19a. Leaves opposite or in whorls of four 119a, in =COMPOSITAE=, p. 123.
19b. Leaves in whorls of 5-12 =CERATOPHYLLACEAE=, p. 34.
20a. Plants small, flattened, rounded or ovate, without distinction of stem and leaf; the whole plant floating on or near the surface =LEMNACEAE=, p. 9.
20b. Plant attached to the soil, with differentiated stem and leaves --21.
21a. Leaves on long slender stalks, which bear also a cl.u.s.ter of slender tubers near the leaf-base; leaves heart-shape 1b, in =GENTIANACEAE=, p. 88.
21b. Leaf-stalks without a cl.u.s.ter of tubers --22.
22a. Leaves all basal --23.
22b. Stem-leaves present --24.
23a. Leaves parallel-veined 1b, in =ALISMACEAE=, p. 3.
23b. Leaves net-veined 1b, in =NYMPHAEACEAE=, p. 35.
24a. Leaves opposite or whorled --25.
24b. Leaves alternate --26.
25a. Leaves less than 2 cm. long =CALLITRICHACEAE=, p. 68.
25b. Leaves more than 2 cm. long 1b, in =NAJADACEAE=, p. 2.
26a. Leaves attached by the center to the stalk 2b, in =NYMPHAEACEAE=, p. 35.
26b. Leaves attached by the margin --27.
27a. Leaves parallel-veined --28.
27b. Leaves net-veined, with a single mid-vein; lanceolate or elliptical in outline 29b, in =POLYGONACEAE=, p. 28.
28a. Leaves not over 2 dm. long 1b, in =NAJADACEAE=, p. 2.
28b. Leaves very long and gra.s.s-like 14a, in =GRAMINEAE=, p. 5.
29a. Brown, yellow, or white plants, without green color --30.
29b. Plants with normal green color, at least in some parts --33.
30a. Stemless and leafless plants, consisting of flowers only and partly underground 4a, in =ARACEAE=, p. 8.
30b. Stems climbing on other plants 1b, in =CONVOLVULACEAE=, p. 91.
30c. Stem and flower-stalks erect, not climbing --31.
31a. Corolla regular; stamens 6-12 1a, in =ERICACEAE=, p. 84.
31b. Corolla irregular --32.
32a. Sepals and petals each 3; flowers in simple racemes 5a, in =ORCHIDACEAE=, p. 16.
32b. Sepals 5; corolla of united petals =OROBANCHACEAE=, p. 106.
33a. Stem thick and fleshy, leafless, th.o.r.n.y =CACTACEAE=, p. 77.
33b. Stem not th.o.r.n.y --34.
34a. Leaves none --35.
34b. Leaves reduced to small scales --40.
34c. Leaves thick and fleshy --44.
34d. Leaves hollow --45.
34e. Leaves small, all basal, bearing large glandular hairs on the upper surface =DROSERACEAE=, p. 48.
35a. Stem none, the flowers appearing at or partially beneath the surface of the soil 4a, in =ARACEAE=, p. 8.
35b. Stem present --36.
36a. Stem freely branched 2a, in =LILIACEAE=, p. 10.
36b. Stem unbranched, except possibly in the flower-cl.u.s.ters --37.
37a. Flowers greenish or brownish, without obvious colored petals --38.
37b. Flowers with conspicuous white or colored petals --39.
38a. Each flower with 6 small chaffy petals 4, in =JUNCACEAE=, p. 10.
38b. Petals none; each flower in the axil of a single chaffy bract 2a, in =CYPERACEAE=, p. 7.
39a. Flowers regular 34a, in =LILIACEAE=, p. 11.
39b. Flowers irregular 10a, in =ORCHIDACEAE=, p. 16.
40a. Stem erect, unbranched or with one or two branches only --41.
40b. Stem freely branched --42.
41a. Plants of moist soil, with opposite scales; corolla regular, with 4 petal-like lobes 1a, in =GENTIANACEAE=, p. 88.
41b. Swamp plants, with a few alternate scales; corolla irregular =LENTIBULARIACEAE=, p. 105.