Sentence:  As might be expected, characters of the <WSD>aspides</WSD> themselves are not of much value in pollen identification, but they are easily recognized and many three-pored, aspidate grains are broadly categorized as "betuloid" in studies of airborne pollen. 
Meaning of the word marked with <WSD> tags:  A prominent ring of thickened exine around a pore on a pollen grain