628 On the Compass Plant [August, 



Compelled to move on a Vertical axis, the leaf can have no upper 

 and no lower face, and thus as they struggle toward the light 

 they face the rising and the setting sun for a position of stable 

 equilibrium. This is facilitated by the number of stomata on each 

 face being equal. If the glare of mid-day should attract one 

 face toward the south, with plane of the leaf east and west, it 

 would be a position of unstable equilibrium. Some of the ob- 

 servers have, in rare instances, found exceptions in which this 

 last named posture of the leaf was found. 1 But it could not 

 remain long in that situation; once diverted, it would settle 

 finally into the meridional position as that of stable equilibrium. 

 This action of light in its full effect is of course best exhibited on 

 the open prairies, where both faces would have the equal light of 

 the sun during the day. 



But in the case of the plants grown by Dr. Gray in his botani- 

 cal garden at Cambridge, they are near houses and trees, and not 

 ia a position assimilated to their native region on the prairies. 

 Their failure to show the property was a fact stated by Dr. Gray 

 in 1849 before the American Association when my paper was 

 read, and it was the cause of his contradiction of the existence of 

 the property in the edition of his " Botany of the Northern U. S. 

 for 1846." Down to the present day these plants in his garden 

 do not show the peculiarity, evidently because they do not, as on 

 the prairies, have the equal light of the sun during the morning 

 and the afternoon. 



Thus I was fully prepared to expect the result of the experi- 

 ment by Stahl in Germany, referred to in the number for Febru- 

 ary, 1882, of the American Journal of Science. It was per- 

 formed not on the Silphium lacmiatum, but on the Lactuca scariola, 

 and it is but just to Dr. George Engelmann, of St. Louis, to say, 

 that he made known the existence of polarity in that plant in 

 August, 1878, to the American Association at St. Louis, also in 

 the number of the Gardeners Chronicle, London, for 26th Febru- 

 ary, 1 88 1. 



Stahl states that he " took two plants of the Lactuca scariola 

 growing in pots and placed one where it would be exposed to 

 direct sunlight from 10 until 3, and kept in the dark for the rest 



