Shape

The basic spore shape among ferns is tetrahedral; the proximal face (the one facing inward during the tetrad, or four-cell, stage following reduction division, or meiosis) is made up of three sloping planes, and the distal, or outer, face consists of a single rounded surface. The tetrahedral structure is commonly obscured in so-called globose spores, the walls of which are thin and soft. Often, the wall is composed of exospore (outer spore layer) only, there being no additional jacket, or perispore. The wall may be either unsculptured and smooth or provided with a variety of sculptured patterns. The tetrahedral spore is formed by cellular divisions in two different planes at right angles to one another during meiosis in the spore mother cell. This results in a three-branched (trilete) tetrad scar where the four products of meiosis were initially joined before the spores matured.

In contrast, the bilateral spore type of many fern species is formed by successive cell divisions in planes along a single axis during meiosis of the spore mother cell. This results in a linear (monolete) scar running parallel to the long axis of the spore. Most bilateral spores in ferns are bean-shaped and jacketed by a perisporial layer, a distinctive covering of the outer wall.