Chapter 4 2 Haustorium Initiation and Early 3 Development 4

6 The ability to develop invasive haustoria is the key feature of parasitic angiosperms. 7 The haustorium attaches the parasite to the host, penetrates the host while keeping 8 its own tissues intact, develops a vascular continuity between the host and parasite 9 and ultimately provides the conduit through which host and parasite materials flow. 10 The ability to make haustoria distinguishes parasitic from non-parasitic plants; 11 indeed, ‘the haustorium embodies the very idea of parasitism’ (Kuijt 1969). 12 This chapter reviews the initiation and pre-attachment development of terminal 13 and lateral haustoria in parasitic Orobanchaceae. Haustoria have been described for 14 many genera of Orobanchaceae, but their initiation and development has been 15 investigated in a relatively small number of species. Most of these studies have 16 investigated the development of terminal haustoria in the weedy species Striga 17 asiatica (Saunders 1933; William 1961; Nickrent et al. 1979; Keyes et al. 2007), 18 S. hermonthica (Okonkwo 1966; Olivier et al. 1991), S. gesnerioides (Okonkwo 19 and Nwoke 1978), Phelipanche aegyptiaca (syn. Orobanche aegyptiaca) and 20 O. cumana (Joel and Losner-Goshen 1994; Zhou et al. 2004) and Alectra vogelii 21 (Nwoke and Okonkwo 1978; Visser et al. 1990). Studies in the development of 22 lateral haustoria have primarily focussed on three facultative species: Agalinis 23 purpurea (Riopel and Musselman 1979; Baird and Riopel 1984), Castilleja exserta 24 (previously known as Orthocarpus purpurascens) (Atsatt et al. 1978) and 25 Triphysaria versicolor (Jamison and Yoder 2001; Bandaranayake et al. 2010).