122 MOVEMENTS OF CURVATURE 



an increase of temperature produced a greater curvature in tendrils than a corre- 

 sponding decrease. In the case of thermonastic flowers, however, if the same 

 peculiarity were always shown, it should be possible, by repeated equal and slight 

 rises and falls of temperature, to make the flower become fully expanded at com- 

 paratively low temperatures. 



The sensation we experience on passing from darkness into intense light is not 

 the same as is produced by the reverse procedure, and the same may apply to plants. 

 Indeed, certain micro-organisms show a different response to increases of illumina- 

 tion to that produced by decreases. In addition, many organisms are able to 

 withstand sudden increases in the concentration of the surrounding medium, whereas 

 corresponding decreases may cause them to burst. Finally, very many chemical 

 changes which are induced by rises of temperature or of illumination are not 

 reversible. 



An analogy is afforded by two metal thermometers of which one responds 

 more rapidly than the other, and which are so arranged that, when warmed, contact 

 is made, and an electric bell-circuit completed when a certain temperature is reached, 

 whereas a fall to the same temperature produces no contact or electrical excitation. 

 In addition, a clock strikes when the hands are moved round in the normal way, but 

 not when they are turned in the opposite direction. 



In the case of the mainly thermonastic flowers of Crocus and Tulipa 

 a slight rise of temperature is sufficient to overcome the tendency to closure 

 induced by darkness, whereas a pronounced fall of temperature is unable 

 to reverse the daily opening movement of the mainly photonastic flowers 

 of Nympkaea alba, Oxalis rosea, Leontodon hastilis and Taraxacum offici- 

 nale '. Similarly, many photonastic flowers do not open at low temperatures 

 such as 1 C. to 3 C., or only experience a slight temporary or permanent 

 opening movement when illuminated under these conditions. Further, 

 many flowers which open early and only close in the evening may remain 

 open only for a short period of the day under special conditions 2 . The 

 heads of Leontodon hastilis and of Taraxacum officinale may open little 

 or not at all during the day if kept during the day and previous night 

 at a temperature of 3 to 4 C., and may open in the evening in darkness 

 as the result of the inductive action of the previous illumination when 

 the temperature is raised to about 20 C. 3 



In addition to these factors the influence of the turgor upon the power 

 of reaction may cause the daily rhythm of the sleep-movements to ex- 

 perience certain modifications which may in some cases become extremely 

 pronounced, while the times of opening and closing of flowers may 



1 Pfeffer, Physiol. Unters., 1873, pp. 195, 206; Period. Bewegungen, 1875, p. 133. 

 a Cf. Oltmanns, Bot. Ztg., 1895, pp. 31, 50. Oltmanns puts forward,, however, a one-sided 

 interpretation of the origin of the early-closing movement. 

 3 Pfeffer, Physiol. Unters., 1873, p. 197 ' 



