1897.] NATURAL SCIENCES OF PHILADELPHIA. 193 



the vertical position is merely the result of vigorous spiral develop- 

 ment? 



"We may get help from the behavior of plants in analogous cases. 

 I observed many years ago, and placed the fact on record in the 

 Proceedings of the Academy, that in going across the prairies of 

 Illinois, the earlier flowers of Helianthus mollis faced the southeast. 

 This I have confirmed by plants in my garden. Not only is it true 

 of this, but of other species of Helianthus. I could travel across the 

 open prairie as well by these flowers as by the leaves of the Com- 

 pass Plant. The opening flowers of many other species of plants 

 are usually in nearly one given direction. Digitalis media of the 

 Old World, has the opening flowers mostly in a direction facing 

 south. I have never found any plant, growing in a fully open place, 

 present any tendency to open its flowers in a northwardly direc- 

 tion. 



Now let us for a moment pass to the anthesis of some very early 

 spring flowers. We will take the Goat Willow, Salix caprea and 

 the Chinese Magnolia Yulan. If the male catkin of this willow be 

 in a position to catch the early morning sun, the catkin has a curve 

 westwardly ; if the aspect is such that it cannot get the sunlight till 

 midday, the curve of the apex of the catkin is northwardly. It is 

 exactly the same with the flowers of Magnolia. A very slight 

 warmth from the sun at that early stage of the season excites 

 growth, and the development of the flower being greater on the 

 warmed side causes the apex to seem to curve in the opposite direc- 

 tion. We may conclude that some such law prevails in the opening 

 of the sunflowers and others. All plants have varying phases of 

 rhythmic rest and growth ; many Composite start their daily growth, 

 as I have shown in various papers, soon after sun-rise. A little 

 extra warmth at this time, would throw the greater growth in the 

 easterly direction. A flower, the period of rest of which ceased at 

 noon time, would have its greatest development encouraged in the 

 westerly line. 



Just how such considerations as these would affect the root-leaves 

 of the Compass Plant is not clear. But if we imagine, as we may 

 from the facts detailed, that growth in the spring is favored in one 

 point more than others, and that there is a favored resting as well 

 as a growing point, we have a good clue to a sound explanation. 

 I feel assured that the final solution of the peculiar position of the 

 leaves in this plant must be sought for in this direction, and not in 

 any peculiar physiological necessity on the part of the plant itself. 



