KEY TO GENERA. THISTLE FAMILY. 349 



Tribe 5. HELIANTHEAE. 



A. Disk-flowers perfect, but sterile. 

 Achenes thick, short, not flattened ; pappus none. 



Achenes merely subtended by the inner involucral bracts. 48. Polymnia. 



Achenes embraced and enclosed by the inner involucral bracts. 



Involucral bracts unarmed. 49. Melampodium. 



Involucral bracts armed with hooked prickles. 50. Acanthospermum. 



Achenes flattened. 



Ray-flowers in 2 or 3 series; achenes falling away free. 51. Silphium. 



Ray-flowers in i series ; achenes adnate to 2 or 3 scales of the receptacle, falling away with them. 

 Rays large, yellow. 



Leaves opposite and basal. 52. Chrysogonum. 



Leaves alternate. 



Pappus none, or of 2 caducous awns. 53. Berlandiera. 



Pappus a persistent irregularly cleft crown. 54. Engelmannia. 



Rays small, the head appearing discoid. 55. Parthenium. 



B. Disk-flowers fertile. 

 * Ray-flowers persistent upon the achenes. 



Achenes compressed, or 3-angled ; leaves entire. 56. Crassina. 



Achenes short, thick; leaves toothed. 57. Heliopsis. 



** Ray-flowers deciduous, or none. 

 t Pappus a cup, or crown, or of a few teeth, awns, or bristles. 



1. Achenes, at least those of disk-flowers, not compressed (except in Ratibida and Phaethusa) ; 

 scales of the receptacle mostly concave, or clasping. 



Scales of the receptacle small, awn-like or bristle-like ; rays white, short. 58. Verbesina. 



Scales of the receptacle broad, larger. 



Involucre of 4 large somewhat united bracts. 59. Tetragonotheca. 



Involucre of several or numerous separate bracts. 

 Receptacle conic, or columnar. 



Ray-flowers fertile, or wanting ; leaves opposite. 60. Spilanthes. 



Ray-flowers sterile, or neutral ; leaves mostly alternate. 

 Rays yellow. 



Achenes 4-angled or terete. 



Achenes 4-angled. 61. Rudbeckia. 



Achenes terete ; leaves cordate-clasping. 62. Dracopis. 



Achenes compressed, winged. 63. Ratibida. 



Rays rose-purple or yellow. 64. Echinacea. 



Receptacle flat, or convex (low-conic in species of Nos. 66 and 68). 



Low fleshy sea-coast shrubs. 65. Borrichia. 



Tall herbs, not fleshy. 



Achenes not much flattened, not winged, nor margined. 66. Helianthus. 



Achenes of disk-flowers flattened and margined, or winged. 



Involucre of a few deflexed bracts. 67. Ridan. 



Involucre of 2 series or more of appressed or spreading bracts. 



Perennials ; bracts erect or appressed. 68. Phaethusa. 



Annuals ; bracts spreading. 69. Ximenesia. 



2. Achenes very flat ; scales of the receptacle flat, or but slightly concave. 



a. Bracts of the involucre all separate. 



Pappus of 2 short teeth or awns, or a mere border, or none. 70. Coreopsis. 

 Pappus of 2-6 awns or teeth, upwardly or downwardly barbed or hispid. 



v Achenes flat, or angled. 71. Bidens. 



Achenes terete ; aquatic, the submerged leaves filiform-dissected. 72. Megalodonta. 



b. Inner bracts of the involucre united to about the middle. 73. Thelesperma. 



tt Pappus of numerous scales. 



Leaves opposite, toothed ; ray-flowers fertile ; rays small. 74. Galinsoga. 

 Leaves alternate, entire. 



Rays large, neutral ; receptacle deeply honey-combed. 75. Endorima. 



Rays none ; scales of the receptacle narrow, rigid. 76. Marshallia. 



Tribe 6. HELENIEAE. 



A. Ray-flowers persistent on the achenes, falling away with them, papery. 



77. Psilostrophe. 



B. Ray-flowers deciduous, or wanting. 

 * Plants not dotted with oil-glands. 



a. Pappus none. 78. Flaveria. 



b. Pappus present (in all our species), of separate scales or bristles. 

 i. Bracts of the involucre petal-like, colored, their margins and apices scarious. 

 Leaves, at least the lower, pinnately parted, or pinnatifid ; rays none ; corolla-lobes of disk-flowers 



ovate. 79. Hymenopappus. 



Leaves entire ; rays present, or none ; corolla-lobes of disk-flowers linear. 80. Othake. 



2. Bracts of the involucre herbaceous, not scarious-tipped, nor petal-like, appressed, or spreading. 

 Receptacle naked. 



Bracts of the involucre appressed. 



Achenes 4-angled, linear or oblong. 81. Picradeniopsis. 



Achenes s-io-ribbed or s-io-angled, top-shaped. 



Involucral bracts separate to the base. 82. Tetraneuris. 



