‖ thallus Bot.
(ˈθæləs)
[L. thallus, a. Gr. θαλλός a green shoot, f. θάλλειν to bloom.]
A 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 tr. 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. |