This section is from the book "Sub-Alpine Plants Or Flowers Of The Swiss Woods And Meadows", by H. Stuart Thompson. Also available from Amazon: Sub-Alpine Plants: Or, Flowers of the Swiss Woods and Meadows.
Leafless brownish root-parasites. Rootstock often tuberous, naked or scaly. Stem stout, solitary, scaly, not often branched. Flowers in more or less dense spikes or racemes. Sepals 4 or 5. Corolla 2-lipped. Stamens 4. Ovary 1-celled. Seeds numerous, minute. A widely distributed family, chiefly in Southern Europe.
Characters of the family. None are truly Alpine, though several, and particularly 0. epithymum DC. (0. rubra Sm.), are found in the sub-Alps. That species has been seen by the author as high as 6600 feet in Savoy and in Switzerland, where it is parasitical upon Thyme, as in parts of England. The following species of Broom-rape, with the host-plant upon the roots of which they grow, have been recorded from Switzerland by A. W. Bennett, Keller and Schinz, etc.:
O. Minor Smith, on various plants, very commonly on Purple Clover. Style violet.
O. Loricata Rchb, on Artemisia campestris. Style yellow.
O. Picridis Schultz, on Picris hieracioides. Plant very pale.
O. Epithymum Dc (O. rubra Sm., O. alba Stephan), on Thyme.
O. Teucrii Holan, on Teucrium and Thymus. Spike short.
O. Caryophyllacea Sm, on Galium. Plant yellow, scented.
O. Flava Mart (O. Frolichii Rchb.), on Adenostyles, Petasites offic. albus and niveus, and Aconitum Lycoctonum.
O. Hederce Duby, on Ivy. Spike elongated.
O. Salvia Schultz, on Salvia glutinosa. Corolla yellow.
O. Lucorum A.Br, on Berberis and Rubus caesius. Corolla brownish yellow.
O. Laserpitii Sileris Reuter, on Laserpitium Siler. Stem swollen at base into a large ball. Corolla brownish yellow, streaked with violet.
O. Elatior Sutton (O. Major L), on Centaurea scabiosa. Plant yellow. Corolla reddish brown.
O. Cervariae Suard. (O. Alsatica Kirchl), on Peucedanum Cervaria and Seseli Libanotis. Corolla fawn-coloured, tinted with violet. Sepals distinct.
O. Rubens Wallr. (O. Gracilis Sm), on Lotus, Hippocrepis, Genista, etc. Corolla reddish brown. Sepals 2-3 cleft.
O. Rapum Genistae Thuill, on Broom and other Leguminosae. Tall, coarse, with large brown flowers.
O. Cruenta Bert, on Medicago sativa and Melilotus. Yellow.
O. Purpurea Jacq. (O. Coerulea VILL), on Yarrow. Whole plant purplish.
O. Iaevis L. (O. Arenaria Berk), on Artemisia campestris. Bluish. Anthers hairy (smooth in purpurea).
O. Ramosa L, on hemp and tobacco. Stem branched.
Laihraa is closely allied to Orobanche, but the calyx has 4 broad, short teeth. Plant fleshy, yellowish white or pale purple.
Raceme, unilateral, drooping.
Europe, Central and Russian Asia except the extreme N. British.