v.] COMPASS PLANT. 117 



would involve a certain amount of waste; but in 

 the black Poplar, as Fig. 73 shows, the leaves when 

 hung in their natural position are quite detached from 

 one another. 



Another interesting case of a species with vertical 

 leaves is the Prickly Lettuce {Lactuca scariold), while 

 those of other species of Lettuce {L. muralis and 

 L. virosa) are horizontal. With this position of the 

 leaves is connected another peculiarity, especially 

 well marked in the so-called " compass " plant of the 

 American prairies {Silphium laciniatum), a yellow 

 Composite not unlike a small Sunflower, and which 

 is thus named because the leaves turn their edges 

 north and south. This has long been familiar to the 

 hunters of the prairies, but was first mentioned by 

 General Alvord, who called Longfellow's attention 

 to it, and thus inspired the lines in " Evangeline : " 



" Look at this delicate plant, that lifts its head from the meadow, 

 See how its leaves are turned north, as true as the magnet ; 

 This is the compass flower, that the finger of God has planted 

 Here in the houseless wild to direct the traveller's journey 

 Over the sea-hke, pathless, limitless waste of the desert." 



The advantage of this position, and consequently 

 the probable reason for its adoption, is that in con- 

 sequence of it the two faces of the leaf are about 

 equally illuminated by the sun ; and in connection 

 with this we find that the structure of the leaf is 

 unusual in two respects. The stomata are about 

 equally abundant on both surfaces, while pallisade 

 cells, which are generally characteristic of the upper 

 surface, are in this species found on the lower opr 

 also. 



