| macronutrient | Collectively, the primary nutrients (nitrogen, phosphorus, and potassium) and secondary nutrients (sulfur, calcium, and magnesium) required for plant growth. |
| macropore | A large soil pore or opening. Macropores include earthworm and root channels and control a soil’s permeability and aeration. |
| mandible | The first pair of jaws on insects. These are stout and tooth-like in chewing insects, needle- or sword-shaped in sucking insects, and the lateral (left and right) upper jaws of biting insects. |
| maturity | Ripeness, usually the state of development that results in maximum quality. |
| meiosis | A cell division process where the chromosome number is reduced by half. |
| meristem | Plant tissue in the process of formation; vegetative cells in a state of active division and growth, e.g., those at the apex of growing stems and roots. |
| mesophyll | A leaf’s inner tissue, located between the upper and lower epidermis, where raw materials (carbon dioxide and water vapor) are held for use in photosynthesis. |
| metabolism | The sum of the biochemical processes of a living cell. |
| metamorphosis | The process by which an insect develops. See complete metamorphosis, simple metamorphosis. |
| microclimate | The climate of a small area within a larger climate area. For example, a backyard can have a different microclimate from that of the surrounding neighborhood, or there may be different microclimates within a single backyard. Microclimates can significantly influence plant growth and should be considered in plant selection and care. |
| microfibril | A fine thread of cellulose in a cell wall. |
| micronutrient | A nutrient used by plants in small amounts, less than 1 part per million. Micronutrients include boron, chlorine, copper, iron, manganese, molybdenum, and zinc. Also called trace elements. |
| micropore | A fine soil pore, typically a fraction of a millimeter in diameter. Micropores are responsible for a soil’s ability to hold air and water. |
| middle lamella | A layer of pectin binding two adjacent cell walls. |
| mitochondria | Cellular bodies in which cellular respiration occurs. |
| mitosis | A cellular division process in which the chromosomes are duplicated. |
| modified central leader | A system of pruning used primarily on fruit trees. The tree’s central leader is encouraged for the first few years, then suppressed. This system allows for well-placed scaffolds and strong crotches but keeps the tree’s crown relatively close to the ground for easy harvesting. Also called modified leader. |
| modified leader | See modified central leader (above). |
| molecule | A chemically bonded group of atoms. |
| molt | The shedding of an insect’s outer covering during growth. The form assumed between molts is called an instar. |
| monocot | A plant having one cotyledon (seed leaf). Also called monocotyledon. |
| monocotyledon | See monocot (above). |
| monoecious | A species having both male and female flowers on the same plant. Pecans, avocados, and squash are examples on monecious plants. See dioecious. |
| morphology | The study of the form of plants or plant parts. |
| mosaic | Nonuniform foliage coloration with more or less distinct intermingling of normal and abnormal colored patches. |
| mottle | An irregular pattern of light and dark areas. |
| mulch | Any material placed on the soil surface to conserve soil moisture, moderate soil temperature, and/or control weeds. Wood chips, bark, and shredded leaves are examples; inorganic materials such as rocks, plastics, or newspapers are also used. |
| multiple fruit | A cluster of mature ovaries from several flowers on a single stem. |
| mutation | A genetic change within an organism or its parts that changes its characteristics. Also called a bud sport or sport. |
| mycelia | Masses of fungal threads (hyphae) that make up the vegetative body of the fungus. |
| mycology | The study of fungi. |
| mycoplasma | See phytoplasma. |
| mycorrhizae | Beneficial fungi that infect plant roots and increase their ability to take up nutrients from the soil. |
