134 



MOVEMENTS OF CURVATURE 



TABLE 4. Leontodon hastilis. Percentage Growth per hour. 



Table 4 shows that after a day's illumination, mainly the outer side of the corolla 

 grew in length during the evening curvature. During the night the flower returned 

 halfway to the day position, owing to the growth of the inner side, and this growth 

 was accelerated by the light at 6 a.m., leading to the assumption of the full day posi- 

 tion by 8 a.m. At 4 p.m. the closing movement begins again, while between 6 and 

 8.45 a.m. the growth of the middle lamella was accelerated. 



SECTION 29. The Mechanics of Variation Movements. 



Most variation movements are photonastic in character, and show 

 a general resemblance to nutation movement except that the curvature 

 is produced by the unequal expansive energy of the turgid tissues instead 

 of by unequal growth. A decrease of illumination produces an increase 

 of the expansive energy in the antagonistic tissues, but this takes place 

 more actively in one-half of the pulvinus than in the other, the tissues 

 of the latter being therefore compressed. Owing to the continued increase 

 of the expansive energy in the compressed half of the pulvinus a partial 

 return to the original position occurs, the fall of illumination exciting 

 a movement in excess of the permanent position adapted to the new 

 constant conditions of illumination. In darkness or in diminished illumina- 

 tion growth in general is accelerated, while the expansive energy of the 

 motile tissues is increased, and to the same extent in both halves of the 

 pulvinus when the leaf returns to its original position. On returning to 

 the previous strong illumination the expansive energy assumes its original 

 value, and possibly a sudden rise of illumination may act as a transitory 

 stimulus and produce an excess of movement. The latter may, however, 

 not be as pronounced as when a fall of illumination occurs, and indeed 

 it may be imperceptible in most cases. 



In constant darkness or illumination the periodic movements are 

 produced by opposed changes of the expansive force of the halves of the 

 pulvinus without any general rise being shown, which indeed is no more 



