484 PHYSIOLOGY OF GROWTH. 



some time. In other cases, however, the paratonic effects may be 

 much more obvious. 



The movements of foliage and flower leaves induced by change 

 in illumination, are due to processes of growth, as Pfeffer 1 showed 

 with complete certainty. The change in illumination acts in the 

 same sense on the growth of the basal parts of the foliar struc- 

 tures which cause the movements, but the opposed tissue com- 

 plexes of the organs are not affected with the same rapidity, and 

 hence the nyctitropic movements here under consideration are 

 brought about. The biological significance of the vertical position 

 assumed at night by many foliage leaves, is clearly to be sought 

 in the fact that it prevents a too rapid radiation of heat, which 

 might easily have disastrous consequences for the plants, while 

 the opening and closing of flowers is related to conditions of 

 pollination. 



In various plant structures changes of temperature, like changes 

 in illumination, induce active movements. We have here to do 

 chiefly with flowers, and excellent objects for investigation are 

 flowering specimens of Ttilipa gesneriana, and above all of Crocus 

 vernus (white flowered variety). I took pot plants of Crocus 

 vernus from a room at a temperature of 19 C. into one at a 

 temperature of 5 C. The flowers, at first open, soon closed. If 

 Crocus plants with closed flowers were taken out of a room at a 

 temperature of 5 C. into one at a temperature of 19 C., the 

 flowers speedily opened. The closing movements of the flower 

 leaves on cooling on the one hand, and their opening movements 

 on warming on the other, proceeded so rapidly in my experiments, 

 that their commencement could be seen with the unaided eye 

 within a few minutes of the change of temperature. Crocus 

 flowers react, moreover, to very slight variations of temperature. 

 The movements induced by change of temperature take place 

 in the light as well as in complete darkness. 



That the above-mentioned movements induced by changes of 

 temperature and illumination are the results of processes of 

 growth, can only be determined by somewhat delicate measure- 

 ments. We select Crocus for investigation. Here the zone in 

 which especially the movement of the perianth leaves takes place, 

 lies just above the perianth tube, in the lower fifth of the perianth 

 segments. We cut off the three inner segments, and make fine 

 ink-dots at intervals of about 3 mm. on the outside and inside of 

 the outer segments, along the middle of the zone of greatest 



