This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
In the following summary 520 genera are included. A few must be regarded as British, only because the species have become established here in various ways. The European flora includes a far larger number of genera; and of the British genera none can be regarded as peculiar, so intimately allied is the British insular flora to that of the mainland. Had one included all the genera noticed in this country, embracing a large number of species (1000 or more) whose occurrence is more or less accidental, sporadic, or ephemeral, the list would have been considerably extended. (See Druce's List of British Plants, Oxford.) But many of the plants that have found a place here are little removed from the garden, their place of origin, or similar habitats, and one cannot include here many species that figure in horticultural lists, since they may persist spontaneously for but a year and then disappear. As, however, such plants come, from time to time, to establish themselves widely and tenaciously in these islands, one can no longer ignore their claim to a place in our flora, cosmopolitan and adventitious as a large part of it must be considered. This is the line adopted here.
1. Clematis. - Climbing shrub; leaves opposite; sepals valvate, 4-6; fruit an achene; styles feathery, persistent, many; stamens many.
2. Thalictrum. - Herbaceous plants; involucre o; sepals imbricate, 4-5, petaloid; fruit an achene, without awns; styles and stamens many.
3. Anemone. - Involucre of 3 leaves; sepals, 5-20, petaloid; carpels in head, not bursting, tipped with persistent styles; styles and stamens many; fruit an achene.
4. Adonis. - Involucre o; calyx of 5-8 sepals; petaloid; petals 5-16; no honey glands; carpels awnless, not bursting; stamens and styles many.
5. Myosurus. - No involucre; sepals 5, spurred; petals 5, with tubular claw; receptacle elongated; stamens 5; carpels imbricate; seed pendulous.
6. Ranunculus. - Sepals imbricate, 5, rarely 3; petals 5, rarely more, with honey glands; with or without a scale; carpels not bursting, in head; fruit an achene; seed ascending.
7. Caltha. - Sepals imbricate 5, petaloid, deciduous; no petals; follicles 5-10, many-seeded, bursting; stamens numerous.
8. Trollius. - Sepals 5, imbricate, deciduous, petaloid; petals 5-15, small, clawed; flowers globular; follicle many-seeded.
9. Helleborus. - Sepals 5, green, persistent; petals 8-10, tubular, with long claw, 2-lipped; follicles 3-10, sessile, many-seeded.
10. Eranthis. - Sepals 5-8, petaloid, deciduous; petals 5, small, 2-lipped, tubular, claw long, inner lips short; follicles stalked, 3-8, many-seeded.
11. Aquilegia. - Sepals 5, petaloid, deciduous; petals 5, funnel-shaped, irregular, with horn-like spur; follicles 5.
12. Delphinium. - Flowers irregular; sepals 5, petaloid, deciduous, dorsal (upper) spurred; petals 4, 2 upper enclosed in spurred sepal; follicles 1, 3, or 5, many-seeded.
13. Aconitum. - Flowers irregular; sepals 5, petaloid, upper one hooded, helmet-shaped; petals 2-5, 2 upper tubular on long stalks enclosed in spurred sepal; follicles 3-5, many-seeded.
14. Actasa. - Flowers more or less regular; sepals 4, petaloid, soon falling; petals 4, small; fruit a berry, many-seeded, not bursting.
15. Paeonia. - Flowers regular; sepals 5, persistent; petals 5, or more; follicles 2-5, many-seeded, opening inwards.
Order 2. Berberidaceae.
16. Berberis. - Shrub, with spines; flowers yellow, in raceme; sepals, petals, stamens, 6; fruit a berry, 2-3-seeded.
17. Epimedium. - Herbaceous, woody plant; sepals 4, deciduous; petals 4; stamens 4; nectaries 4; fruit a capsule, many-seeded.
18. Nymphaea. - Flowers regular, yellow; sepals 5-6; petals numerous, on receptacle; stigma sessile, with many rays; fruit a berry, many-seeded.
19. Castalia. - Flowers white; sepals 4, green; petals numerous, on disk or base of ovary; fruit a berry, many-seeded.
Order 4. Papaveraceae.
20. Papaver. - Petals 4; stamens numerous; stigma a 4-20-rayed, nearly sessile disk; fruit a capsule; opening by pores.
21. Meconopsis. - Petals 4; stamens numerous; stigma 5 6-rayed, free; style short, club-shaped; fruit a capsule, oblong, opening by pores.
22. Glaucium. - Flowers large; petals 4; stamens numerous; stigmas 2; fruit a capsule, 2-celled; seeds not crested.
23. Roemeria. - Flowers violet; petals 3; stamens numerous; stigmas 2-4, radiant; fruit a capsule, 2-4-valved, 1-celled; seeds not crested.
24. Chelidonium. - Flowers yellow, small; petals 4; stigmas 2; fruit a capsule, 2-valved; seeds crested.
25. Corydalis (Capnoides). - Flowers irregular; sepals 2, or o; petals 4, upper spurred below; stamens diadelphous; fruit a pod, 2-valved; many-seeded, opening.
26. Fumaria. - Sepals 2; petals 4, upper spurred below; stamens diadelphous; fruit a nut, I-seeded, not opening.
Order 6. Cruciferae.
27. Matthiola. - Flowers purple; stigma 2-lobed, lobes erect, gibbous or horned on back; fruit a pod, round or flattened, linear.
28. Cheiranthus. - Flowers yellow; lateral sepals saccate; lobes of stigma spreading; podcompressed or 2-edged, with rib on each valve; seeds in single row in each cell.
29. Radicula (Nasturtium). - Pods round in section, short, linear; valves turgid, nerveless; seeds in 2 irregular rows, small.
30. Barbarea. - Pod round or 4-angled in section, linear; valves convex, with longitudinal rib; seeds in 1 row, oblong.
31. Arabis. - Pods flattened, linear; valves not elastic, I-nerved with prominent rib; seeds in 1 or 2 rows.
32. Cardamine. - Pod flattened; valves elastic, nerveless; funicle slender.
33. Dentaria. - Like 32, but funicle dilated.
34. Alyssum. - Petals entire; filaments simple or toothed, shorter with a gland each side below, pouch compressed, oval; seeds 2-4 in each cell.