2] UPON THE DIRECTION OF GROWTH 439 



of one meter from the organism (meter-candle). A flame of one candle 

 power at a distance of five meters has, therefore, an effective intensity 

 of 1 -4- 5' 2 = 0.04: " meter-candles." An ordinary flame of gas, kept at 

 constant pressure by means of a manometer, was employed in the earlier 

 experiments; a " microburner" in the later ones. 



The following table gives the optimum intensity in the case 

 of various seedlings, in units of the meter-candle, at a tempera- 

 ture of 20 to 27 C. and at a humidity of 75 to 77% : - 



TABLE XLV 

 THE OPTIMUM INTENSITY FOR PHOTOTROPISM IN VARIOUS SPECIES OF PLANTS 



Lepidimn sativum, hypocotyl .... 0.25-0.11 



Pisum sativum, epicotyl 0.11 



Phaseolus multiflorus, epicotyl .... 0.11 



Vicia faba, epicotyl 0.16 



Helianthus annuus, hypocotyl .... 0.16 



Vicia sativa, epicotyl 0.44 



Salix alba, etiolated sprout 6.25 



This table shows also the effect of preceding conditions of illu- 

 mination ; the etiolated plant has a very high optimum. 



At an intensity of light above the optimum the phototropic 

 response is less pronounced until, finally, at between 100 and 

 800 meter-candles it disappears. At an intensity below the 

 optimum a similar diminution in response occurs ; but the 

 minimum lies often remarkably low. Thus FIGDOR ('93) has 

 found the minimum to lie for the different species at or just 

 below the following intensities (in meter-candles). The tem- 

 perature was 15 to 24 C., and the humidity between 58 and 

 80%. 



TABLE XLVI 



THE MINIMUM INTENSITY FOR PHOTOTROPIC RESPONSE IN VARIOUS SPECIES OF 



PLANTS 



Lepidium sativum, Amaranthus melancholic us ruber, *Papa- 



ver pseoniflorum, and fLamularia biennis . limit below 0.00033 



*Vicia sativa 0.0026 



Salpiglossus sinuata, Reseda odorata, Iberis forestieri 0.004 to 0.16 

 Mirabilis jalappa, * Helianthus annuus, *Dianthus chinensis 0.016 

 * Xeranthemum annuum, *Eaphanus sativus, * Helichrysum 

 monstrosum, * Capsicum annuum, Cynoglossum offici- 

 nale 0.016 to 0.06 



