicots include, hydrangea, sunflowers, pansies, oak tree, maple tree, and petunia