THESIUM 
103 
Subclass 2. PETALOIDEAE 
Petaloideae nobis; Archichlamydeae b Engler Syll. ed. 2, 105 (1898). 
For characters, see page 2. 
British orders of Petaloideae 
Order- 1. Santalales (see below). Flowers cyclic, “calyculus” present or not, usually homo- 
chlamydeous. Perianth usually petaloid, sometimes sepaloid. Stamens usually as many as the 
perianth-segments and antisepalous, sometimes twice as many. Ovary subinferior or inferior, with 
1 — 3, usually 2 — 3 carpels, loculi as many as the carpels. Ovules either 1 — 4 to each loculus and 
pendulous from the apex or from a central placenta, or not differentiated and embryo-sacs filling 
up the interior of the ovary. 
Order 2. Aristolochiales (p. 106). Flowers cyclic, homochlamydeous, actinomorphic or zygo- 
morphic. Perianth petaloid. Ovary usually inferior, either with 3 — 6 loculi and axile placentation 
or 1 loculus and parietal placentation. Ovules 00 to each loculus. 
Order 3. Polygonales (p. 108). Leaves usually with stipular sheaths or “ochreae.” Flowers 
either partly spiral or cyclic, actinomorphic. Perianth homochlamydeous or heterochlamydeous, petaloid 
or sepaloid. Ovary superior, unilocular, uniovulate. Ovules basal, orthotropous, rarely anatropous, 
with 2 integuments. True fruit an achene. 
Order 1. SANTALALES 
Santalales Lindley Nat. Syst. ed. 2, 192 (1836); Engler Syll. ed. 1, 98 (1892); in Pflanzenfam ., Nachtr. 
346 (1897); Ascherson und Graebner Syn. iv, 640 (1911). 
For characters, see above. 
British families of Santalales 
Family 1. Santalaceae (see below). Ovules 1 to each loculus, pendulous from the apex 
or from a free-central placenta. 
Family 2. Loranthaceae (p. 105). Ovules and placentae not differentiated from the placenta, 
and the embryo-sacs in the tissue filling up the interior of the ovary. 
Family 1. SANTALACEAE 
Santalaceae R. Brown Prodr. FI. Nov.-Holl. 350 (1810); Lindley Nat. Syst. ed. 2, 193 (1836); Hierony- 
mus in Engler und Prantl Pflanzenfam. iii, pt. i, 202 (1889); Ascherson und Graebner Syn. iv, 641 (1912). 
Trees, shrubs, or herbs; hemiparasitic, some being stem-parasites and others root-parasites. 
Leaves alternate or opposite, entire, exstipulate. Inflorescence various, but primitively cymose. 
Flowers monoecious or dioecious, usually with an epigynous disc. Perianth monochlamydeous, 
petaloid (in the British species) or sepaloid, with 4 or 5 divisions. Stamens equal in number to 
the sepals, epiphyllous. Ovary semi-inferior, with 1 loculus. Placentation free-central. Ovules 
suspended, 1 — 4 in each loculus, all but 1 aborting; integument absent. Fruit an achene or drupe. 
Seeds 1 to each ovary. Testa absent. Endosperm present. 
26 genera; 250 species; tropical and temperate zones. Only British genus: — Thesium. 
Genus 1. Thesium 
Thesium L. [Gen. PI. ed. 1, 60 (1737)] Sp. PI. 207 (1753) et Gen. PI. ed. 5, 97 (1754); Hieronymus in 
Engler und Prantl Pflanzenfam. iii, pt. i, 212 et 223 (1894); Ascherson und Graebner Syn. iv, 644 (1912). 
Hemiparasitic herbs. Roots attached to the host-plants by means of suckers. Leaves alternate, 
narrow, decurrent. Flowers monoclinous. Disc minute or absent. Bract adnate to the peduncle, 
and, with the 2 bracteoles, usually forming a kind of involucre. Perianth petaloid, with 3 — 5, 
usually 5 segments. Fruit a nutlet. 
1 15 species; old world, chiefly in the north temperate zone. 
