IRRITABILITY 213 



ferent depths in a reservoir. Taking the numbers present 

 as an index of the suitableness of the illumination at differ- 

 ent depths — a standard not above criticism — the largest 

 number, and inferably the best illumination, are found a 

 few inches below the surface. The accompanying diagram 

 indicates the results of the experiment. On the surface, 

 where there was no absorption of light, and where the 

 organisms could therefore receive most light, the growth ( i.e. 

 the reproduction ) was less than when a thin layer of water 

 absorbed the actinic rays. 



Certain seeds and spores appear to germinate better, and 

 perhaps only, in light. * The European mistletoe ( Visciim 

 album ) , the seeds of some grasses, and the spores of some 

 of the vascular cryptogams are said not to germinate in 

 darkness. In these cases we have the stimulating influence 

 of light upon processes which precede growth ( e.g. respira- 

 tionf ) . After their germination, the influence of light upon 

 growth is the same as in other plants. 



The influence of light upon the rate of growth is evident 

 from the foregoing; light^lowers the growth-rate. If un- 

 equal amounts of light fall on different parts of a growing 

 organ, the growth of the organ will be unequal. By this 

 means the direction of growth of an organ or organism is 

 influenced bj^ light. The stems of plants growing indoors 

 near a window turn toward the light, the leaves spread 

 themselves at right angles to the incident rays, the roots 

 (when visible at all) turn away from the light. The be- 

 havior of the parts of such plants is readily accounted for 

 on the basis of what has already been said. The growing- 

 cells on the side of the stem away from the window receive 

 less light and are less checked in growth than those on the 

 opposite side, and hence push the tip of the stem over to- 

 ward the window. The influence of light upon the direction 

 of growth, though we see that it is due to the effect of light 

 on the rate of growth, is known as heliotropism or photo- 

 tropism. 



* Davenport, C. B. Experimental Morphology, Part II., pp. 423 - 5, 1899. 

 f Kolkwltz, R. fiber den Einfluss des Lichtes auf die Athmung der 

 niederen Pilzen. Jahrb. f. wies. Bot., Bd. 33, 1898. 



