M O R P H O L O G Y
The Wall
The exine, which constitutes the wall of the pollen grain, is further divided into a set of layers. The stratification of the exine has a controversial terminology. Both are presented and they can be contrasted in the following figure:
The typical exine is composed of two layers, which can be differentiated by staining techniques. The inner layer, which is not sculptured, is called nexine and the outer layer, which is sculptured, is called the sexine. The other nomenclature describes the two layers as the endexine and the ektexine, for the inner and outer layers respectively. For simplicity and clarity the former terminology will be used. The nexine is differentiated based on staining characteristics into an inner layer, which is called nexine 2, and an outer layer, called nexine 1. The sexine usually exhibits a structure of "pillars" supporting an outer "roof" that may or may not bear sculpture. The "pillars" are called columellae and the "roof" is called the tectum. The tectum may be complete, partial, or absent. The sculptural elements are classified as sexine 3, the tectum as sexine 2, and the columellae as sexine 1. If the tectum covers at least 75% or more of the surface of the grain, the latter is said to be tectate. The grains having no tectum are referred to as intectate and the columellae, if present, are not fused. Grains showing an intermediate fusion are termed semitectate.
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