Dec, 1902.] Compass Plants of Ohio. 333 



COMPASS PLANTS OF OHIO. 



Harriet G. Burr. 



The name ' ' compass plant ' ' was first given to Silphhc^n 

 Jaciniatiim, commonly called Compass plant or Rosin-weed, one 

 of the famil}- Compositae 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 cloud}- 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 exceedingl}" injurious to the plant and might cause its 

 death. To avoid this the leaves twist so that they 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 laciniatuni L. Compass-plant, Rosin-weed. 



Erigeron Philadelphicus 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 Philadelphiacus are very conunon. 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 

 laciniatuni is foimd on the prairies in the northwestern part of 

 the State. 



