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progression. The intensity of white light given by a half-watt incandescent electric lamp of 200 candle power, placed at a distance of a metre, is taken as the unit. Much feeliler light would have been sufficient, but it would have required much longer exposure. The intensity was increased by bringing the lamp nearer the plant: marks were made on a horizontal scale so that the intensity of incident light increased at the successive.) marks of the scale as l 2 2: 3: and so on. The duration of exposure was same in all cases, namely, 5 minutes. After each experiment. suitable periods of rest Were allowed for the plant to recover its normal rate of growth. Records in Fig. 77 show increasing retardation 'induced by stronger intensities of light. Table XVIII gives the result of a different experiment.

Flu. 77. -.-\ction of light. of increasing intensities: 1:2:3 in retardatium of growth.

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TABLE XVlll.-l‘ll“|"l5t"l' HI“ l.ltill'l' HF INI'RHANING IN'II‘INSI'I'Y

“N THE RATE Ul" (lltOW'l‘lI.

I Intensity of light. Rate of growth. i ” (Normal) 0'47 In, per sec.

l IllllI ... ... “'23 ,1 v

i 2 ,, . .. 0-17 ,L ,,

} 1i .. ‘ 0'10 ,1 ,,

I ‘l ,, Arrest of growth.

E.1:pm'inu'nl 81.—-—The continued effect of light of moderate intensity in bringing about increasing retardation of growth