538 LIFE MOVEMENTS IN PLANTS 



numerous plants at all seasons o£ the year. The autograph 

 of the plant is often so characteristic that it is possible 

 to name it by mere inspection of its daily record. 



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 the attached lever. The oscillation of the plate 

 takes place once in fifteen 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 makes 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, 

 corresponding to the particular variation of temperature 

 at any period, may thus be easily determined. I shall 

 now give a typical example of diurnal movement induced 

 by variation of light and darkness. 

 DIURNAL MOVEMENT OF THE LEAFLET OF Cassici alcita. 



The leaflet of Cassia niata exhibits a movement of 

 opening in the morning, and it remains outspread 

 throughout the day. It then begins to close before 

 evening and remains closed throughout the night. The 

 problem before us is to find out the relative import- 

 ance 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 at night. As regards 

 temperature there is a continuous rise from morning till 

 the thermal noon at 2 p.m., after which the fall of 

 temperature is continuous till next morning. The open- 

 ing of the leaflets in the daytime may therefore be due 

 to the summated efflects of rising temperature and increas- 

 ing light, the closure, on the other hand, being due to 



