Sporoderm defines the shape of a pollen grain and spore. Erdtman (1952) distinguishes nine common types of shape (Fig. 4.33). The shape classes are based on the ratio between the length of polar axis (P) and equatorial diameter (E). P and E are measured from the equatorial view of a pollen grain and spore.
Measurements are taken from the optical section of a pollen grain and spore. The imaginary straight line (Fig. 4.31A & B) that occurs between the proximal pole and distal pole of a pollen grain is called polar axis. Equatorial axis/diameter are also an imaginary straight line that occur halfway perpendicular to polar axis. In measuring the P and E axes longest side of pollen and spore are considered in optical section.
The followings are the common shapes according to Erdtman (1952): peroblate (ratio of P to E < 0.50), oblate (ratio of P to E between 0.50 and 0. 75), subspheroidal (ratio of P to E between 0.75 and 1.33), suboblate (ratio of P to E between 0.75 and 0.88), oblate spheroidal (ratio of P to E between 0.88 and 1), prolate spheroidal (ratio of P to E between 1 and 1.14), subprolate (ratio of P to E between 1.14 and 1.33), prolate (ratio of P to E between 1.33 and 2) and perprolate (ratio of P to E > 2). Erdtman in 1943 mentioned another shape class termed spheroidal (ratio of P to E between 1.14 and 0.88).
Shape of a pollen grain has taxonomic significance. It is used in the formulation of pollen keys. As for example 3-colpate grains of Rumex dentatus are differentiated from the 3-colpate grains of Leonurus sibiricus, Tectona grandis and Vitex negundo on the basis of shape. Rumex dentatus has subspheroidal to spheroidal pollen and the rests have subprolate to prolate pollen.