VIII. c, 5 Copeland: Growth of Lagerstroemia 297 
all of the more active growing twigs had assumed a more or less 
spiral form, although none of them had made more than a single 
turn. Within the following two days all actively growing twigs 
had drawn out, making a single twist by which the distal part 
of the twig had returned to its original position with regard to 
the vertical. 
The curves assumed during the first day showed that in the 
case of actively growing twigs, gravity and internal tendencies 
are jointly responsible for the curvatures and that the shares of 
the two are on the whole not very unequal. The first curvatures 
executed were performed under the influence of the internal 
directive factors. In two of the active twigs whose position 
was most exactly reversed, epinasty caused the distal part of 
the twig to pass the vertical so that on this part positive geo- 
tropism and epinasty then acted together, in the same direction. 
Except in this case the action of gravity seemed after a few 
hours to be more powerful with regard to the younger parts of 
the twig than did the epinasty. On the older parts epinasty 
retained the ascendency through the day with the result that 
twigs which had almost ceased to grow executed almost the 
usual daily curvature in the usual direction with regard to the 
parent stem, and without any evident regard to the directive 
action of gravity. 
The observations which have just been reported, on the be- 
havior of stems whose natural position with regard to gravity 
had been reversed, furnish a satisfactory explanation of at 
least the one class of torsions, which resulted when the twig 
r 
again became straight, with all but the oldest parts twisted into its 
original position. In this case, it is perfectly obvious that the 
torsion results from the conflict of two opposing directive agents, 
of which, in this case, one is external and one internal. 
Finally, it is of interest to note that the diurnal movements of 
Lagerstroemia have a very obvious biological significance. The 
young parts of twigs growing with such rapidity are inevitably 
delicate and very easily broken. In the day position, they are 
obviously much more exposed to danger from almost every en- 
vironmental factor which might injure them than they are in 
the night position. Mechanical injury due to the wind, and the 
attacks of animals larger than insects, are the most evident 
dangers from which the assumption of the night position affords 
a measure of protection. The movements continue after the 
leaves on the curving part of the stem develop an appreciable 
area, and probably need to be exposed to ordinary illumination 
