In the former experiment (MITSIIDERA and NUMATA 1960), the first factor for the fair growth of bamboo stand was analysed statistically. As the result of such an exper-iment and analysis it was the amount of rainfall in the growing season of bamboo sprouts (May-June). Using a two-year-old bamboo (Table 1, Figs. 1, 2), the water economy of bamboo was examined by the new transpirometer (Fig.3). Meteorological values and the amount of transpiration are shown in Table 2. The variation of the amount of transpiration is elucidated by the recording of the transpirometer (Figs.4, 5, 6). The relationships between the amount of transpiration and the air temperature or the relative humidity, and the one between the increment of transpiration and the relative humidity are shown in Figs. 7, 8, 9. The characteristics of the soil moisture meter used in the experiment (Fig.10), the daily variation of the soil moisture content in the field tanks (Fig.11), and the relation of the calibration of the instrument to the amount of transpiration (Fig.12) were examined.
According to these data and the variation of the water content of bamboo leaves, the soil in the root circle 20 cm deep became dry at first in the morning, then the amount of transpiration reached to the maximum 1 hour later, and at last the water content of leaves dropped.
The formula for the water economy is as follows:
M=(P+C)-(I+E+R)
where M: soil moisture, P: precipitation, C: codensation water, I: interception,
E: evapotranspiration, and R: run off. Here we assumed P=160 1/m², C=O, 1=16 1/m², R=48 1/m², and E=158 1/m². Then M=-62 1/m². Though we assume I=10% of P at the lowest estimate, M showed the negative value.