MOVEMENTS OF CURVATURE 97 



PART IV 



PHOTONASTIC, THERMONASTIC, AND HYDRONASTIC CURVATURES 



SECTION 21. General. 



Since the growth of the different cells and tissues of an organ is 

 unequally affected by temperature, light, and the percentage of water, 

 physiologically dorsiventral organs are often caused to perform thermonastic, 

 photonastic, or hydronastic curvatures by variations in one of the above 

 factors. These curvatures, though often trifling in amount, may be in many 

 cases pronounced, as in the case of the daily movements l dependent upon 

 variations in the intensity of the light, or upon changes of temperature. 

 Instances of these movements are afforded by those flowers which open and 

 close at definite periods of the day, and by those leaves which perform sleep 

 movements at night when the light is feeble, or at midday when it becomes 

 intense. In such cases the organ assumes a position best suited to the 

 external conditions, and within certain limits the amount of movement 

 corresponds to the degree of change in the external conditions, such as 

 illumination, temperature, or supply of water. At low temperatures or 

 under feeble illumination the peduncles of certain plants curve downwards 

 instead of being erect, while in other cases the foliage or floral leaves remain 

 pressed together, so that the flower of such a plant under these conditions 

 becomes cleistogamous and never opens. Such flowers may be said to be 

 facultatively cleistogamic. 



It often happens that during these aitionastic movements the curvature 

 is at first excessive, so that the ultimate position of equilibrium is only 

 attained after a few oscillations. Thus a sudden rise of temperature causes 

 the flowers of Crocus and Tnlipa to open widely at first, and this is followed 

 by a gradual assumption of the less expanded position which they maintain 

 so long as the new conditions remain unaltered. The same progress of the 

 reaction can be traced when the temperature is lowered, if by removing 

 five of the perianth-segments the remaining one is allowed to perform its 

 full amplitude of movement. When all the segments are present they press 

 against one another, and so prevent any movement in excess of that required 

 to close the flower. Similar results are obtained by illuminating or darken- 

 ing flowers and foliage-leaves capable of photonastic reaction. Hence it may 

 happen especially in the cases of foliage-leaves that the change from light 



1 Since the term ' tropism ' is reserved for curvatures produced by unilateral stimuli, it becomes 

 necessary to change the term ' nyctitropic' used by Darwin (The Power of Movement in Plants, 

 1880, p. 281) into that of ' nyctinastic.' 



PFEFFER. Ill 



