Experimental taxonomy on the genus Euphrasia L. (Scrophulariaceae - Rhianthoideae).

Euphrasias are semi-parasitic annuals whose roots attach themselves to the roots of other plants. Because of their similarities, the many species present taxonomic difficulties. Development and parasitism were studied by cultural means. Seed germinates in spring and the plants flower in summer. Euphrasias can produce seed in isolation; when grown together they parasitize one another; in either case they are usually less vigorous than Euphrasias grown in pots in their natural turf, or planted as seedlings with a chosen host-plant. Some hosts are more favourable than others. With garden cultivation many plants became extraordinarily vigorous. Five related species have diploid chromosome-numbers, but the majority are tetraploid; one wild triploid was found. The possibility of cross-pollination increases with flower-size, but even the largest flowers have a good chance of eventual self-pollination. The insects seen visiting the flowers were chiefly Apidae and Syrphidae. Presumed Euphrasia hybrids are frequent in nature. Many were found in the progeny of open-pollinated Eu-phrasias in cultivation, including two from a very small-flowered species. Apomixis has not been detected. Self-pollination gave approximately normal seed-yields, as did the crossing of species alike in chromosome-number. Cross-pollination between diploids and tetraploids gave no normal seed. Closely-related parents gave hybrids more or less normal in fertility, but sterility increased with increasing parental dissimilarity; this also applied to F 2 hybrids. Several E. nemorosa populations, one of E. stricta and two of E. anglica were grown in the garden and compared. Some populations showed conspicuous differences, others slight but statistically significant floral differences. Geographical and ecological isolation are probably the chief agents of speciation in Euphrasia, as in most flowering plants. Explanations of the poor definition of the species are tentative; Euphrasia is in an active phase of evolution. The taxonomy of the British species, as left by Pugsley, is largely accepted. Suggestions for future work are made.