The first volume of this series was published as Monograph 8 of the National Institute of Sience and Technology in Manila, the second as Supplement 5 of Blumea. Originally I intended to publish the third volume as another supplement of Blumea, but following the advice of the Director of the Rijksherbarium Dr. C. Kalkman, I decided to issue it separately. After this chequered history the series will hopefully be continued as an irregular publication of the Rijksherbarium. It is a pleasure to see that already in this early stage P.P.A. has induced thought and has been made use of, e.g. by Thorne (1972)¹.