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 7iieters has, therefore, an effective intensity 

 of 1 H- 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 



Lepidium sativum, hypocotyl .... 0.25-0.11 



Pisum sativum, epicotyl 0.11 



Phaseolus multiflorus, epicotyl .... 0.11 



Vicia faba, epicotyl 0.16 



Helianthus animus, 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 fLunularia biennis . limit below 0.00033 



*Vicia sa,tiva 0.0026 



Salpiglossus sinuata, Keseda odorata, Iberis forestieri 0.004 to 0.16 

 Mirabilis jalappa, * Helianthus animus, *Dianthus chinensis 0.016 

 * Xeranthemum aimuum, *Raphanus sativus, * Helichrysum 

 monstrosum, * Capsicum annuum, Cynoglossum offici- 

 nale 0.016 to O.OG 



