1898] NUTATION OF HELIANTAUS ANNUUS 493 
are taken only in the morning. The nodding of the heads takes 
place about the time when the ligulate florets have all expanded ; 
sometimes, however, it begins earlier. When the disk nods 
toward the northeast, the bracts of the head are brought with 
their outer surfaces into the most advantageous position for the 
process of photosyntax, and can thus continue to manufacture 
_ food which can no longer be transferred so readily through the 
elongated and hardening stem. This is of great advantage to 
the developing seeds of the large disk. There is also another 
advantage. The disk is turned away from direct sunlight, bring- 
ing the conditions to an optimum for the processes of pollination 
and fertilization. 
Meehan‘ has observed an eastward turning of the heads of 
Helianthus mollis. He states that it has a southeasterly face on 
opening, and that it turns not with the sun but eastwardly as the 
head progresses toward maturity. 
HELIANTHUS RIGIDUS. 
During the summer of 1897, observations were also made on 
the nutation of Helianthus rigidus. Nutation in this plant is even 
More marked than in H. annuus. Young leafy stems and some 
With young heads were observed to nutate from 70° to 80° east 
in the morning and go® west in the evening. The point of bend- 
mg is quite low, usually about eight inches from the tip. 
Because of the naked and slender stems which bear the heads 
aid the Vigor with which they nutate, this plant should be an 
*xceedingly favorable object for the study of curvature in stems, 
since it could be manipulated almost as readily as can the roots 
of seedlings, 
CoLumnus, Onto. 
*Bor, Gaz, 9: 49-50. 1884. 
