One of the main aims of the botanical gardens is natural and cultivated plants preservation. From 1994 the investigations of rare species and phytocenosises take part in the BG of PSU. Reservation of BG (350 ha) is one of those rare areas in the middle taiga zone where series of ecological conditions appeare to be favourable for the existing of phytocenosises with nemoral species. Three types of phytocenosises with Tilia cordata Mill. and some other species of platyphyllous forests were discovered. The distinction features of this phytocenosises are species diversity, complicated structure, significance of nemoral and boreal-nemoral species and eutrofic plant species. The importance of studied phytocenosises is caused by providing the ecological niche for rare species of Karelian flora (Brachypodium pinnatum (L.) Beauv.). Several types of phytocenosises with Alnus glutinosa (L.) Gaerth. (the relict species in Karelia) were detected in the territory of BG. The "curly rocks" are the habitations of 4 species from Karelian Red Book (Origanum vulgare L., Dracocephalum ruyishiana L., Veronica spicata L., Woodsia alpina (Bolt.) S.F.Gray). In whole 9 species forming natural in the BG are included into Karelian Red Book (1995). Some species of Orchidaceae family (Cyprepedium calceolus L., Gymnadenia conopsea (L.) R.Br.) were planted in the natural habitations. Moreover 11 species of Karelian Red Book is planted in the collection plots. Much attention is devoted to creation of programm "Calypso"- plant records database management system for botanical gardens.