Bot. [L. thallus, a. Gr. θαλλός a green shoot, f. θάλλειν to bloom.] Α vegetable structure without vascular tissue, in which there is no differentiation into stem and leaves, and from which true roots are absent.
1829. Loudon, Encycl. Pl. (1836), 874. (Lichenes) … the thallus … is either pulverulent, crustaceous, membranous, foliaceous, or branched and shrub-like.
1846. Lindley, Veg. Kingd., 2. A thallus is a fusion of root, stem and leaves, into one general mass.
1854. Thoreau, Walden, xvii. (1857), 326. The lobed and imbricated thalluses of some lichens.
1875. J. H. Balfour, in Encycl. Brit., I. 508/1. Algæ … consist of a brown, red, or green, flattened, cellular, leaf-like expansion, called a thallus.
b. attrib. and Comb.
1861. Bentley, Man. Bot., 67. Such are … termed Cormophytes or stem-producing plants, to distinguish them from the thallus-forming plants or Thallophytes.
1875. Bennett & Dyer, trans. Sachs’ Bot., 160. The flat extension of the thallus or thallus-like stem. Ibid., 130. In contradistinction to Thallus-plants (Thallophytes), all plants in which leaves can be … distinguished might be termed Phyllophytes.
