Plants of the common reed (Phragmites australis (Cav. Trin. exSteud.) were treated with high doses of nitrogen in an experiment performed in a sand culture and lasting one vegetation period. Nitrogen treatment resulted in alterations of growth; the increased growth of rhizomes and stalks was connected with production of significantly shorter roots. The differences in root growth were connected with changes in the internal root structure. In rhizome-borne roots, treatment with high nitrogen reduced the formation of aerenchyma and lignified layers in subapical regions. No substantial differences were observed in starch occurrence in the roots under the two treatments.