Thesiaceae Vest.

Genera Included:  Buckleya Torrey, Kunkeliella Stearn, Osyridicarpos A. DC, Thesidium Sond., and Thesium L.

Habit:  Shrubs and herbaceous perennials.

Parasitism: Root hemiparasitism has been documented in Buckleya (Shirai 1877 quoted in Kusano 1902, Piehl 1965), Thesidium (Fineran 1985), and Thesium (Mitten 1847). 

Roots: The structure of the haustorium has been described for Buckleya quadriala (Kusano 1902) and various Thesium (Pitra 1861, Solms-Laubach 1968, Benson 1910, Abou-Raya and El-Sharkawy 1977, Weber 1977).

Stem: For Buckleya, branching distichous to nearly decussate, pubescent.  In Thesium, Thesidium and Kunkeliella, branching is quite variable depending upon the habit of the plant which ranges from herbs to woody shrubs.  Osyridicarpos is a much-branched, somewhat climbing shrub with scattered, spreading, ribbed branches.  

Leaves: Alternate or opposite, persistent, caducous or deciduous, not dimorphic, developed or as scales (squamate), estipulate.

Inflorescence: axillary and terminal of various types: flowers solitary, monochasial, spikes, racemes, cymes (simple and compound dichasia) and umbels. Floral and inflorescence trichomes absent or present.  Floral bracts and/or bracteoles separate, persistent, caducous or deciduous.  Pedicel or stipe present or absent (flowers then sessile).

Plant Sex: Bisexual flowers or unisexual (plants then dioecious)

Flowers: Sessile or pedicellate, monochlamydous or dichlamydous.

• Calyx: Absent (fused to inferior ovary) or present as a calyculus. In some taxa the calyx is developed into sepal lobes, e.g. in Buckleya, Osyridocarpos, and Thesium libericum.
• Corolla: Petals 4-5, corolla tube absent or very short and cup-shaped (campanulate or urceolate). Post-staminal hairs present on petals of bisexual or male flowers or absent (Buckleya).
• Nectary: Glandular disk on bisexual and female flowers present or absent (Osyridicarpos), lobed in Buckleya.
• Androecium: Stamens 4-5. Filament equal to or slightly shorter than anther or longer than anther (Buckleya, Osyridicarpos).  Anther dorsifixed, 4-locular (tetrasporangiate), positioned in one plane, locule isomerous, arranged parallel (longitudinal) to filament, dehiscence longitudinal by slits with both thecal locules using a common opening. 
• Staminodia: Carpellate flower staminodes (and staminate flower pistillodes) absent.
• Pollen: examined in Thesium (Lobreau-Callen 1980) and Kunkeliella (Stearn 1972).
• Gynoecium: Ovary position inferior (flower epigynous), unilocular throughout (Buckleya, Osyridocarpos) or multilocular below, unilocular above; tricarpellate (Thesium).  Style short cylindrical to absent or medium to long cylindrical in Osyridicarpos, stigma variously shaped: 2- to 4-lobed, globular or capitate.
• Ovule: Ovules 2-5, ategmic or unitegmic.
• Embryo, etc. Embryo sac extending beyond ovule or placenta.  Suspensor present. Embryology documented for Thesium (Rao 1942, Paliwal 1956, Bhatnagar 1991) and Buckleya (Raj 1964).

Fruit: Buckleya, Osyridicarpos, and Kunkeliella have drupes (pseudodrupes) whereas in Thesium nutlets (achenes) are seen. Mature fruit crowned by persistent perianth parts. Mesocarp hard, bony, endocarp wall parenchymatous and crustaceus (hard, thin, brittle), not dividing into segments. The fruiting pedicel becomes swollen in some species of Thesium thus functioning as an elaiosome for myrmechochory.

Seed: one per fruit, enclosed in the hard mesocarp.

Chromosomes: Chromosome numbers have been reported for several species of Thesium: T. alpinum, n = 6 (Vachova, 1976; Nikolov, 1991), n = 7 (Franzen & Gustavsson, 1983); T. bergeri, n = 8 (De Montmollin, 1984); T. divaricatum, n = 8 (García Martín & Silvestre, 1985), n = 8 (Pastor & al., 1990); T. ebracteatum, n = 12 (Semerenko, 1990); T. humile, n = 9 (Ruíz de Clavijo, 1990); T. italicum, n = 8 (Corrias, 1980); T. linophyllon, n = 12 (Vachova, 1978). 

Alternate Family Names: none