Some cytological, ecological and evolutionary aspects of pollination

Different types of pollen dispersing units are described. The pollen of angiosperms may be dispersed in monads, tetrads, polyads, massulae or compact pollinia. The monads and tetrads may form larger clumps of pollen with the aid of the following substances: a) pollenkitt, a product of in situ degeneration of the tapetum; b) tryphine, a product of tapetal extra situm degeneration, found only in Cruciferae; c) elastoviscin, a highly viscous product of the degeneration of a limited number of tapetal cells, found only in certain Orchidaceae; d) viscin threads or sporopollenin filaments arising from the pollen exine. The number of pollen grains composing the clumps depends on pollen diameter and the cohesive forces holding the pollen grains together. The various types of pollen dispersing unit are discussed in relation to the type of pollination and number of ovules per ovary.