Within the various collections housed in the Herbarium, two deserve to be specially mentioned for their peculiar historical or scientific interest: Hortus Nereidum and the Cryptogamic Collections of De Notaris.
The Hortus Nereidum is a Prelinnean collection, set up in the first half of the 18th century and received by the Institute of Botany in 1875, after the dismantling of the naturalistic collections in the Kircherian Museum. The author, Monsignor Antonio Baldani (1691-1765), an erudite prelate, archaeologist and naturalist, set up an original collection of algae, seed plants and marine invertebrates, identified by mythological names. The Hortus consists of 592 "plates" preserved in four large parchment folders. In each plate, the samples are included between two thin transparent sheets of unspecified organic nature, and then inserted in windows cut out on two sheets of paper glued together. The contours of the windows roughly reproduce the shape of the sample itself making the plate valuable also from an aesthetic point of view.
The Cryptogamic Collectionsof Giuseppe De Notaris (1805-1877), considered one of the most important Italian botanists especially for his important contributions in the field of Briology and Mycology, have a great scientific value linked to the presence of numerous type specimens with their wealth of information. Detailed descriptions and accurate drawings of microscopic structures accompany the exsiccata of the rarest species and new taxa. Drawings and descriptions that, with very few modifications, are reproduced in print in his publications.
The cryptogamic collections of De Notaris together with the Cesati Herbarium are the most requested collections among those housed in the Sapienza Herbarium of Rome, both for the presence of numerous type specimens and for the richness and importance of the materials preserved in them.