MAASTRICHTIAN AGE
The Maastrichtian age was the last part of the Cretaceous period, about 71 to 65 million years ago, at the very end of the Mesozoic Era.

MAGNETIC FIELD
The Earth's magnetic field is aligned with the north and south poles, and has reversed many times during geologic history. A fossil's magnetic orientation can give clues to its date.

MALM EPOCH
The Malm epoch was the late (or upper) part of the Jurassic period, about 159 to 144 million years ago.

MANGROVE
Mangroves are tropical evergreen trees and shrubs. They live near the shore in tropical marshes and tidal shores with their adventitious roots in the salt water. Classification: genus Rhizophora. Some mangroves include the red mangrove (Rizophora mangle), the black mangrove (Avicennia germinans), and the white mangrove (Laguncularia racemosa).

MANGROVE FOREST
A mangrove forest (also called a mangrove swamp) is an expanse of mangrove trees. These trees live near the shore in tropical marshes and tidal shores with their adventitious roots in the salt water..

MARGINS
The margins of a leaf are its edges, which very from plant to plant. The margins can be smooth, serrated, or toothed; they can also be lobed or entire.

MASS EXTINCTION
Mass extinction is the process in which huge numbers of species die out suddenly. The dinosaurs (and many other species) went extinct during the K-T extinction, probably because of an asteroid that hit the Earth.

MEDULLARY RAYS
Medullary rays (also called wood rays) are bundles of cells that radiate from the center of a tree like the spokes of a wheel (they are most easily seen in the cross section of a tree trunk). They store food and transport it horizontally within the tree.

MEIOSIS
Meiosis is a process in which an eukaryotic cell's (diploid) nucleus divides twice and produces four daughter cells (called gametes or reproductive cells), each of which have half the genetic complement of the parent cell (they are haploid). This process was named by Farmer and Moore in 1905.

MERISTEM
The meristem is a group of plant cells that can divide indefinitely. The meristem provides new cells for the plant.

MESIC
Mesic environmental conditions are ones with medium moisture (water). Compare with very dry conditions (xeric) and very wet conditions (hydric).

MESIC SOIL
Mesic soil is a medium type of soil (hence the name, which means middle) that drains well yet retains some water. Mesic soil is exellent for farming.

MESOPHYLL
Mesophyll is the internal tissue of a photosynthetic leaf.

MESOPHYTE
A mesophyte is a plant that has moderate water requirements. (Compare with hydrophytes and xerophytes.)

MESOZOIC ERA
The Mesozoic Era ("The Age of Reptiles"), occurred from 248-65 million years ago. It is divided into the Triassic, Jurassic, and Cretaceous periods. Dinosaurs, mammals, and flowering plants evolved during the Mesozoic, and Pangaea broke up. The era ended with the K-T mass extinction.

METAMORPHIC ROCK
Metamorphic rocks are compacted by pressure and heat from deep inside the earth.

METEOR
A meteor is a meteoroid that has entered the Earth's atmosphere, usually making a fiery trail as it falls. It is sometimes called a shooting star. Most burn up before hitting the Earth.

METEORITE
A meteorite is a meteor that has fallen to Earth. Meteorites are either stone, iron, or stony-iron.

METEOROID
Meteoroids are tiny stones or pieces of metal that travel through space.

MICROPYLE
The micropyle is the a small pore in a seed that that allows water absorption.

MIDRIB
The midrib (rachis) is the central rib of a leaf. It is usually continuous with the petiole and is often raised above the lamina (the leaf blade). On a compound leaf, the midrib extends from the first set of leaflets (where the petiole ends) to the end of the leaf.

MILANKOVITCH THEORY
The Milankovitch Theory attempts to explain major temperatures changes on Earth, especially ice ages, by slight variations in the amount of sunlight reaching the Earth caused by the eccentricity of the Earth's orbit around the Sun. The Earth's orbital eccentricity changes the Earth's average distance from the sun and therefore slightly changes the amount of sunlight reaching the Earth. Milankovitch looked over the past 600,000 years and correlated summer temperature mimnima with four major ice ages in this time. Eccentricity cycles last over 100,000 years. This theory was proposed by Milutin Milankovitch in 1938. Recently, scientists (Richard A. Muller and Gordon J. MacDonald, July 11, 1997, Science) have found that changes in the axal tilt of the Earth's orbit more closely match glacial cycles for the past million years.

MILDEW
Mildew is a parasitic, filamentous fungus that grows on a host plant.

MILKWEED
Milkweed (genus Asclepius) is a common plant that contains toxins (poisons). There are more than 100 species of this perennial herb, containing varying concentrations of toxic chemicals (glycosides). Monarch butterflies (and other milkweed butterflies) lay their eggs on milkweed leaves; the caterpillars eat milkweed leaves to incorporate the milkweed toxins into their bodies in order to poison their predators.

MILKWEED BUTTERFLIES
Milkweed butterflies are members of the family Danaidae. As larva, they eat the milkweed plant. Milkweed butterflies include the Monarch (Danaus plexippus), the Queen (Danaus gilippus), and others.

1,000,000

MILLION
A million is a thousand thousand. The dinosaurs lived millions of years ago.

MINERAL
A mineral is a naturally-occurring solid of definite chemical composition whose atoms usually form a regular pattern.

MITOCHONDRION
Mitochondria (the plural of mitochondrion) are organelles within most eukaryotic cells - they power cells by generating energy (in the form of ATP, adenosine triphosphate) by breaking down glucose (a type of sugar). Mitochondria are self-replicating and bound by membranes.

MITOSIS
Mitosis is a process in which an eukaryotic cell's nucleus divides after having duplicated its chromosomes - it produces two daughter cells that are genetically identical to the parent cell and to each other. Walter Flemming (1843-1905), discovered and named this process in 1870.

MIXED WOODLAND
A mixed woodland is a woodland that has both conifers and broadleaf trees.

MOLD
Mold is a type of fungus (and not a plant). Like other fungi, molds do not contain any chlorophyll (and cannot make their own food); molds live off the food produced by plants or animals, or decaying matter. Molds are often parasites on plants, animals, or even other fungi. Molds reproduce with spores. Some molds spoil our food, but other foods are produced by the action of mold (for example, blue, Roquefort, and Camembert chesses have mold growing in them, giving them their flavor). The anti-bacterial drug penicillin is made from the Penicillium mold (Alexander Fleming discovered penicillin in 1928). Classification: Kingdom Fungi, Division Eumycota (septate fungi), Classes Hyphomycetes, Oomycetes, and Zygomycetes.

MONOCOT
A monocot is a type of flowering plant (an angiosperm) whose seed has one embryonic leaf (cotyledon). The leaves of monocots generally have parallel venation (the veins are parallel to one another). The roots of monocots are usually fibrous and the flower parts are often in multiples of three.

MONOCULTURE
Monoculture is a system of agriculture in which a single type of crop is grown in an area.

MONOECIOUS
Monoecious plants have the male and female reproductive organs on the same plant. (Compare with dioecious.)

MONOPHYLETIC
A monophyletic group consists of all organisms that share a particular common ancestor (and therefore have similar features). The members of a monophyletic group are more closely related to one other than they are to any organism outside the group. A monophyletic group is also called a clade. An example of a monophyletic group is mammals.

MONOPODIAL
Monopodial (meaning "one foot") is a type of plant growth in which the plant has a single main stem. Compare with sympodial.

MORPHOLOGY
Morphology is the study of the external structure of organisms (for example, the arrangement of leaves on a plant).

MORRISON FORMATION
The Morrison Formation is a rock outcropping located in Utah, USA. This exposed sedimentary rock dates from the late Jurassic period, when this area was similar to a savanna (without the grass, since flowering plants hadn't evolved yet). Dinosaurs like Allosaurus, Seismosaurus, Ceratosaurus, Ornitholestes, and Diplodocus have been found at the Morrison foundation.

MOSS
Moss is a small, low-growing, green plant that has a stem, leaves, and rhizomes (but no vascular system). Mosses reproduce with spores or by forming gametes. Classification: Kingdom Plantae (plants), Plylum Bryophyta (mosses), Four Classes: Class Sphagnopsida (Peat Mosses), Class Takakiopsida ("Takakiophytes"), Class Andreaeopsida (Granite Mosses), Class: Bryopsida or Musci ("True" Mosses). The true mosses (Class Bryopsida/Musci) are divided into three subclasses: Subclass Polytrichidae, Subclass Tetraphidae, Subclass Bryidae.

MUCIGEL
Mucigel is a slimy, protective substance that is secreted by plant roots.

MULTICELLULAR
Multicellular organisms consist of many cells. (Compare with unicellular.)

MUTUALISM
Mutualism is an association between two different species of organisms in which both benefit from the association. An example of mutualism is the relationship between bees and the flowers they sip nectar from- the bees get nectar from the flower and the flower gets pollinated by the bees. Mutualism used to be called symbiosis (which is now a more general term).

1,000,000

MYA
"mya" stands for millions of years ago.

MYCORRHIZAE
Mycorrhizae is a fungus that grows in a symbiotic relationship with the roots (or rhizoids) of a plant.

NANOFOSSIL
Nanofossils are microscopic fossils that are very abundant, widely distributed, and time-specific (because of their high evolutionary rates). They are very useful index fossils.

NATURAL SELECTION
Natural selection is the process in which some organisms live and reproduce and others die before reproducing. Some life forms survive and reproduce because they are better suited to environmental pressures, ensuring that their genes are perpetuated in the gene pool.

NATURALIST
A naturalist is a person who studies plants and animals.

NECTAR
Nectar is the sweet liquid produced by many flowers. Nectar attracts many insects (like butterflies and bees) who go from flower to flower sipping nectar, causing the pollination of the flowers.

NEEDLE
A needle is a long, thin, pointed, needle-shaped leaf, like that of the pine. The word acicular is sometimes used to describe needles.

NEOCOMIAN EPOCH
The Neocomian epoch was the early (lower) part of the Cretaceous period, about 144 to 127 million years ago.

NEOGENE
The Neogene (24 million to 1.8 million years ago) was the later part of the Tertiary Period. It is divided into the Miocene Epoch (24 million to 5 million years ago, when many mammals appeared, including the horses, dogs, bears, South American monkeys, apes in southern Europe, and Ramapithecus; also, modern birds appear) and the Pliocene Epoch (5 million to 1.8 million years ago, when the first hominids (australopithecines) developed, modern forms of whales appeared, and Megalodon swam the seas).

NEOTROPICAL
The neotropical region extends from the southern tip of South America up to Mexico. This vast region includes many different habitats, including tropical rainforests.

NICHE
A niche is a place or position that an organism occupies in an environment.

NODE
A node is a part of the stem of a plant from which a leaf, branch, or aerial root grows.

NOXIOUS WEED
A noxious weed is a plant that is considered by local authorities to be a problem, growing where it is not wanted.

NUCLEAR ENVELOPE
(also called the nuclear membrane) The nuclear envelope is double membrane that surrounds the nucleus.

NUCLEUS
The nucleus is a membrane-bound organelle in each plant cell which contains the genetic material of the cell (DNA in chromosomes). It is where DNA (deoxyribonucleic acid) replicates itself, and where RNA (ribonucleic acid) is made. The plural of nucleus is nuclei.

NUCLEOLUS
The nucleolus is a spherical or ovoid organelle in the nucleus of a eukaryotic cell. It consists of granules of DNA and RNA, and synthesizes ribosomal RNA. The plural of nucleolus is nucleoli.

NURSE LOG
A nurse log is a fallen tree trunk that nourishes new life on the forest floor as it decomposes. It provides a rich, moist habitat for ferns, lichens, mosses, and tree seedlings.

NUTRIENT
A nutrient is a chemical that an organism need to ingest in order to survive (like fats, carbohydrates, vitamins, minerals, etc.).

NUCLEAR MEMBRANE
(also called the nuclear envelope) The nuclear membrane is double membrane that surrounds the nucleus of the cell.

OMNIVORE
Omnivores are animals that eat both animals and plants.

ORDER
In Linnean classification, an order is a group of related or similar organisms. An order contains one or more families. A group of similar orders forms a class.

ORDOVICIAN PERIOD
(pronounced or-da-VISH-ee-en) The Ordovician Period lasted from 505 to 438 million years ago. Primitive plants appear on land, as do the first corals, primitive fishes, seaweed and fungi. Graptolites, bryozoans, gastropods, bivalves, and echinoids also appear. There were high sea levels at first, then global cooling and glaciation, and extensive volcanism. North America was under shallow seas. The Ordovician ended in huge extinction, probably due to glaciation. The Ordovician was named for an ancient tribe (the Ordovices) that lived in northern Wales.

ORGAN
An organ is a specialized tissue (a group of cells) in multicellular organisms that performs a particular function. For example, the ovary is an organ that produces ovules.

ORGANELLE
An organelle is membrane-bound structure within a plant's cell (and all eukaryotic cells) where specialized metabolic tasks occur. Some organelles include the nucleus, nucleolus, mitochondria, the ER (endoplamic reticulum), and lysosomes.

OSMOSIS
Osmosis is a process in which water moves across a semi-permeable membrane in order to equalize the concentration of the solution on both sides. For example, water will move across a cell membrane from areas of low salinity (a hypotonic solution) to areas of high salinity (a hypertonic solution) to equalize the concentrations (and create two isotonic solutions).

OSMOTIC PRESSURE
Osmotic pressure is a phenomenon in which water exerts pressure against a membrane (when it is bordered by solutions of differing concentrations).

OSMOTIC POTENTIAL
Osmotic potential is the potential of water to move into a region by the process of osmosis, the potential of the water to travel from a hypotonic (low concentration) solution to a hypertonic (high concentration) solution.

OUTCROPPING
An outcropping is a place where the bedrock (the underlying rock) is exposed on the Earth's surface.

OVARY
An ovary is a female reproductive organ in plants that produces ovules.

OVERBURDEN
The overburden is the rock that lies on top of a fossil.

OVULE
An ovule is the female reproductive cell of flowering plants and cone-bearing plants. After the ovule is fertilized by the male pollen, the ovule becomes a seed.