Mesophyll. Internal tissue of left blade differentiated into palisade and spongy mesophyll. In dorsi-ventral leaves palisade mesophyll consists of elongated cells immediately below upper epidermis with their long axes at right-angles to leaf surface; these cells contain numerous chloroplasts and are chiefly concerned in photosynthesis. Spongy mesophyll is a tissue of irregular, loosely arranged cells, containing fewer chloroplasts, with large air spaces between the cells, communicating through stomata with atmosphere outside.

Merogamete. Any protozoan Gamete formed by fission of parent cell.

Mesocephalic. Having a medium-sized head.

Merophilic. Having an optimal growth temperature from 20⁰C to 45⁰C

Contain various enzymes; one kind of microbody (peroxisome), contains oxidases and catalyse (enzyme destroying hydrogen peroxide). Found in a number of different plant and animal cells.

Micrometre or micron. A unit of length equal to 10^-6 metre Symbol=µm

Microorgonism. (Microbe). Any organism that can be observed only with the aid of a microscope. Micro-organisms include bacteria, viruses, rickettsiae, protozons, and some algae and fungi.

Microphyll. A type of foliage leaf seen in certain peteridophytes (e.g. Selaginella, Lycopodium) that has a single unbranched mid-rib. Such leaves are generally no more than a few millimeters long.

Micropyle. A small opening in the surface of a plant ovule through which the pollen tube passes prior to fertilization It results from the incomplete covering of the nucellus by the integuments. It remains as an opening in the testa of most seeds through which water is absorbed.