23.+Plants

Origin and Evolution of Plants
The first plants evolved from bacterial and protistan photoautotrophs and were producers for ecosystems. The first simple stalked plants grew in muddy sediments along the coasts and streams. As plants have evolved, several trends have developed. The first is a shift from nonvascular to **vascular** plants. Vascular plants have **xylem** and **phloem** which deliver water and nutrients to different parts of the plant, as plants have grown bigger. This growth was made possible by the development of the capacity to use lignin to strengthen cell walls. The second shift is from haploid-dominant to diploid dominant phases in the plant life cycle, meaning that a diploid **sporophyte** is dominant to a haploid **gametophyte**. This made it easier for plants to adapt from life underwater to land because terrestrial environments are more likely to have changes in season in which water and minerals are available. The plant could wait for favorable conditions before fertilizing and dispersing a new generation. On land, a waxy layer called a **cuticle** developed to help retain water in the plant, and **stomata**, openings on surfaces of leaves and stems, developed to help with absorbing carbon dioxide and containing water. The third shift is the development of pollen and seeds. Seed-bearing plants produce two different spores which is called **heterospory**. Macrospores result in the female gametophytes in which eggs form and get fertilized (**seeds**). Microspores result in **pollen grains**, cellular structures that become pollen grains (sperm-bearing male gametophytes).

Bryophytes
 Of over 295,000 species of plants, less than 19,000 are bryophytes. Bryophytes lack xylem and phloem, and are therefore **nonvascular** plants. There are three groups of bryophytes: mosses, hornworts, and liverworts. They grow best in moist habitats and are less than twenty centimeters tall. Bryophytes are able to dry out and revive after absorbing water. Most bryophytes have structures called **rhizoids**, which are elongated cells or threadlike absorptive structures. Rhizoids also attach the gametophytes to the soil. Bryophytes display three features that evolved in early land plants. The **cuticle** prevents water loss from aboveground parts. A **cellular jacket**, which surrounds the parts that produce sperm and eggs, holds in moisture. Lastly, their gametophytes are notably larger than the sporophytes, which dominates the life cycle. Their gametophytes do not get nutrients from sporophytes; embryo sporophytes develop inside gametophyte tissues and they are never distributed. Instead, they stay attached to the gamete-producing body and get nutrients from it.

**Life Cycle of Bryophytes**
 Diploid sporophites release spores, some of which grow into haploid male gametophytes and others grow into haploid female gametophytes. The reproductive structures of bryophytes consist of sperm-producing structures at shoot tip of male gametophyte and an egg-producing structure at shoot tip of female gametophyte. Rain or a film of water on the plant move the sperm to enable it to reach the egg so that fertilization can occur. Diploid zygotes, formed in fertilization, remain attatched to the gametophytes as they grow and develop. Those gametophytes are then able to produce sperm and eggs, and the cycle continues. Bryophytes are haploid for the majority of their life cycle, and the diploid sporophite is dependant on gametophyte.-

Seedless Vascular Plants
 Whisk ferns, lycophytes, horsetails, and ferns are seedless vascular plants that are still in existence. Seedless vascular plants and bryophytes differ in that their sporophytes do not stay attached to a gametophyte, it has vascular tissues, and it dominates the life cycle.  Most seedless vascular plants reside in wet, humid habitats. They must live in places with water because for them to reproduce, the sperm must swim through the water to the egg. 

**Lycophytes**
 Lycophytes commonly have **strobili**, which are conelike reproductive structures made of modified leaves. The leaves contain hollow chambers in which spores form. Each chamber is a sporangium, and the sporophylls form tight clusters on the ends of stems. The spores form by meiosis. After they are distributed, they germinate and become small, free-living gametophytes. //Selaginella// is a lycophyte group that is heterosporous; it produces two kinds of spores in the same strobilus.

 **Whisk Ferns**
 Whisk ferns have **rhizomes**, short, branched, mainly horizontal absorptive stems that grow underground. All whisk ferns have xylem and phloem.

**Horsetails **
 Horsetails grow best in disrupted habitats like streambank muds and vacant lots. Their spores produce gametophytes from 1 millimeter to 1 centimeter across. The sporophytes of most horsetails have rhizomes, hollow photosynthetic stems, and scale-shaped leaves. The stems consists of a ringlike array of xylem and phloem strands.  Silica reinforces the ribs that structurally support the stems. Young fern leaves, called fronds, are coiled while they develop and are often divided into leaflets when maturing.

 **Ferns**
 Ferns are the largest and most diverse group of seedless vascular plants with about 12,000 species. All are native to the tropics except for about 380 species. Their sizes range from leaves of less than one centimeter across to tropical tree ferns that are 25 meters tall. Most have vascularized rhizomes that produce its roots and leaves. <span style="color: rgb(252, 252, 252);"> A **sorus** (plural, sori) is a cluster of sporangia which show rust colored patches on the lower surfaces of many fern fronds.

<span style="color: rgb(252, 252, 252);"> **Life Cycle of Seedless Vascular Plants**
In the haploid stage of a fern's life cycle, spores are released from the sporangia. These spores germinated and grow into gametophytes, which, once they mature, produce either sperm or egg. Upon fertilization, a diploid zygote forms and grows into a sporophyte. The sporophyte remains attached and dependent to the gametophyte as it grows and develops. Once fully grown, it forms a sorus, a spore producing structure. After, meiosis occurs and the process starts, again.

The Evolution of Seed-Bearing Plants

 * Pollen grains**, sperm bearing male **gametophytes**, are produced by seed-bearing plants. **Microspores** are one of the two spores that plants produce. Microspores, the smaller spores, start of pollen grains, which become sperm-bearing gametophytes. **Pollination** occurs when pollen grains land on and enter an ovule (carried by wind, birds, etc). **Megaspores** are the larger of the two spores produced. These megaspores give rise to male gametophytes in which eggs form and become fertilized. **Ovules** are the female reproductive structures which mature into seeds. The formation and development of zygotes occurs inside the ovule after sperm fertilizes the egg after which the embryo sporophyte will then develop. Seed bearing plants have water retention. The development and changes of the stomata and cuticle aid in the plants water retention. The **cuticle** is a waxy coat that protect shoots where as the **stomata** (tiny openings) controls water and CO2 absorption. Plants have adapted to the new climates by shifting to diploid dominance as the moved further and further on land. Also having to adapt to the land meant having to adapt to the water and nutrient supplies which gave way to new roots.

<span style="font-size: 10pt; line-height: 115%; color: rgb(252, 252, 252);">

Gymnosperms

 * Gymnosperms** are plants with “naked seeds,” meaning that the seeds are perched on a spore-producing structure, instead of in a hard chamber like that of and angiosperm. The most well known group of gymnosperms is the **conifers** (Coniferophyta). These are woody trees or shrubs with needlelike or scalelike leaves and true cones. **Cones** are reproductive structures that bear exposed ovules on their upper surface. Trees like these can be either evergreen or deciduous. Evergreen means they remain leafy all year; deciduous means they shed leaves in the fall.

Lesser known gymnosperms include **cycads**, of which about 100 species are still around. They are seed and pollen bearing and very vulnerable to extinction. Another lesser known group of gymnosperms is the **ginkgos**, once a diverse group of deciduous plants, during dinosaur times, but now with only one remaining species. They are very hearty, resistant to insects, disease, air pollution, and have fan-shaped leaves. Their heartiness leads them to be favored in an urban setting, but the female tree seeds are the size of plums, and stink when stepped on, so usually only male trees are used. A final lesser known group of gymnosperms is the **gnetophytes,** 3 groups of woody plants including //Gnetum// that are leathery leafed vines that thrive in humid tropics and other arid regions, //Ephedra// which lives in deserts and arid regions and in which photosynthesis occurs in the green stems, and //Welwitschia mirabilis// which grows in African deserts and consists of stems and cones with a few strap-shaped leaves which split the entire length over time.

**Life Cycle of Gymnosperms**
The life cycle of gymnosperms, for example a pine tree, is mostly in the diploid stage. In the haploid stage, gametophytes who are in the n phase of their life cycle form. The male gametophytes are pollen grains, developed from microspores. Female gametophytes are megaspores that develop into the egg inside the ovule. When fertilization occurs, the male and female gametophytes combine and form a zygote. The gymnosperm is now a diploid sporophyte. The diploid stage dominates a gymnosperm's life cycle. The zygote develops into a seedling, grows into a plant, and grows cones to contain ovules (female sporophytes) and pollen producing sacs to contain male sporophytes. The spores that form restart the cycle of the alternation of generations.

Angiosperms
Angiosperms have the greatest diversity of all types of plants. 90 percent of all existing species of plants are flowering plants. They vary in size from 1 millimeter long to 100 meters tall and have the greatest diversity in traits of any plant group. There are three main types of **angiosperms**, flowering plants. The first is **magnoliids**. There are 9,200 kinds, including magnolias, avocado trees, nutmeg trees, and pepper plants. The second is **monocots**, which are plants with one cotyledon or "seed leave." There are 80,000 kinds of monocots, including orchids, palms, lilies, grasses, and crop plants like rice and corn. The third type of angiosperm are the **eudicots** (true dicots), of which there are 170,000 kinds, including herbaceous (nonwoody) plants like daisies and lettuces, as well as flowering shrubs and trees from roses to maples and oaks. These have two cotyledons.

While most plants are **phototrophs**, some of them are **heterotrophs**. Characteristics of these heterotrophs are that they flower. This is caused by a specialized reproductive shoot. Coevolution with pollinators occurs when two or more species evolve together because of their close ecological interactions. The fruits that these plants produce aid in seed dispersal. These are created by nutrient rich tissues packaging seeds called endosperm and portions of the ovaries to mature into the fruit structrues. The diversity of the fruits is astounding with many of them be fragrant, colorful, small, large, hard-shelled, winged, and/or sticky.

**Life Cycle of Angiosperms**
<span style="color: rgb(252, 252, 252);"> The reproductive cycle is a three step process. First the seed matures into an ovule where its coat protects the embryo when conditions force apparent sporophyte to enter dormancy. Then the flowering stem of the mature sporophte produce the flowers which produce the ovaries and pollen sacs. Inside the ovaries, meiosis occurs which forms the female gametophyte. Inside the pollen sacs, the male gametophyte is formed through the process of meiosis as well. After, pollination occurs which is where the pollen arrives on the female reproductive parts of the seed plant. After pollination occurs, a pollen grain develops into a pollen tube which grows toward the ovary. The tube, or male gametophyte, contains two sperm.

Plants and People
Coal is the compressed organic remains of decaying plants, particularly from the thick Carboniferous forests. When burned, coal is a valuable resource which can be used in place of other important resources like wood. If we use coal instead of wood, it prevents regional climate caused by deforestation, which can effect evaporation, runoff, and rainfall patterns. Carbon dioxide levels increase and oxygen levels decrease with deforestation. However, there is only a finite amount of coal in the world, so coal resources will eventually be depleted and unrenewable.

For more than 10,000 years, humans have been growing domestic plants to have reliable food sources. Over time, humans have found various functions for different plants, including using wood for lumber and papermaking and using Mexican cockroach plants as a pesticide. Plants are used for medicinal purposes and rituals, particularly in Eastern cultures and Chinese medicine, even today. We have also learned to abuse plants, such as tobacco, cocaine, or poisons like henbane.

//<span style="color: rgb(156, 156, 156);">Outline: A collaborative effort from Hashop, Yociss, and Fishman //

Works Cited: Starr, Cecie, and Ralph Taggart. __Biology : The Unity and Diversity of Life__. 10th ed. Belmont: Brooks/Cole, 2003.

Images: 1. Life Cycle of a Moss: http://scitec.uwichill.edu.bb/bcs/bl14apl/images_bryos/moss_life_cycle.jpeg 2. Life Cycle of a Fern: https://eapbiofield.wikispaces.com/file/view/Fern_life_cycle2copy.jpg 3. Angiosperm Life Cycle: http://www.mun.ca/biology/scarr/139450_Angiospermae.jpg