Dec., 1902.] 
Compass Plants of Ohio. 
333 
COMPASS PLANTS OF OHIO. 
Harriet G. Burr. 
The name ‘ ‘ compass plant ’ ’ was first given to Silphium 
laciniatum, commonly called Compass plant or Rositi-weed, one 
of the family Composite and found on the prairies of North 
America. It is the best known of this class of plants and affords 
perhaps the best illustration of them. The name originated with 
the hunters on the prairies who observed that the leaves had a 
peculiar vertical position with the edges pointing north and south. 
Even under cloudy skies they were able to make use of this pecu- 
liarity as a guide to directions. 
The vertical position of the leaves is due to the twisting of 
the leaf-blades ; this seems to be confined to those plants which 
prefer open, sunny places and which grow in the warmest parts 
of the year, and to this we may look for an explanation of the 
phenomenon. The full effect of the sun’s rays upon the leaf 
during the hottest part of the day, especially on the prairies 
where the temperature sometimes rises very high, would cause an 
over-heating and an excessive amount of transpiration which 
would be exceedingly injurious to the plant and might cause its 
death. To avoid this the leaves twist so that the} - have what is 
called the “ profile position,” the sun’s rays falling directly upon 
the edges at mid-day and upon the surfaces only at morning and 
evening. It is interesting to note that when compass plants are 
found growing in damp, shady places, the leaves have the ordin- 
ary position, i. e., not vertical or twisted. This placing of the 
leaves horizontally in the shade and vertically in dry, sunny 
places may be seen in many plants, including shrubs and trees, 
though in only a few of them do the leaves have a north and 
south position. The name ‘‘compass plant,” however, is not 
restricted these last. 
The following is a list of our Ohio compass plants : 
Lactuca Scariola L. Prickly Lettuce. 
Lactuca virosa L. Strong-scented Lettuce. 
Silphium laciniatum L. Coinpass-plant, Rosin-weed. 
Erigeron Pliiladelpliicus L. Philadelphia Fleabane. 
Lacinaria scariosa (L.) Hill, Button Snakeroot. 
Grindelia squarrosa (Ph.) Duval. Broad-Leaved Gum-plant. 
Of these, Lactuca virosa (formerly confused with L. Scariola ) 
and Erigeron Philadclphiacus are very common. In the latter the 
twisting of the leaves is especially noticeable in the spring, 
although they do not have a north and south position. Silphium 
laciniatum is found on the prairies in the northwestern part of 
the State. 
