48] CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 213 



c. Irritability to Variations in the Intensity of Light (Paratonic 

 Effect of Light ; p. 162). This is exhibited in a marked manner 

 by the majority of motile members, more especially leaves. When, 

 for instance, the intensity of the light is diminished, the perianth- 

 leaves of many flowers and the foliage-leaves and cotyledons of 

 many plants perform movements which are termed nyctitropic or 

 sleep-movements (see p. 173). Thus, the flowers close; and the 

 foliage-leaves change their position in various ways, assuming 

 what is known as the nocturnal position, so that thsy no longer 

 present the surface, but the margin of the blade to the sky. Con- 

 versely, when flowers or leaves which have assumed the nocturnal 

 position are exposed to light, or to brighter light than before, they 

 resume their normal expanded (diurnal) position. 



Another remarkable manifestation of this irritability is that 

 movements of variation in some cases, and movements of growth 

 in most cases, are retarded or arrested by exposure to light of a 

 sufficient intensity. 



In illustration of the effect of light upon movements of varia- 

 tion, it may be stated that certain members, such as the leaves 

 of the Sensitive Plant, which perform spontaneous movements of 

 variation, are unable to do so when exposed to bright light : 

 under this condition the leaves become fixed, as it were, in the 

 diurnal position. This is not, however, the case with all mature 

 motile members : for instance, the movement of the lateral leaf- 

 lets of the Telegraph-plant (Desmodium gyrans) continues even in 

 bright sunlight. 



The paratonic action of light on movements of growth is strik- 

 ingly exhibited in various ways. It is well demonstrated by etio- 

 lated plants (see p. 162), that is, by plants which have been kept 

 in darkness for some considerable time. A characteristic feature of 

 etiolated shoots is the excessive length of their internodes, as com- 

 pared with those of a shoot which has been growing for the same 

 period exposed to the normal alternation of day and night. This ex- 

 cessive elongation in darkness which occurs as a rule in all radial 

 and isobilateral members which usually grow exposed to light is 

 the result of the absence of the retarding paratonic action of light. 



The effect of the paratonic action of light can also be estimated 

 by direct measurement of the growing member. As the result of 

 a great number of comparative measurements, it has been found, 

 in regard to members of all kinds, that the rate of growth is more 

 rapid in darkness than in light. 



