420 MODIFIED CIRCUMNUTATION. Chap VIH 



rate of bending towards it, and the accuracy with 

 which they point towards its source, &c., will be 

 given. Afterwards it will be shown — and this seems 

 to us a point of much interest — that sensitiveness to 

 light is sometimes confined to a small part of the 

 plant; and that this part when stimulated by light, 

 transmits an influence to distant parts, exciting them 

 to bend. 



Heliotropism. — When a plant which is strongly 

 heliotropic (and species differ much in this respect) 

 is exposed to a bright lateral light, it bends quickly 

 towards it, and the course pursued by the stem is 

 quite or nearly straight. But if the light is much 

 dimmed, or occasionally interrupted, or admitted in 

 only a slightly oblique direction, 



Fig. 168. ^. •' ° -^ , .^ ' 



the course pursued is more or less 

 zigzag ; and as we have seen and 

 shall again see, such zigzag move- 

 ment results from the elongation or 

 drawing out of the ellipses, loops, 

 &c., which the plant would have de- 

 scribed, if it had been illuminated 

 fvoni above. On several occasions 

 Beta vnhja •S: civcuranu- we were uiuch struck with this fact, 

 tntion of hypocotyl, de- ^^•hilst observing the circumnuta- 



flected by the light . r> i ■ i i •,• ^^• 



being slightly lateral, tion 01 highly Sensitive seedlings, 

 traced on a horizontal ^hich were Unintentionally illu- 



glass from K.30 A.M. to . •' 



5.30p.m. Directionofthe minated rather obliquely, or only 

 lighted taper by which ^^ successive intervals of time. 



it was illuminated, 



shown by a line joining p^^, jjigtance, two youBg seedlings of 



the first and penultimate i -, • ■, -im 



dots. Figure reduced to J^eta vulgaris were p\a,cea in the jnidale 



one-third of the original of a'Toom with north-east windows, and 



*°^'^- , were kept covered up, except during 



o?^li observation which lasted for only a minute or two ; but the 



rosult was that their hypocotyls bowed themselves to the side, 



whence some light occasionally entered, in lines which were 



