210 CHAP. XIX. RECURRENT LIGHT AND DARKNESS 



simultaneous records under identical external conditions 

 of leaves of different age of the same plant, or of leaves 

 of three different species. I have for several years past 

 taken records of numerous plants and at all seasons of the 

 year. The plant's autograph is often so characteristic that 

 it is possible to name it by mere inspection of its daily record. 

 The Thermograph. — For obtaining a continuous record of 

 diurnal variation of temperature, I use a compound strip, 

 T, made of brass and steel. Variation of temperature 

 induces a curvature of the compound strip, which is recorded 

 by means of an attached lever. The oscillation of the plate 

 takes place once in 15 minutes, and the successive dots 

 thus produced give time records of the diurnal curve. The 

 record thus consists of a series of dots. An additional 

 device sometimes employed is to make the plate oscillate 

 three times in rapid succession at the end of each hour ; 

 the hourly dot is thus thicker than others. The movement 

 of the plant-organ corresponding to any particular variation 

 of temperature at any period may thus be easily deter- 

 mined. I shall now give a typical example of diurnal 

 movement induced by variation of light and darkness. 



Response of the Leaflet of Cassia alata 



These leaflets remain tightly closed during the night, but 

 from early morning onwards they begin to open and remain 

 widely spread out throughout the day. The problem is to 

 find out the relative importance of variation of temperature 

 and of light in the diurnal movement of the leaflets. 



In the daytime the light is increasing till midday ; 

 there is, on the other hand, a rapid decline of light after 

 5 P.M. and uninterrupted darkness during the night. As 

 regards temperature there is a continuous rise from morning 

 till the thermal noon, after which the fall of temperature 

 is continuous till next morning. The opening of the leaflets 

 in the daytime will therefore be due to the summated effects 

 of rising temperature and increasing light ; the closure in 



