2 THE POLARITY OF THE COMPASS PLANT. 
from Texas on the south to Iowa on the north, and from Southern 
Michigan on the east to three or four hundred miles west of Mis- 
souri and Arkansas ; its chief habitat being rich prairie land. * 
At the same meeting, Dr. Gray stated that ‘‘ there were plants 
then growing in the Botanic Gardens here, and these did not pre- 
sent the edges of their leaves north and south, or in one plane 
more than another.” He thought ‘‘ that the hypothesis of electri- 
cal currents was hardly probable, as rosin was a non-conductor of 
electricity ; but that it was due to the fact that the leaves were 
inclined to be vertical, and the direction of their edges north and 
south was the one in which their faces would obtain an equal 
amount of sunlight.” 
The statement of General Alvord was confirmed by the Rev. 
Mr. Morris, ‘‘ who had observed the fact while running lines for 
surveys on the prairies.” 
-= At the nineteenth meeting of the American Association for 
the Advancement of Science, Rev. Dr. Hill presented a paper 
on ‘“‘ The Compass Plant,” in which he gives additional evidence 
for the truth of General Alvord’s statement. + 
In November, 1870, Dr. Gray received a letter from Mr. Charles 
E. Bessey, of the Iowa State Agricultural School, in which he 
Says: ‘fwe have the curious ‘Compass Plant, S. laciniatum, 
growing in great abundance throughout all this region. The 
polarity of its leaves is very marked. Use is made of it by the 
settlers when lost on the prairies in dark nights. By feeling the 
direction of the leaves they easily get their bearings.” 
From the record of these observers there can be little doubt that 
the leaves on the prairies do assume a meridional bearing; and 
the cause assigned for this by Dr. Gray is undoubtedly the correct 
one, viz.: that both sides of the leaf are equally sensitive to 
light. It only remains to be shown what renders its two sides 
thus equally sensitive. It is well known that the two sides of a 
leaf usually differ in structure, that the number of stomata, or 
breathing-holes, is much greater on the under than the upper sur- 
face; and that the tissue of the upper is denser than that of the 
lower stratum. As the two surfaces of the leaf of S. laciniatum ap- 
peared somewhat alike, Dr. Gray suggested that it would be well to 
ee T . 
*For his paper see the Proceedings of the Second Meeti A 
si eeting of the American Asso- 
ciation for the Advancement of Science. * j ei 
For an abstract of his paper see the AMERICAN NATURALIST, Vol. IV, p. 495. 
