The pollen grains are covered with two concentric wall layers, the outer exine and the inner intine. The stratification of pollen wall is primarily based upon optical microscopy of whole as well as sectioned grains and upon staining reactions of the strata. Recently such stratification is complemented by the biochemical analysis of the various pollen wall layers.
The intine is pectocellulosic in nature and acid degradable, while the exine is composed of sporopollenin which is found to be resistant to both physical and chemical decay.
The pectocellulosic intine can again be divided into two layers the outer pectic polysaccharide layer, called exintine and the inner cellulosic layer, called endintine. Cytochemically these two layers are distinguishable.
The outer exintine layer stains positively with alcian blue, while the inner endintine reacts strongly to the PAS test and calcoflour white. In some graminaceous pollen a pectic polysaccharide rich middle layer (called Z layer) is distinguishable which is thickened at the germinal aperture and termed “Zwischenkörper”.
This layer is comparable to the exintine. In some members (e.g., Amphobolis) intine shows uniform staining reaction, hence cannot be distinguished. Intine of different species vary greatly in thickness. In some species, the cellulosic layer is too thin to be observed under microscope, while in others it occludes practically the whole lumen of the pollen grain.
The cellulose of the intine shows a microfibrillar structure and the microfibrils are oriented parallel to the surface of the grain showing random deposition except near the apertural areas. Hence the intine depicts negative spherical birefringence in polarizing microscope where maximum extinction plane is tangential. In electron micrograph the intine often appears lamellated because of the interbedding of the cellulose layers with the layers of protein.
The exine is further divided into two layers, the outer ektexine (sexine and foot layer) and the inner endexine which is often well developed in dicots, but virtually absent in monocots. These two layers can be demarcated on the basis of their different staining property. Ektexine is generally dissolved with 2- and 3- ethanolamine and stains deeply with basic fuchsin and auramine 0.
Endexine is resistant to hydrolysis with 2- and 3- ethanolamine and does not stain or often stains weakly with basic fuchsin and auramine 0. The homogenous ektexine is formed early.
It shows positive spherical birefringence where submicroscopic structural elements are radially arranged. The ultrastructure of endexine is comparable to intine showing negative spherical birefringence. This layer is formed later by the apposition of tangentially oriented lamellae.