THE POLARITY OF THE COMPASS PLANT. 3 
examine the leaf microscopically in order to see if it corresponded 
with ordinary leaves in the above respects, or with truly vertical 
leaves, the two surfaces of which are usually similar or nearly so; 
also to compare with it the leaves of other species of Silphium, 
in which no tendency to assume a north and south position is 
shown. The species observed were the S. laciniatum, or Compass 
Plant in question, S. perfoliatum, S. compositum and S. terebinthi- 
naceum; the magnifying power used was about four hundred di- 
ameters; and the results obtained may be tabulated as follows : 
Average’number of stomata in the field of 
the microscope at one time, on the 
NAME OF SPECIES. > 
UPPER SURFACE. | UNDER SURFACE. 
S. laciniatum, vers ene wee ; 20 20 
S perfoliatum s si oe enile a 10 30 
S. compositumy, ans veg eR e 3° 9 
S. terebinthinaceum,. ... +. +4 10 20 
The cellular structure of the leaf of S. laciniatum, on making 
a traverse section, appeared to be homogeneous throughout; but, 
in the herbarium-specimens this could not be determined with cer- 
tainty. For this, and for more extensive comparison of the sto- 
mata of the two surfaces, further examinations should be made in 
summer upon the fresh plant. 
But the observations here recorded- appear to show: that the 
meridional position of the edges of the leaf is to be explained by 
the structure of the two surfaces, which being identical, at least 
in the important respect of the number of the stomata, seek an 
equal exposure to the light ;—the mean position of equal exposure, 
in northern latitudes, being that in which the edges are presented 
north and south, the latter to the maximum, the former to the 
minimum of illumination. 
NoTE.—In Longfellow’s reference to this plant, in “ lnm or 7 
ee at thia Aallaata nt, that lifta ita h Af 
See how its leaves all ern to the north, as true as the aguas 
ot is the a cise wih, mar the finger of God has suspended 
th HL hT ws oy 
Over p t waste of the desert,” > 
it is enrions ti } } } . pi a ddi a 
and stout plant. — Eps. 
pect of this coarse 
