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 



