CHAP. IX. SENSITIVENESS TO LIGHT. 465 



therefore conclude that the bending back of plants 

 from a light, when this becomes obscure or is extin- 

 guished, is wholly due to apogeotropism.* 



In our various experiments we were often struck 

 with the accuracy with which seedlings pointed to a 

 light although of small size. To test this, many seed- 

 lings of Phalaris, which had germinated in darkness in 

 a very narrow box several feet in length, were placed 

 in a darkened room near to and in front of a lamp 

 having a small cylindrical wick. The cotyledons at 

 the two ends and in the central part of the box, would 

 therefore have to bend in widely different directions 

 in order to point to the light. After they had become 

 rectangularly bent, a long white thread was stretched 

 by two persons, close over and parallel, first to one and 

 then to another cotyledon ; and the thread was found 

 in almost every case actually to intersect the small 

 circular wick of the now extinguished lamp. The 

 deviation from accuracy never exceeded, as far as we 

 could judge, a degree or two. This extreme accuracy 

 seems at first surprising, but is not really so, for an 

 upright cylindrical stem, whatever its position may 

 be with respect to the light, would have exactly half 

 its circumference illuminated and half in shadow ; and 

 as the difference in illumination of the two sides is 

 the exciting cause of heliotropism, a cylinder would 

 naturally bend with much accuracy towards the light. 

 The cotyledons, however, of Phalaris are not cylin- 

 drical, but oval in section ; and the longer axis was 

 to the shorter axis (in the one which was measured) 

 as 100 to 70. Nevertheless, no difference could be 



* It appears from a reference heliotropically is at the same time 



in Wiesner (' Die Undulirende striving, through apogeotropism, 



Nutation der Internodien,' p. 7), to raise itself into a vertical posi- 



that H. Miiller of Thurgau found tion. 

 that a stem which is bending 



