Some crop plants are hybrids from different genera (intergeneric hybrids), such as Triticale, × "Triticosecale", a wheat–rye hybrid. Most modern and ancient wheat breeds are themselves hybrids; bread wheat, "Triticum aestivum", is a hexaploid hybrid of three wild grasses. Several commercial fruits including loganberry ("Rubus" × "loganobaccus") and grapefruit ("Citrus" × "paradisi") are hybrids, as are garden herbs such as peppermint ("Mentha" × "piperita"), and trees such as the London plane ("Platanus" × "acerifolia"). Among many natural plant hybrids is "Iris albicans", a sterile hybrid that spreads by rhizome division, and "Oenothera lamarckiana", a flower that was the subject of important experiments by Hugo de Vries that produced an understanding of polyploidy.