744 PART IV. THE PHYSIOLOGY OF PLANTS. 



tected in various leaves : for instance, a rise of temperature causes 

 certain flowers (e,g. Tulip, Crocus) to open, and a fall of tempera- 

 ture causes them to close : similarly a fall of temperature causes 

 the leaves of such plants as the Sensitive Plant and the Wood- 

 Sorrel (Oxalis Acetosella) to fold up, whereas a rise of temperature 

 causes them to expand (see Fig. 471). 



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

 Effect of Light ; p. 674). 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. 689). Thus, the flowers close; and the 

 foliage-leaves change their position in various ways, assuming 

 what is known as the nocturnal position, so that they 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 (Besmodium gyrans) continues even in, 

 bright sunlight. 



The paratonic action of light on movements of growth is 

 strikingly exhibited in various ways. It is well demonstrated by 

 etiolated plants (see p. 674), 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 

 excessive elongation in darkness which occurs as a rule in all 

 radial and isobilateral members which usually grow exposed to 



