LACTUCA IN IOWA 241 



The leaf-forms of the two plants are strikingly different and ap- 

 pear very uniform throughout our range. L. virosa appeared in 

 Iowa some ten years previous to L. scariola, but the latter is rapidly 

 supplanting it, especially in the central and southern portions of 

 the state, and threatens to become one of our most persistent weeds. 

 L. virosa seems to prefer drier conditions, and is the prevailing 

 form in the arid regions of the west. 



The common garden lettuce L. sativa L. which, at least in our 

 region, does not possess sufficient vitality to exist without cultiva- 

 tion, is not included in this list. It is very closely related to L. virosa 

 and by some is thought to be derived from that species. 



All the species contain an acrid juice which probably accounts 

 for their seldom being eaten by stock. The dried milky juice of 

 several old world species, especially L. virosa, is a mild anodyne or 

 narcotic, and has frequently been used in medicine, under the name 

 Lactucarium. 



The genus Lactiica may be characterized as follows : Leafy 

 herbs, two to ten feet tall, with rather open, panicled heads of yel- 

 low, white, blue or violet flowers, and variously formed alternate 

 leaves ; bracts disposed in two or more rows, the successive inner 

 ones longer. Akenes flat or somewhat thickened, contracted at 

 the summit into a long slender beak, or in the third section decidedly 

 contracted at the summit, or beakless in L. villosa; the dilated sum- 

 mit of the beak or neck bearing a copious pappus of soft capillary 

 bristles, which are white, except in one of our species ; heads six to 

 thirty flowered ;• receptacle flat and naked. 



The name Lactnca is from the Latin Lac, milk, in allusion to the 

 milky juice. 



The species, which number about one hundred, are most numerous 

 in the old world. Besides those here listed five others occur in the 

 Atlantic coast states, one of which, L. saligna, is a recent introduc- 

 tion from Europe, and two other species have been described from 

 the Rocky Mountain region. 



Key to the Species. 

 1 Section, Scariola D. C. Akenes flat, orbicular to linear-oblong, 

 with filiform beak, cauline leaves sagittate-clasping, annual or 



biennial. 



Heads 6 to 12 flowered, akenes linear-oblong, several nerved, 



flowers yellow. 



Leaves mostly entire, midrib usually prickly- 

 setose ^- ^^-^^ 



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