The paper presents results of analyses of selected physico-chemical and biological parameters in four rivers (the Buczynka, the Kosówka, the Ugoszcz, the Osownica) situated in the Nadbużański Landscape Park. Chemical assessment of the rivers was performed in accordance with the water classification valid in Poland while their biological assessment was carried out basing on the taxonomic composition of macroinvertebrates and in accordance with the Belgium Biotic Index (BBI). The chemical parameter decisive for the final classification of water was the concentration of phosphates. Particularly high substandard values of these ions were recorded in the Buczynka and the Kosówka rivers in places located below the outlets of sewage treatment plants. These stations also turned out to be deficient in macroinvertebrates sensitive to water pollution (plecopteran, ephemeropteran, caddis-fly larvae). According to the 5 class scale of water quality the Ugoszcz and the Osownica carry the 2^nd class water in the majority of analysed stations, while the Buczynka and the Kosówka carry the 3^rd class water. These tributaries bring to the Bug river moderately or slightly polluted waters. The results indicate that investigated rivers are higher classified according to biological indices than to chemical ones.

Presence of two associations belonging to Panico-Setarion alliance (order Polygono-Chenopodietalia) was observed in the root cultivations of the Siedlce city. The largest area of the city arable lands was covered by plots of association Echinochloo-Setarietum, differentiated internally into 3 variants (typical, with Matricaria maritima subsp. inodora and with Galinsoga parviflora). Variant with Matricaria was divided into two subvariants, different as to moisture conditions. Association Digitarietum ischaemi, occurring on the poorest habitats was noted very seldom in root cultivation of the Siedlce city, considering the diminish of its habitat acreage. The similar process is observed in the whole Poland. The vegetation described in the paper is characterized by a large flora biodiversity and large participation of ruderal species originating from surrounding communities.

Results of an analysis of herbarium and fresh materials from the area of Poland (Czarna 2002) showed that among three taxa of the genus Lamiastrum, two should be regarded as species (L. galeobdolon and L. montanum), and one as a subspecies (L. montanum ssp. flavidum). In Poland L. montanum ssp. montanum is quite common. So far, 99 localities of this taxon distributed in all parts of Poland (but mainly in the south) have been documented as herbarium specimens. By contast, Lamiastrum montanum ssp. flavidum is very rare, recorded in only 6 localities, both on lowlands and in mountains.

Occurrence of Ptilium crista castrensis at 53 localities in central-eastern Poland was noted. The species was the most frequently found in Peucedano-Pinetum association, more seldom in Molinio-Pinetum and Pino-Quercetum. At 53 localities it was accompanying by Picea excelsa (about 70%). Among from 21 localities of Goodyera repens occurrence of the species together with Ptilium crista-castrensis at 17 localities was noted.

In central Wielkopolska, 15 species of the genus Taraxacum were recorded. The most common in the study area are: T. scanicum (7 localities) and T. proximum (6 localities). One of the most interesting species is T. subalpinum, which was earlier known from only three localities in Poland: Wrocław, Bojanowo and Lubiń. T. disseminatum was earlier recorded only near Szczecin, as a new species for the Polish flora. Taraxacum tenuilobum and T. sellandii is also rare in Poland.

A new station of rare, protected fungus - Phallus hadriani was found in Siedlce in 1999. The station consists of 8 fructifications, growing on a lawn, just by Robinia pseudacacia trunk. The locality in Siedlce will be monitoring.

The aim of this experiment was to study the influence of various concentrations of selenium added to the soil on the changes in plant enzymes. The wheat plants of the Kobra variety were grown in 0.5 dm3 pots filled with silt loam, of 1.92% organic C content. The soil was previously supplemented with the following concentrations of selenium: 0; 0.05; 0.15 and 0.45 mM · kg^-1. After 7 days the activity of acid and alkaline phosphatase, catalase and peroxidase were examined. The results of the study showed that selenium application caused the activation of oxireductive enzymes. The maximum catalyse activity was observed at concentration of 0.45 mM · kg^-1 and peroxidase at 0.15 mM · kg^-1. The concentration of 0.15 and 0.45 mM · kg^-1 of selenium slightly increased the activity of alkaline phosphatase and inhibited the activity of acid phosphatase.