Parasitic Flowering Plants

Chapter 5. Establishment of the parasite

This chapter primarily deals with how a parasite becomes established, how the haustoria are initi- ated and develop, and how water and nutrients are transferred from host to parasite. In this con- text the structure of the interface (Fig. 333) is of particular interest. In other words, the focus will be on what it means to be a parasitic plant, and * how this comes about. It is natural, however, first to look at how at least some of the parasite seeds end in a suitable place for germination near a po- tential host. Then we will develop a further un- derstanding of the fascinating and complex haus- torium, which parasites use to extract materials from their hosts. The general structure of mature haustoria is described along with the presenta- A tion of the parasitic families, and the terminol- ogy is outlined in Chapter .

Seed dispersal strategies Parasitic plants have adopted one of five differ- ent strategies for seed dispersal. The purpose of all four strategies is to bring seeds either in di- rect contact with a suitable host, or to bring * seeds within a critical distance beyond which a seedling has no chance to reach the host through its own growth. This is simply because the stored nutrients in the seed are limited, and therefore the seedling will otherwise die. The critical dis- tance varies of course with species. The five germination strategies are as follow: B A. The seeds are relatively large and have enough storied nutrients in the form of starch, Fig. 334. SEM of seed and folded embryo of Cuscuta. fat, and protein in the endosperm to sustain the *, Endosperm. A, C. pedicellata. Cross section of seed. seedling for a certain period (Fig. 334). For ex- Arrow, Protein-rich aleuron layer of endosperm. B, C. ample Cuscuta (dodder) seedlings can live for 3-7 campestris. Opened seed. - Lyshede 1992. 263 Parasitic Flowering Plants

Fig. 335. Seeds deposited by birds. A, Scurrula parasitica (Loranthaceae) with fruit (left) and seeds in a rosarie. Nepal. B, Viscum monoicum. Germination of seeds deposited in masses. Sri Lanka. - G Glatzel. weeks, and the young stem of Cuscuta gronovii food. In this case it is the behaviour of the dispers- can search for hosts up to a distance of 35 cm be- er which determines how successful the strategy fore it begins to wilt (pp. 54-55). The critical is. The method is very common in Santalales and distance is likely to be much larger for certain root crucial for the stem parasitic loranths and mistle- parasitic Santalaceae such as the record large Ou- toes, because it assures that some seeds obtain di- koubaka which also has record large seeds (Fig. rect contact with a suitable host (Fig. 335). There 48). However, usually the critical distance is is usually only one seed per flower. In Tristerix much shorter and may be in the order of millime- aphyllus the critical distance has increased to per- tres for most holoparasitic root parasites. Besides haps 12 cm (Fig. 79), but in other stem parasites Cuscuta, Cassytha and all root parasitic mem- the seeds must have direct contact with the host bers of Santalales, the ‘large seed’ strategy is also surface, since the radicles have very limited growth practised be several hemiparasitic Orobanchaceae potential. Many of the species have photosynthesis (formerly Scrophulariaceae) such as Euphrasia, from the very beginning of germination since the Rhinanthus, Melampyrum, and Castilleja. endosperm contains chlorophyll (Fig. 76). The Many kinds of fruit-eaters from rodents to special adaptations such as the stickiness of the birds may serve as dispersers. The seeds pass the seeds, structure of digestive canal, and behaviour digestive canal alive, and in some cases the ger- of the birds were discussed in the section ‘Disper- mination percentage is higher after the passage sal biology’ (p. 50). Also the behaviour of the silky since digestive enzymes have softened the seed flycatcher which deposits Phoradendron seeds in coat. However, the birds involved have no spe- masses may be recalled (pp. 119-121). cial adaptations to dispersal of parasites as is of- It is not always birds which are part of the ten the case for the following strategy. ‘sticky seed - edible fruit’ strategy. In a few cases B. The seeds are sticky and spread by animals, marsupials or mammals do the same job and the primarily birds, which find the fruits attractive as dispersal of Tristerix corymbosus by the marsu- 264