Flowerless plants are referred to as cryptogams. Cryptogams include a wide range of plant species that reproduce without producing flowers or seeds. Option 1, phanerogams, is incorrect because phanerogams are plants that reproduce through the production of seeds contained within fruits or cones, such as flowering plants and gymnosperms. Option 2, bryophytes, is also incorrect because bryophytes are a group of plants that include mosses, liverworts, and hornworts, which have simple reproductive structures but not true flowers. Option 3, thallophytes, is incorrect as well because thallophytes are plants that have a simple body structure without differentiation into roots, stems, and leaves, but they may still produce flowers. Therefore, the correct answer is option 4, cryptogams, as it specifically refers to flowerless plants.