The New Student's Reference Work/Tetrad

Tetrad (tĕt'răd) (in plants). In most of the higher plants each mother-cell organizes four asexual spores within itself. This group of four is called a tetrad. These four sister-spores cling together for a time, and sometimes always, but generally they soon separate from one another. For example, if a very young anther be sectioned and examined under the microscope, each spore-bearing cell (mother-cell) within it will be seen to contain just four forming pollen-grains. This tetrad formation of asexual spores is almost without exception from mosses to seed-plants.

A MOTHER-CELL ORGANIZING FOUR SPORES WITHIN ITSELF
(A TETRAD).