Of recent years a few investigators have thrown out hints as to the stimulant action exerted by boron compounds on plants. Roxas indicated that M/100,000 (M = molecular weight) of boric acid exercised a favourable action on rice. Nakamura (1903) tested the point by means of pot cultures. Peas and spinach plants were grown in soil which received 1 and 5 mg. borax per kgm. With peas the 1 mg. exerted evident stimulant action, as determined by the increase in height of the shoot over that of the control, 5 mg. seeming to be slightly depressant in action. With spinach a stimulation was observed both in weight and height with a dose of 5 mg. borax per kgm. Average weight Average length of leaves 5 mg. borax 10·35 38·2 Control 7·2 34·0 Agulhon (1910 c and d) took the matter up still more definitely and made many tests of various kinds, in water, sand and pot cultures. (_a_) _Water cultures._ His water cultures were made under sterile conditions, the seeds when possible being sterilised with corrosive sublimate, the germinating apparatus being also sterilised. With wheat a stimulant action was evident, maximum growth being obtained with between 2·5 and 10 mg. boric acid per litre, though the dry weight increase did not quite keep pace with that of the fresh weight, a fact to which previous reference has been made. The chief improvement is in the root, the stem/root ratio falling to 5, as against 6 in the control series. Visual observation indicated that the roots of plants receiving 5–10 mg. boric acid per litre are longer than the others, though they are less rich in adventitious roots. The increased dry weight due to boron may amount to as much as 30%. (_b_) _Sand cultures._ Agulhon again observed stimulation in this case. 2 kgm. of sand were moistened with 500 c.c. nutritive solution, varying quantities of boric acid being added in addition. ·1 mg. boric acid per litre of N.S. (·05 mg. per pot) gave an increase of 25% fresh weight, and 7·5% dry weight. The stimulating doses seem to be weaker than in the experiments with liquid media, probably because the evaporation from the sand increases the concentration of the boric acid at the surface. It was also noticed that the increase of weight varied in experiments made at different times. With oats the stimulating influence is greater than with wheat, showing that some plants are more sensitive than others to the influence of boron. With radish 1 mg. boric acid per litre exercised a stimulating effect, the enormous average increase of 61% in fresh weight occurring with this strength, though this only represented an average increase of 9·6% dry weight. (_c_) _Soil cultures._