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A detailed description is given of the growth of Impatiens parviflora DC in the laboratory at 15° C and saturation deficits of 1.5 or 2.0 mm Hg, illuminated for a 16-hour day with light quantities of 11.2, 13.8, 20.6 and 26.2 cal./cm2/day. Growth under these conditions is compared with growth under artificial shades in the field, described in paper I of this series. The description and comparison cover growth in dry weight and leaf area, relative growth rate and relative leaf growth rate, unit leaf rate and leaf-area ratio, specific leaf area, the distribution of dry matter between leaf, stem and root, and the dry weight/fresh weight ratios of leaf, stem and root. In all these account is taken of ontogenetic drifts up to dry weight of 500–600 mg (reached at an age of about 6 weeks) above which analysis is complicated by self-shading.

Nearly all the comparisons reveal closely similar plants, passing through similar ontogenetic drifts, and make it possible to use the laboratory studies, which in general reveal relationship with greater detail and precision, to help to interpret growth in the field.

Observations are included on the effects of transferring plants from the lowest to the highest light and vice versa. The complications which this causes in the calculation of unit leaf rate are discussed. When they have been overcome it is shown that plants transferred to a given intensity have a unit leaf rate almost identical with that of plants grown in this intensity, differences in growth being due to differences in leaf-area ratio (mainly reflecting differences in specific leaf area). In other respects adaption to a new light climate is largely complete within a week. The leaf growth of adapted and transferred plants is in harmony with the hypothesis that leaf expansion is controlled by a balance between extension due to water uptake by cells with high osmotic pressure and loss of cell wall plasticity by growth in wall thickness.

Both field and cabinet observations yield a similar relationship between specific leaf area and unit leaf rate. It is shown that this relationship accounts for the relatively slight changes in relative growth rate with changing light at medium intensities. A comparison is also made of unit leaf rates in the field and in the cabinets, for equal mean daily quantities of visible radiation expressed in absolute units, problems of such measurements having been discussed. While closely similar at very low intensities, the curves rapidly diverge at higher intensities, unit leaf rate around 3.3 mg/cm2/week being reached at 26 cal./cm2/day in the cabinets and 51 cal./cm2/day in the field. Finally, the efficiency of conversion of visible radiation is shown to be about 15% for the lowest intensity in the cabinets, falling to 12.5% at the highest, and 3% in full daylight.