Abstract:Water-cultured young plants of Phragmites communis Trin. were used in this experiment. They were treated with boric acid and gibberellic acid, separately or in combination, by method of foliage spraying. The results are summarized as follows: 1. In comparation with the controls, the length of the fibre cells was remarkably increased following the treatment with boric acid at concentrations ranging from 1 to 200 μmol/l. The optimum concentration of boric acid was found to be 100 μmol/l. Gibberellic acid alone at the concentration of 50 μmol/l also enhanced the length of the fibre cells. However, the best result was obtained in the group treated with a combination of boric acid and gibmberellic acid at a ratio of 200:50 μmol/l. A maximum increasement of fibre length and a greater ratio of length to width were observed in this experimental group. 2. Microscopic study indicates that the increase in both the number of the fibre cells and ,the thickness of the fibre layer are apparent in histological preparation. 3. In addition, The authors have also found that the incorporation of H3-glucose into the fibre cell wall was greatly enhanced by such combination treatment. 4. In conclusion, it has been demonstrated clearly that the boric acid and the gibberellic acid, when used in combination, produces an augmentative effect on both the growth and the development of the fibre cells of Phragmites communis Trin.