The elasticity of the exine

We pressed pollen grain exines of ten genera with sizes ranging from about 20 to over 100 mu m in diameter past a piston in a close fitting cylinder. The clearance between piston and cylinder was about 20 mu m. Except for exi nes of Betula all the other pollen types were at least twice the clearance diameter and could be expected to be greatly deformed, crushed or fractured . Cracks were evident with the light microscope in some grains and a few we re clearly deformed but most appeared intact, even exines of Zea mays at a diameter of 100-110 mu m. With scanning electron microscopy cracks were app arent in most of the large grains (Zea, Lilium, Pinus, Crinum and Epilobium ) but not in the smaller grains (Betula, Ephedra, Tulipa, Fagus and Typha). We also found many exines within exines. In some cases, e.g., Lilium, the exines entered through apertures but in other grains such as Zea and Pinus, exines came in through cracks which had opened during acetolysis or centri fugation, then closed so tightly that the cracks were difficult to see with light microscopy. This opening and closing of cracks in exines means that the pollen grain exine is very flexible and resilient and capable of withst anding shock without permanent deformation. To regain their original form t he exine components that were severely cracked, ruptured or partly separate d must spring back together like the partly separated halves of a tennis ba ll.