Mistletoes

Mistletoes are unique enough among canopy flora to merit separate treatment. Certain relic terrestrial forms parasitize roots of other plants, but they will be mentioned only in passing; the principal focus will be on aerial mistletoes which are here defined as shrubby hemiparasites growing attached to branches. These unusual plants deviate from "true" epiphytes in form, diversity, physiology, and impact on hosts. Most mistletoes belong to San-talales, a sizable, predominantly tropical, order. Xylem rather than phloem supply is reputed to be the usual consequence of santalalean parasitism, but, as noted later, advanced forms as well (e.g., Arceuthobium) take host substrates. Mistletoes have long occupied a place in European folklore and continue to figure prominently in certain holiday rituals of the Western world. Their destructive qualities are widely recognized. Fortunately, enough scientific curiosity has been aroused by these remarkable organisms to encourage a hard look at their biology. In fact, vegetative and reproductive activity is better known for these plants than for any other like-size assemblage of forest-canopy residents. In this chapter, that information is summarized and aerial mistletoes are contrasted with the true, fully autotrophic, epiphytes.

Systematics and biogeography

The mistletoe habit is polyphyletic, having arisen at least three or four times in Santalales and again in Laurales. The largest mistletoe family is santalalean Loranthaceae with some 900 species distributed unevenly among about 65 genera. Second in size and much more uniform in floral structure is Vis-caceae, a group of perhaps 400 species in just seven genera. Eremolepidaceae and Myzodendraceae are small satellite taxa; the larger Santalaceae contain mostly root parasites. Elevation of Loranthaceae and Viscaceae from subfamily to family status has been argued convincingly on cytological, morphological, and phytogeographic grounds (Barlow 1983). Restriction of numerous primitive loranthaceous genera - including terrestrial Atkinsonia and Nuytsia, and arboreal Cecarria and Muellerina, to Australia/Papuasia; Alepis, Peraxilla, Trilepidea, and Tupeia to New Zealand; and, among others, Desmaria, Gaiadendron, and Tripodanthus to South America - points to a Gondwanan origin much like that attributed to several other predominantly Southern Hemisphere families, including Casuarinaceae, Myrtaceae, and Proteaceae. If Barlow is correct in his proposed mid-Cretaceous ancestry, Loranthaceae must be considered one of the oldest extant angiosperm families. Current distribution indicates that substantial secondary radiations occurred in Africa, Asia, South America, and Australia/New Zealand after elements of an already diverse stock were isolated by shifting continental plates. It is not clear when parasitism evolved.

Viscaceae are, by contrast, Laurasian and more recently divergent and specialized, having originated in eastern Asia (Barlow 1983) and subsequently achieved wide distribution in the Paleocene. The family is predominantly tropical today but maintains a notable temperate presence in North America, Asia, and Australia/New Zealand. Some dwarf mistletoes extend well into Northwest Canada, making this group by far the most frost-hardy of the vascular epiphytes. Viscaceae, like Loranthaceae, are parasites of dicots, but with a few exceptions. Radiation of highly reduced and specialized Arceuthobium as a conifer parasite seems to be rather recent and was probably favored by territorial expansion of Pinaceae during the late Tertiary global cooling. Viscaceae contrast sharply with ornithophyllous Loranthaceae (Fig. 6.1F,G) in that the former produce small, inconspicuous flowers (Fig. 6.ID,E.J) and what is usually a more reduced mycetoid endo-phyte (Fig. 6.2E). The two families are more alike in fruit and seed structure, dispersal, and germination. Both taxa achieve greatest diversity in humid tropical forests.

Vegetative morphology

Exposed structures

All mistletoes are autotrophic to some degree; photosynthesis occurs primarily in simple, usually opposite, leaves (Fig. 6.1A,B) or aphyllous green stems. The endophytic part of the mistletoe ranges from a discrete but often much expanded haustorium with a ball-in-socket configuration (Fig. 6.2F) to a largely internal thalloid apparatus composed of ramifying parenchymatous strands; examples are Arceuthobium (Fig. 6.2D,E), some other Viscaceae, and a few Loranthaceae. Exposed growth among the loranthacean mistletoes is usually extensive and persistent from germination onward. There are intervals, notably juvenile stages, when advanced viscaceans are completely embedded in host tissue. Recorded lifespans based on observation of shoots has exceeded 100 years.

Floral Diagram Sapota

Figure 6.1. Aspects of mistletoes. The mimetic foliage of (A) Amyema pendulum (left) and Eucalyptus paucifolia; (B) Dendrophthoe homoplastica and Eucalyptus shirleyi; (C) Amyema cambagei (left) and Casuarina torulosa. Staminate and pistillate flowers of (D) Arccuthobium campylocentrum and (E) A. minutissimum, respectively. Omithophilous flowers of (F) Macrosolen platyphyllus and (G) Actanthus macranthus. (H) Longitudinal section of pseudoberry of Psittacanthus cuneifolius. (I) Germination and initiation of parasitism by Tristerix tetrandrus. (J) Small fruiting shoot of Arceuthobium sp. on Picea sp. twig. (C after Barlow and Wiens 1977; D-G after Kuijt 1969; H after Bhatnagar and Johri 1983; I after Hoffmann et al. 1986.)

Figure 6.1. Aspects of mistletoes. The mimetic foliage of (A) Amyema pendulum (left) and Eucalyptus paucifolia; (B) Dendrophthoe homoplastica and Eucalyptus shirleyi; (C) Amyema cambagei (left) and Casuarina torulosa. Staminate and pistillate flowers of (D) Arccuthobium campylocentrum and (E) A. minutissimum, respectively. Omithophilous flowers of (F) Macrosolen platyphyllus and (G) Actanthus macranthus. (H) Longitudinal section of pseudoberry of Psittacanthus cuneifolius. (I) Germination and initiation of parasitism by Tristerix tetrandrus. (J) Small fruiting shoot of Arceuthobium sp. on Picea sp. twig. (C after Barlow and Wiens 1977; D-G after Kuijt 1969; H after Bhatnagar and Johri 1983; I after Hoffmann et al. 1986.)

Figure 6.2. Aspects of mistletoes: (A) Haustorial penetration of bark of two incompatible and one host species by Tristerix tetrandrus - only the infection of Kageneckia oblonga succeeds; (B) Eremolepis sp. growing on branches of Weinmannia sp.; (C) apex of cortical strand of Viscum album; (D) Endophytic system of Arceuthobium americanum; (E) distribution of endophyte of A. americanum in dormant bud of broom on Pinus contorta; (F) cross section of haustorium of Macrosolen cochinchinensis on Achras sapota; (G) germinating mistletoe illustrating adhesive disc. (H) Psittacanthus schiedeanus seedling. (A after Hoffmann et al. 1986; B-H after Kuijt 1969.)

Figure 6.2. Aspects of mistletoes: (A) Haustorial penetration of bark of two incompatible and one host species by Tristerix tetrandrus - only the infection of Kageneckia oblonga succeeds; (B) Eremolepis sp. growing on branches of Weinmannia sp.; (C) apex of cortical strand of Viscum album; (D) Endophytic system of Arceuthobium americanum; (E) distribution of endophyte of A. americanum in dormant bud of broom on Pinus contorta; (F) cross section of haustorium of Macrosolen cochinchinensis on Achras sapota; (G) germinating mistletoe illustrating adhesive disc. (H) Psittacanthus schiedeanus seedling. (A after Hoffmann et al. 1986; B-H after Kuijt 1969.)

Mistletoe foliage, often borne on pendent stems, is evergreen with the exception of a few deciduous Loranthus forms. Xeromorphy, or at least a similar structural quality often including considerable succulence, is characteristic of many northern temperate natives. In humid tropical zones, host and hemiparasite more often share comparably mesomorphic foliage. Profuse generation of determinate shoots accounts for the compact form of most bushy species, but host canopies may be shrouded by festoons of elongate branches in some cases (e.g., Phthirusa). Another striking exception to the usual mistletoe habit is Phrygilanthus acutifolius: Its roots grow down from a primary host and extend through the soil for many meters in several directions, only to creep up again and parasitize bases of neighboring trees via multiple haustoria. Costa Rican Gaiadendron punctatum lives in wet forests where it infects diverse epiphytes, both flowering plants and ferns, without attacking the common support! Infestation by some arboreal tropical mistletoes (e.g., Phthirusa and Struthanthus) spreads locally via creeping epicortical rootlike stems that generate secondary haustoria (Fig. 6.2B) and occasional leafy shoots. These versatile appendages feature thigmotro-pism, photophobic growth, and neutral geotropism, all of which promote host penetration.

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