Yellow and Orange 



wide midrib. After what we learned when studying the bar- 

 berry and the prickly pear cactus, for example, about plants that 

 choose to live in dry soil, it is not surprising to find that this is a 

 lower, less leafy, and more hairy plant than the moisture-loving 

 tall lettuce. 



An European immigrant, naturalized here but recently, the 

 Prickly Lettuce (Z,. Scariola) has nevertheless made itself so very 

 much at home in a short time that it has already become a trouble- 

 some weed from New England to Pennsylvania, westward to 

 Minnesota and Missouri. But when we calculate that every 

 plant produces over eight thousand fluffy white- winged seeds on 

 its narrow panicle, ready to sail away on the first breeze, no 

 wonder so well endowed and prolific an invader marches triumph- 

 antly across continents. The long, pale green, spiny-margined, 

 milky leaves, with stiff prickles on the midrib beneath, are doubly 

 protected against insect borers and grazing cattle. 



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

 See how its leaves all point to the North as true as the magnet." 



While Longfellow must have had the coarse-growing, yellow- 

 flowered, daisy-like prairie rosin-weed {Silphium laciniatum) in 

 mind when he wrote this stanza of " Evangeline," his lines apply 

 with more exactness to the delicate prickly lettuce, our eastern 

 compass plant. Not until 1895 did Professor J. C. Arthur discover 

 that when the garden lettuce is allowed to flower, its stem leaves 

 also exhibit polarity. The great lower leaves of the rosin-weed, 

 which stand nearly vertical, with their faces to the east and west, 

 and their edges to the north and south, have directed many a 

 traveller, not from Acadia only, across the prairie until it has earned 

 the titles pilot-weed, compass or polar plant. Various theories 

 have been advanced to account for the curious phenomenon, some 

 claiming that the leaves contained sufficient iron to reader them 

 magnetic — a theory promptly exploded by chemical analysis. 

 Others supposed that the resinous character of the leaves ma^le 

 them susceptible to magnetic influence ; but as rosin is a non-con- 

 ductor of electricity, of course this hypothesis likewise proved 

 untenable. At last Dr. Asa Gray brought forward the only sensi- 

 ble explanation : inasmuch as both surfaces of the rosin-weed 

 leaf are essentially alike, there being very nearly as many stomata 

 on the upper side as on the under, both surfaces are equally sensi- 

 tive to sunlight ; therefore the leaf twists on its petiole until 

 both sides share it as equally as is possible. While the polarity 

 of the prickly lettuce leaves is by no means so marked. Dr. 

 Gray's theory about the rosin-weed may be applied tp them as 

 well. 



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