66 BOTANY. [CHAP. n. 



the rays of the red end of the spectrum, no attempt 

 at twining will take place, but the plant, if previously 

 yellow, will soon become green, and thus continue to 

 grow, as its chlorophyll will enable it to assimilate. If 

 a similar seedling be exposed to the rays of the violet 

 end it will commence to twine round its support, but 

 not being able to develop chlorophyll will soon perish, 

 thus proving that although all the rays of light are of 

 service to plants, yet special kinds of work depend 

 entirely on the influence exerted by particular rays. 



Heliotropism. Paradoxical as it may appear in face 

 of the above statements, it is nevertheless a fact that 

 light retards the growth of plants, or in other words, 

 when plants are unequally exposed to light the side with 

 least light grows fastest. This is clearly shown in the 

 case of plants grown in a window, which always bend 

 towards the light, not because, as usually believed, they 

 like the light and are bending towards it, but, on the 

 contrary, because the shaded side of the stem grows 

 much faster than the one exposed to the bright light, 

 and consequently becomes longer and convex, the 

 shorter side becoming concave. If a mirror, or even a 

 sheet of white paper is placed behind a plant growing in 

 a window so that the light is reflected on to its dark 

 side it continues to grow erect, whereas another plant 

 of the same kind not so provided will bend towards the 

 light. Plants that are influenced in this particular way 

 by light are said to be lieliotropic, and when the bending is 

 towards the light, positively heliotropic. A few plants, 

 as the common ivy, when unequally lighted, bend away 

 from the light, owing to the side exposed to most light 

 growing fastest. This habit is of value to the ivy, as 



