562 LIFE MOVEMHljfTS IN PLaKTS 



variation are far more delicate than ordinary thermo- 

 meters. 



Effect of restrict''d pliahiliti/ of the organ. — A leaf is 

 more pliable in one direction than in the other. The 

 pulvinus of Mimosa, for example, allows a greater amount 

 of bending downwards than upwards ; in consequence of 

 this the leaf in its fall becomes almost parallel to the 

 internode below ; the up-movement is, however, far more 

 restricted. The leaf in its most erect position still makes 

 a considerable angle with the internode of the stem above 

 it. If the leaf-stalk of a plant be restricted in its rise 

 the erectile movement at night will reach a limit, and 

 the top of the curve will remain flat. This is seen 

 illustrated in the record of the leaf of Croton (Fig. 202), 

 which attains its maximum erection at 9 p.m. and the 

 subsequent curve remains flattened till 7 a.m. ; after this 

 thi) leaf begins to execute its downward movement. In 

 other cases, the range of up-movement is very great 

 and the plant-organ erects itself continuously till morn- 

 ing. In certain cases the impulse of up-movement carries 

 the organ beyond the stable position of equilibrium ; 

 after this the leaf begins to retrace its path slowly ; the 

 down-movement due to rise of temperature is, however, far 

 more abrupt, and e9,sily distinguishable from the previous 

 slow return. 



It will thus be seen that though the diurnal record 

 consists of an alternating up and down curve, yet these 

 minor characteristics or ' personal equation' of the plant 

 confers on the record a certain stamp of individuality. 



Effect of age. — In the floral leaves of Nymphcea the 

 thermonastic movement is of positive sign ; that is to say^ 

 an erection of the petal during rise, and a fall during the 

 lowering of temperature. The corresponding movement of 

 leaves would therefore be an erection of the leaf in 



