36 OIECUMNUTATION OF SEEDLINGS. Chap. L 



and at last dropped off. Their rising at night so as to stand 

 almost vertically appears to depend largely on temperature; 

 for when the seedlings were kept in a cool house, though they 

 still continued to grow, the cotyledons did not become vertical 

 at night. It is remarkable that the cotyledons do not generally 

 rise at night to any conspicuous extent during the first four or 

 five days after germination; but the period was extremely 

 variable with seedlings kept under the same conditions; and 

 many were observed. Glass filaments with minute triangles of 

 paper were fixed to the cotyledons (li mm. in breadth) of two 

 seedlings, only 24 h. old, and the hypocotyl was secured to a 

 stick; their movements greatly magnified were traced, and they 

 certainly circumnutated the whole time on a small scale, but 

 they did not exhibit any distinct nocturnal and diurnal move- 

 ment. The hypocotyls, when left free, circumnutated over a 

 large space. 



Another and much older seedling, bearing a half-developed 

 leaf, had its movements traced in a similar manner during the 

 three first days and nights of June ; but seedlings at this age 

 appear to be very sensitive to a deficiency of light; they were 

 observed under a rather dim skylight, at a temperature of 

 between 16° to 174° 0. ; and apparently, in consequence of these 

 conditions, the great daily movement of the cotyledons ceased 

 on the third day. During the first two days they began rising 

 in the early afternoon in a nearly straight line, until between 

 6 and 7 p.m., when they stood vertically. During the latter 

 part of the night, or more probably in the early morning, they 

 began to fall or open, so that by 6.45 a.m. they stood fully 

 expanded and horizontal. They continued to fall slowly for 

 some time, and during the second day described a single 

 small ellipse, between 9 a.m. and 2 p.m., in addition to the 

 great diurnal movement. The course pursued during the 

 whole 24 h. was far less complex than in the foregoing case of 

 Cassia. On the third morning they fell very much, and then 

 circumnutated on a small scale round the same spot ; by 8.20 

 p.m. they showed no tendency to rise at night. Nor did the 

 cotyledons of any of the many other seedlings in the same pot 

 rise ; and so it was on the following night of June 5th. The 

 pot was then taken back into the hot-house, where it was exposed 

 to the sun, and on the succeeding night all the cotyledons rose 

 again to a high angle, but did not stand quite vertically. On 

 each of the above days the line representing the great nocturnal 



