236 Field, Forest, and Wayside Flowers 
around continuously. As the growth of the plant 
causes the ends of its main stem and branches to 
ascend, the motion of each vine is not a series of 
circles, but one close, continuous spiral. This re- 
volving movement is quicker by day than it is by 
night. It is accelerated by the warmth of sun- 
shiny summer noons, and retarded by overcast or 
chilly weather. It is most rapid, generally speak- 
ing, in June heats, when all plant-life reaches its 
uttermost fulness, and it slows down gradually with 
the waning of the year. But all summer long, in 
glad or in gloomy weather, this strange movement 
goes on in growing tips of twining and_ tendril- 
bearing vines. 
Decrease in temperature always has the effect of 
retarding the revolution of a vine-tip. When twin- 
ing plants grow in a window the sprays travel faster 
when in the sunlight, and their speed slackens as 
they twine into the shadow. Thus, a morning- 
glory, living in a sunny window, has been found to 
make a complete revolution in five hours and thirty 
minutes, but the half of its orbit which lay in the 
light was traversed in one hour, and all the rest 
of the time was spent in getting around the semi- 
circle which lay in shadow. 
When a hop begins to grow, the two or three 
