Stranded Arrangement of Sporopollenin in the Exine of Microspores of Poa annua.

In the early microspore wall of the grass Poa annua L., sporopollenin is arranged in bundles of anastomosing strands. Each bundle contains at least two strands, each about 50 A in diameter, which anastomose or intertwine and look like a meshwork with pores of the mesh up to 50 A in diameter. After the early microspore stage the exine becomes homogeneous; it is similar in this respect to the exines or ornamented parts of the exines of pollen of other angiosperms examined by electron microscopy.