IRRITABILITY. 



433 



which the heliotropic effect ceased to be exhibited was, in 

 very sensitive organs, higher, and in less sensitive lower, than 

 that which, as mentioned in the previous lecture (pp. 380, 

 396) sufficed to arrest growth altogether. 



Wiesner obtained his results by placing the plants at known dis- 

 tances from an artificial source of light of known intensity, and by 

 determining the time at which the first trace of curvature could be 

 detected. 



In the following table his determinations of the upper and lower 

 limits and the optima of intensity of light are given. The unit of inten- 

 sity is that of the normal flame at a distance of one metre. 



Passing, now, to the relative heliotropic effect of rays of 

 different wave-length, it has been ascertained by experiments 

 with light that had passed through absorbent media (coloured 

 liquids or glasses) that the heliotropic effect of the rays of high 

 refrangibility is much greater than that of the rays of low 

 refrangibility (Payer, Dutrochet, Zantedeschi, Sachs). Ex- 

 periments made with the spectrum (Poggioli, Gardner, Guille- 

 min, Wiesner) shew that all the visible rays, excepting the 

 yellow, and, according to Wiesner, the invisible ultra-red and 

 ultra-violet rays, produce heliotropic effects. Wiesner gives 

 the following account of the relative heliotropic effect of the 

 different rays. Sensitive organs, such as the stem of etiolated 

 Vetch-seedlings, curve most strongly at the junction of the 

 ultra-violet and violet rays ; from this point the heliotropic 

 effect diminishes until, in the yellow, it disappears ; it begins 

 to manifest itself again in the orange, and increases until it 

 reaches a small secondary maximum in the ultra-red. The 

 V. 28 



