Introduction

　One of the most important collections of the National Herbarium of the Netherlands is undoubtedly a collection of Japanese plants, mainly collected in a period when Japan was closed to all western nations except the Netherlands.

　Already in 1829 P.F. von Siebold (1796-1866) had published about the Japanese Hydrangea's, a genus he was very interested in (Von Siebold, 1829). Together with the German botanist J.G. Zuccarini (1797-1848) he described many new genera and species based on his herbarium collections (Von Siebold & Zuccarini, 1843; 1845; 1846). In 1835 he started the publication of the Flora Japonica. After the death of Zuccarini in 1848 this project came to a halt, but after Von Siebold died in 1866, Miquel published some new parts.

　F.A.W. Miquel (1811-1871), who succeeded C.L. Blume (1792-1862) in 1862 as director of the Rijksherbarium (now the Leiden branch of the National Herbarium of the Netherlands), was also very interested in the Japanese collections. Thanks to Von Siebold and later collectors these were at that time the most extensive in the world. Publication on the Japanese Flora was impossible without consulting the Leiden collections.

　An important incentive for Miquel to make haste with the study of the Japanese collections, might have been the opening of Japan to other nations beside the Dutch nation in 1854. After that also the Americans, British and Russians started to make extensive botanical collections. It may well be that Miquel was of the opinion that the description of the Japanese Flora, which had started in Leiden, should be continued there. This research on the Leiden collections towards a Japanese Flora would certainly increase their scientific importance.

　Besides in the last parts of the Flora Japonica, Miquel also published on these Japanese collections in the Annales Musei Botanici Lugduno-Batavi (1863-1870), in which he described many new Japanese taxa, and in his Prolusio florae Japonicae (1866-1867). The latter is mainly a reprint of the Annales. Miquel considered the Japanese collections in Leiden so important, that he placed them in a separate room (Miquel, 1865). This must have been the collection, which was referred to as the Herbarium Japonicum. Because Miquel had catalogued all plants and objects in this collection we know exactly what was present then (Miquel, 1870). On the first pages of this catalogue and the Prolusio, he gave a short description of its contents. It should consist then of collections made by C.P. Thunberg (1743-1828), the collections of Von Siebold and his Japanese collaborators and the collections made by H. Bürger (1806?-1858), J. Pierot (1812-1841), C.J. Textor (1816-x) and O.G.J. Mohnike (1813-1887). Furthermore it should consist of duplicate collections made by S. Wells Williams and J. Morrow, and C. Wright and I. Small during the first American expeditions to Japan. Miquel further mentions duplicate collections made by the British botanist R. Oldham, and the Russian botanist C.J. Maximowicz (1827-1891).

　In 1871 W.F.R. Suringar (1832-1898) succeeded Miquel as director of the Rijksherbarium. He was especially interested in the Japanese algae and published on them in the Algae Japonica (1870), and in the first two volumes of the Musée botanique de Leide (1871-1875). He enlarged the Herbarium Japonicum with algae he had received from the chemist K.W. Gratama (1831-1888) and the army physician C.G. van Mansvelt (1832-1912).

　Not only in the past, but also in recent times the Herbarium Japonicum has been the subject of ongoing research. From a source of new species it now has become important in the study of the history of early Japanese botany. In the 1980's M. Omori published on the Keiske albums and the album with maple leaves (Omori, 1985; 1989). Since 1995 T. Yamaguchi and N. Kato visited the Leiden herbarium many times, often accompanied by other Japanese botanists. The results of their extensive studies are published among others in the Japanese periodical Calanus, and many facts mentioned here are borrowed from their publications (Yamaguchi, 1997; 1998; 2003).