420 MODIFIED CIROUMNUTATION. 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. ,, ■' ° ■' , P- , ' 



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 

 from above. On several occasions 

 Betavuiga'-is: circumnu- we Were much struck with this fact, 

 tation of hypocotyi, de- whilst observing the circumnuta- 



flected by the light . « i • i i 



being slightly lateral, tion 01 highly Sensitive seedlings, 



dassVom8 3olT*to '^^^^^ ^^re Unintentionally illu- 



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



ii-'wt'^^'inun^iS ^* ^^"'^es^i^e intervals of time. 



fhStldiiruS „ For instance, two young seedlings of 



dots. Figure reduced to ^^^ vulgaris were placed in the middle 



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



^"^^^^ were kept covered up, except during 



each observation which lasted for only a minute or two ; but the 



result was that their hypocotyls bowed themselves to the side, 



whence some light occasionally entered, in lines which were 



