Cannabaceae, moraceae and urticaceae

Although this is not the procedure followed in most of the contributions to the Northwest European Pollen Flora, this part combines three families. The reason for this is the close resemblance of the pollen grains in these three families that often makes identification of the pollen a problem. It is of practical importance to have similar pollen types combined into a single key and considerations of this kind also led to the amalgamation of the families Sparganiaceae and Typhaceae (Punt, 1975). In Northwest Europe the family Urticaceae is represented by five species in two genera. The Cannabaceae comprises two genera each with a single species while the Moraceae has one genus and two species. Pollen grains of the three families often show a thickening around the porus margin. It is, however, difficult to decide whether this is caused by a thickening of the nexine (costa) or of the sexine (annulus). Various authors have used different terms for this phenomenon. Sorsa and Huttunen (1975) chose the term "annulus", whereas Birks (1973) used "costa" and Hyde and Adams (1958) "aspis", a term introduced by Wodehouse (1935)