During the course of a revision of the species of Tradescantia indigenous to the United States, published several years ago,1 I found it advisable to make some correlative observations upon the genus as a whole and the American representation of the family generally. These rather casual excursions afield were both comforting and disquieting, for they showed that, although the Tradescantias of the United States are relatively homogeneous phylogenetically, those of the tropics are extremely heterogeneous, and also that the systematics of the family, at least in North America, is very precarious indeed. Such being the case, I fixed my attention upon the limited job in hand with a profound sense of thanks to Providence for my lot at that time, and a nebulous vow of propitiation by a revision of the tropical Spiderworts in the indefinite future. The future has arrived rather unexpectedly at last, for I find my vow exacted by the needs of the 'Flora of Panama' upon which I have been working for some years past. The Commelinaceae always have been difficult subjects for herbarium study because; of their deliquescent flowers. It is not easy to understand, therefore, why previous systematists of the family have focused almost their whole attention upon floral structure in the delimitation of subfamilies, tribes, and genera. In his account of the family for de Candolle 's 'Monographiae', C. B. Clarke2