488 



MANUAL OF BOTANY 



receive, for when both are equally well lighted the palisade paren- 

 chyma appears on both sides, while etiolated leaves, as we have 

 seen, do not develop this tissue at all. 



From a general consideration of the facts briefly summarised 

 above we may come to the conclusion that light tends to retard 

 growth in length. This is definitely borne out by actual 

 measurements of growing organs which have been simultaneously 

 cultivated under the two conditions of light and darkness. This 

 action of light is sometimes spoken of as a ' paratonic ' mfiu- 



FiG. 1193. 



Fiff. 1193. yicoluam glattcn. A. Shoots witli leaves expaiideil during the 

 (lay. B. Tlie same in tlie nocturnal position, pointing vertically upwards. 

 (After Darwin.) 



ence. It has been further ascertained that the rays of the 

 spectrum which exercise this influence are those of high re- 

 frangibility, the blue and violet. To these rays the protoplasm 

 seems to be excessively sensitive. No mere physical explanation, 

 such as a dependence on turgidity, or change in mode of nutri- 

 tion, is sufficient to account for the facts observed ; they can 

 only be referred to the power of the protoplasm to respond to 

 the influence of the environment. 



Khythmic differences in the intensity of the illumination to 

 which a plant is exposed, in some cases, exercise a very powerful 



