GAMETANGIUM
(plural gametangia) A gametangium is a reproductive organ that is in some plants (especially algae, fungi, mosses, and ferns). The gametangium produces gametes (reproductive cells).

GAMETE
A gamete is the male or female reproductive cell of an organism (the sperm or the egg). Each gamete has only half the number of chromosomes that the other cells of that organism have (it is haploid).

GALLIC EPOCH
The Gallic epoch was the middle part of the Cretaceous period, about 127 million to 89 million years ago.

(pronounced GAS-troh-liths) Gastroliths are stones that some animals swallow and use to help grind up tough plant matter in their digestive system. They're also called gizzard rocks.

GENOME
The genome of an organism is made up of the set of chromosomes that contain all of its genes.

GENUS
(pronounced GEE-nus) In classification, a genus is a group of related or similar organisms. A genus contains one or more species. A group of similar genera (the plural of genus) forms a family. In the scientific name of an organism, the first name is its genus (for example, people are Homo sapiens - our genus is Homo and our species is H. Sapiens).

GEOLOGICAL TIME
The history of the earth is described in geological time, which is measured in millions of years and billions of years. The divisions used are: eon, era, period, and epoch.

GEOLOGICAL TIME PERIODS
Geologic time is divided into divisions based on some distinguishing feature of that time (like an Ice Age). The divisions used are: eon, era, period, epoch, and age.

GEOLOGY
Geology is the study of the Earth's structure, including rocks.

GEOTROPISM
Geotropism is a plant's reaction to gravity in which the roots go towards the pull of gravity, and the shoots go in the opposite direction.

GERMINATION
Germination is the beginning of growth of a plant from its seed.

GIGANTOPTERID
Gigantopterids were ancient seed plants that lived during the Permian period, over 250 million years ago. It is thought that the gigantopterids evolved into the flowering plants. One species of gigantopterid is the broad-leafed climbing plant Vasovinea tianiia; it was found in 1993 in China.

GINKGO or GINGKO
Ginkgo or Gingko (also called the maidenhair tree) is a primitive seed-bearing tree (a gymnosperm) that was common during the Mesozoic Era, but has only one existing species now. Ginkgos peaked during the Jurassic and Cretaceous periods. This deciduous (losing its leaves in cold weather) tree has fan-shaped leaves divided into two lobes. Classification: Division Pinophyta (Gymnosperms) , Subdivision Pinicae, Class Pinopsida, Order Ginkgoales, Family Ginkgoaceae (ginkgos).

GIZZARD ROCKS
Gizzard rocks are stones that some animals swallow and use to help grind up tough plant matter in their digestive system. They're also called gastroliths.

GLABROUS
A glabrous surface lacks hairs (and has a smooth surface).

GLOCHID
Glochids are tufts of short, barbed spines that are found at the areoles of opuntia cacti.

GLOSSOPTERIS
Glossopteris (from the Greek glossa, meaning tongue, because the leaves were tongue shaped) is a genus of extinct seed fern (a Pteriosperm) whose fossils are found throughout India, South America, southern Africa, Australia, and Antarctica. Glossopteris was about 12 ft (3.6 m) tall. The distribution of this fossil plant throughout the southern hemisphere led the Austrian geologist Eduard Suess to deduce that there had once been a land bridge between these areas. He named this large land mass Gondwanaland (named after a district in India where the plant Glossopteris was found). This was the southern supercontinent formed after Pangaea broke up during the Jurassic period. It included what are now the continents South America, Africa, India, Australia, and Antarctica. These deciduous (losing their leaves in the cool season) gymnosperms arose during the late Permian and became dominant, but went extinct by the end of the Triassic period.

GOLGI BODY
(also called the Golgi apparatus or Golgi complex) a flattened, layered, sac-like organelle that looks like a stack of pancakes and is located near the nucleus. It produces the membranes that surround the lysosomes. The Golgi body packages proteins and carbohydrates into membrane-bound vesicles for "export" from the cell.

GONDWANALAND
Gondwanaland, also known as Gondwana, was the southern supercontinent formed after Pangaea broke up during the Jurassic period. It included what are now the continents South America, Africa, India, Australia, and Antarctica. Gondwanaland was named for a district in India where the fossil plant Glossopteris was found; this plant led E. Suess to deduce that the southern continents were once joined, supporting Wegener's continental drift theory

GRAFT
A graft is a shoot or bud that has been joined to another plant.

GRAIN
A grain is a single particle of pollen.

GRANUM
(plural grana) A stack of thylakoid disks within the chloroplast is called a granum.

GRASS
Grasses are a group of flowering plants (angiosperms) that belong to the family Graminae. Classification: Division Magnoliophyta (angioperms), Class Magnoliopsida (dicots), Class Liliopsida (monocots), Subclass Commelinidae (grasses, sedges and rushes, Order Cyperales, Family Poaceae (Gramineae) (grasses).

GRAZER
A grazer is an animal that eats low-lying vegetation, such as grasses and other low plants.

GRAZING FOOD CHAIN
The grazing food chain is a model that describes the flow of organic energy through organisms in an ecosystem. A trophic level is a level of this grazing food chain. Plants (called primary producers) occupy the first trophic level. Plant-eaters (also called primary consumers) occupy the second trophic level in the grazing food chain. The third level contains animals that eat primary consumers (first-level carnivores = secondary consumers). The fourth trophic level contains tertiary consumers, generally larger animals (like eagles) that eat primary consumers.

The greenhouse effect is an increase in the temperature of a planet as heat energy from sunlight is trapped in the atmosphere. Excess carbon dioxide and water vapor increase this effect. The greenhouse effect is strong on Earth, maintaining (and possible exacerbating) warm temperatures.

GREEN REVOLUTION
The green revolution was a dramatic increase in agricultural yields that occurred in the 1950s through 1960s. The green revolution was based upon many improvements in plant science, including the genetic improvement of many plants (including new, high-yield hybrid varieties), improved irrigation, more efficient machinery, new fertilizers, and pest controls that increased plants' disease-resistance, improved their hardiness, and increased their productivity (especially rice, wheat, and corn).

GUARD CELL
Each stoma has two crescent-shaped guard cells that control the size of the opening of the stoma using turgor pressure. This changes the amount of water vapor and other gases that can enter and leave the plant.

GUM
Gum is a sticky substance that is secreted by some plants. Gum hardens when it dries.

GYMNOSPERM
Gymnosperms (meaning "naked seeds") are seed-bearing plants that that don't produce flowers. They release pollen into the air to the female ovule, causing fertilization. Their seeds develop without a protective covering. The earliest gymnosperms were seed ferns from the Devonian period (408-360 million years ago). Conifers (like pines, redwoods, and fir), gingkos, seed ferns, cycadeoids, and cycads are gymnosperms. These plants were very important to plant-eating dinosaurs.

HABITAT
A habitat is a space (which includes food, water and shelter) suitable for the survival and reproduction of an organism.

HALF-LIFE
The half-life of a radioisotope is the amount of time it takes for half of the radioisotope to decay.

HALOMORPHIC
Halomorphic characteristics are adaptations to a saline (salt water) environments.

HALOPHTYE
A halophyte is a plant that has adapted to salty soils.

HAPLOID
A haploid cell has only half the number of chromosomes that the other cells of that organism have (most are diploid). Gametes (like the sperm and the egg) are haploid.

HARDWOOD
Hardwood trees are trees whose wood is dense, strong, and relativel non-absorbent of water. These slow-growing trees include oak and maple. Hardwood trees have broad leaves (and not needles).

HARDY
A hardy plant is one that can withstand the extremes of climate, like frost.

HAUSTORIAL ROOT
An haustorial root is one that absorbs water and nutrients from another plant (and not the soil).

HEATH
A heath is an area of open land that is covered with low-growing shrubs like heather.

HECTARE
A hectare is metric unit of area. A hectare is equal to 10,000 square meters.

HERB
An herb is a seed plant that does not have a woody stem. Every year, herbaceous plants produce a completely new stem. Herbaceous plants are generally short lived and relatively short (compared to woody plants). Some herbaceous plants include the banana, grasses, and forbs.

HERBICIDE
An herbicide is a chemical that kills certain plants. Herbicides are usually intended to kill weeds.

HERB LAYER
The herb layer is another name for the floor of a rainforest.

HERBIVORE
Herbivores (also called primary consumers) are animals that eat plants.

HERMAPHRODITE
A hermaphrodite is a plant that has both female and male reproductive organs.

HETEROTROPH
(pronounced HET-er-oh-TROFE) A heterotroph (also called a consumer) is a living thing that eats other living things to survive. It cannot make its own food (unlike plants, which are autotrophs, making their own food). Animals are heterotrophs.

HETEROPHYLLOUS
A heterophyllous (meaning "other leaf") plant or branch has two or more different leaf shapes on it. Although these leaves have different shapes, they have similar internal anatomical organizations.

HILUM
The hilum is the scar on a seed coat at the location where it was attached to the plant's stalk during development.

HOLDFAST
A holdfast is the root-like structure of kelp that holds the base of the plant in place on the sea floor. Unlike real roots, however, the holdfast does not obtain and supply nutrients to the rest of the plant.

HOMOLOGY
Homology is the similarity of characters found in different species that are due to common descent. Examples include the flippers of whales and our arms.

HOMOPLASY
Homoplasy is the similarity of characters found in different species that are NOT from common descent. Examples include the wings of insects and the wings of pterosaurs. These characters derive from convergent evolution, parallel evolution, or character reversal.

HORMONE
A hormone is a chemical in plants (and other organisms) that regulates the plant's growth, reproduction, and other functions.

HORSETAIL
Horsetail is a primitive, spore-bearing plant (a sphenopsid and a pteridophyte) with rhizomes that was common during the Mesozoic Era. Its side branches are arranges in rings along the hollow stem. Horsetails date from the Devonian period 408-360 million years ago, but are still around today and are invasive weeds. Classification: Division Equisetophyta, Class Equisetopsida, Order Equisetales, Family Equisetaceae (Horsetails).

HORTICULTURE
Horticulture is the science of growing fruits, vegetables, flowers, and ornamental plants.

HOST
A host is an organism which a parasite uses for food and/or shelter.

HUMUS
Humus is the rich, organic portion of the soil. It is composed of decayed plant and animal materials.

HUXLEY, THOMAS H.
Thomas H. Huxley (1825-1895) was a British scientist and friend of Charles Darwin. He was the first scientist to notice the similarity between birds and dinosaurs. He named: Acanthopholis (1865), the family Archaeopteryglidae (1871), Euskelosaurus (1866), Hypsilophodon (1869), and the family Megalosauridae (1869).

HYBRID
A hybrid is the offspring of two organism that belong to different breeds, varieties, species or genera.

HYDRIC
Hydric environmental conditions are ones that are very wet. Compare with very dry conditions (xeric) and medium moisture conditions (mesic).

HYDPOCOTYL
The hypocotyl is the part of the stem of a sprouting plant that is above the root and below the stalk of the cotyledons (seed leaves).

HYDROLOGIC CYCLE
The Hydrologic Cycle (also known as the water cycle) is the journey water takes as it circulates from the Earth to the sky and back again.

HYDROPHYTE
A hydrophyte is a plant that grows in water or in water-logged soil. Hydrophytes have a reduced root system, reduced support and vascular systems, and specialized leaves. Some hydrophytic plants include waterliles and Wolffia (which is the smallest flowering plant). Anchored hydrophytes have a rooting system that is embedded in the soil and they often have floating leaves. (Compare with mesophytes and xerophytes.)

ICE AGE
An ice age is a time lasting thousands of years during which the Earth is very cold and largely covered by ice and glaciers.

(pronounced IK-nites) Also known as trace fossils or ichnofossils, these are fossilized footprints, nests, dung, gastroliths, burrows, stomach contents, etc., but not actual body parts. Ichnofossils record the movement and behavior of animals. Ichnology is the study of ichnites.

Also known as trace fossils or ichnites, these are fossilized footprints, nests, dung, gastroliths, burrows, stomach contents, etc., but not actual body parts. Ichnofossils record the movement and behavior of animals. Ichnology is the study of ichnofossils.

When molten rock cools, igneous rock is formed.

IMBIBITION
Imbibition is the process in which a seed takes up water from its surrounding and swells. This powerful process precedes germination and can spit the seed coat.

IMPACT CRATER
Impact craters are the remains of collisions between an asteroid, comet, or meteorite and the Earth.

IMPERFECT FLOWER
An imperfect flower has either male (stamen) and female (ovary) reproductive organs on the same flower, but not both. (Compare with perfect flowers.)

INCOMPLETE FLOWER
An incomplete flower is missing one of the four major parts of the flower, the stamen, pistil, petals, or sepals.

INDEHISCENT
A fruit that remains closed at maturity is indehiscent. Nuts, acorns, sunflowers, grains, and maple pods are examples of indehiscent fruits.

Index fossils are commonly found fossils that are limited in time span. They help in dating other fossils. For example: trilobites were common during the Paleozoic, but not found before the Permian period, and ammonites were common during the Mesozoic Era, but not found after the Cretaceous period. Another example: the oldest-known ostracods are from the Cambrian period; they became widespread during the Ordovician and remain so.

INDIGENOUS
An indigenous organism is one that lives naturally in a particular region and were not introduced there by man.

INFERIOR OVARY
An inferior ovary (also called an epigynous ovary) is an ovary located below the flower parts (the calyx, corolla, and androecium). The flower parts are attached to the top of the ovary.

INFLORESCENCE
Inflorescence is the a type of flower in which there is more that one flower in a single structure.

INSECTIVOROUS
Insectivorous organisms eat insects. Insectivorous plants trap and digest insects for nourishment.

Insects have exoskeletons and six legs. They evolved during the Silurian Period, 438 to 408 million years ago, long before dinosaurs existed.

INTERNODE
The part of a plant's stem between two nodes is the internode - the distance along the stem between the leaves.

INTERSPECIFIC COMPETITION
Interspecific competition is competition between two different species of organisms (competing for food, water, territory, etc.).

INTRASPECIFIC COMPETITION
Intraspecific competition is competition among members of a species (competing for food, water, territory, etc.).

IRIDIUM
(pronounced irr-ID-ee-um) Iridium is a heavy metal element that is rare on the Earth's surface, but abundant on chondritic meteors and in the Earth's core.

IRIDIUM ANOMALY
The iridium anomaly is a layer of Earth's crust (the K-T layer, which is about 65 million years old) in which there is excess of iridium (a relatively rare element). The presence of this extra Iridium supports the Alvarez asteroid theory, since this iridium may have come from an asteroid.

IRRADIANCE
Irradiance is the amout of light energy from the Sun that hits the Earth in an area.

ISOTOPE
An isotope of an element is another form of the same element that has a different number of neutrons in the nucleus (giving it a different atomic weight).