462 TRANSFORMATION OF ENERGY 



he found the sporangiophores exhibiting positive phototropism at a distance 

 of 80 cm. from the source of hght (the hght intensity being the equivalent 

 of 8,000 Hefner lamps) ; at a distance of 20-30 cm., on the other hand, with 

 a light intensity equal to about 100,000 Hefner lamps, they showed negative 

 curvature. As a result of these experiments it is obvious that there is a certain 

 intermediate light intensity which has no phototropic effect at all. Indeed 

 Oltmanns' experiments have proved that sporangiophores remain erect if 

 they be farther than 30 cm. from and nearer than 80 cm. to the source of light. 

 An hour after the beginning of the experiment the negative curvatures had 

 become still more apparent, but at a distance of 60-70 cm. no phototropic 

 reaction was induced. 



Each organism may, therefore, be found in one of three different conditions 

 determined by light intensity, viz. (i) a condition of positive heliotropism ; 

 (2) a condition of indifference ; (3) a condition of negative heliotropism. 



Before considering Oltmanns' experiments further in detail it must be 

 first of all emphasized that the heliotropic effect depends essentially on a factor 

 we have not as yet noticed, viz. the intensity of the light. Wiesner, some time 

 before (1878), studied the influence of the intensity of light on heliotropic 

 curvature by illuminating, unilaterally, etiolated seedlings by means of a gas flame 

 of constant but very limited intensity. He placed the plants at different 

 measured distances from the source of light and calculated the intensity of the 

 light which fell on the exposed face of the plant. He noted further the begin- 

 ning of the curvature and the inclination from the vertical which each had 

 attained after four hours. He found that in the case of Vicia sativa, with a light 

 intensity equal approximately to a half spermaceti candle power, curvature was 

 most quickly induced (after seventy minutes), and reached its maximum (to the 

 horizontal position). The increase or decrease of the light afforded a measure 

 both of the time before reaction commenced and of the angle of inclination, 

 conforming with a decrease of the heliotropic stimulus. Quite close approxima- 

 tion to the flame induced no bending nor even growth, a result which at first does 

 not appear to correspond with the results obtained by Oltmanns with Phyco- 

 myces. Closer examination shows that the intensity of the light in Wiesner' s 

 experiments was very low (about 200 candles), while Oltmanns, both in respect 

 of Phy corny ces and of the etiolated seedlings of cress and barley, first saw curva- 

 tures taking place when the intensity of the light was the equivalent of from 

 10,000 to 15,000, likewise 400,000-500,000 and 500,000-600,000 Hefner lamps. 

 In Wiesner' s experiments injurious effects must have been induced by the gas 

 flame, and this exhibits itself in the fact that Wiesner found growth ceased 

 in the indifferent position, which was not the case in Oltmanns' experiments. 

 Wiesner attempted to determine the lower limits of light intensity also, but 

 in place of his results we will quote the more recent experiments of his pupil 

 FiGDOR (1893). This latter investigator found that light intensity approxi- 

 mately equal to 0-002 of normal candle power was no longer sufficient to 

 induce any heliotropic curvature in the seedlings of Vicia sativa. At the same 

 time other plants behaved in a different manner; Lepidiiim sativum, for instance, 

 still exhibited heliotropism with light of 0-0003 candle power, and in Raphanus 

 sativus light of o-oi6 candle power had no longer power to induce heliotropic 

 curvature. 



Summarizing these results we may say that there is a certain minimum 

 intensity of light at which heliotropic curvature commences but that this 

 liminal stimulus varies in different plants ; and that there is also a certain 

 light intensity at which the curvature takes place in the shortest time and 

 exhibits the greatest angle; and, thirdly, a light intensity at which no heliotropic 

 curvature follows, an intensity which Oltmanns regards as the best for the 

 organism or organ in question, i. e. the optimum illumination for its general 

 welfare, while the second intensity referred to above is to be regarded as the 



