This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. CHAPTER 10 Anatomy of the Dwarf Mistletoe Shoot System Carol A. Wilson and Clyde L. Calvin * In this chapter, we present an overview of the Morphology of Shoots structure of the Arceuthobium shoot system. Anatomical examination reveals that dwarf mistletoes Arceuthobium does not produce shoots immedi­ are indeed well adapted to a parasitic habit. An exten­ ately after germination. The endophytic system first sive endophytic system (see chapter 11) interacts develops within the host branch. Oftentimes, the only physiologically with the host to obtain needed evidence of infection is swelling of the tissues near the resources (water, minerals, and photosynthates); and infection site (Scharpf 1967). After 1 to 3 years, the first the shoots provide regulatory and reproductive func­ shoots are produced (table 2.1). All shoots arise from tions. Beyond specialization of their morphology (Le., the endophytic system and thus are root-borne shoots their leaves are reduced to scales), the dwarf mistle­ (Groff and Kaplan 1988). In emerging shoots, the toes also show peculiarities of their structure that leaves of adjacent nodes overlap and conceal the stem. reflect their phylogenetic relationships with other As the internodes elongate, stem segments become mistletoes and illustrate a high degree of specialization visible; but the shoot apex remains tightly enclosed by for the parasitic habit. From Arceuthobium globosum, newly developing leaf primordia (fig. 10.lA). Two the largest described species with shoots 70 cm tall oppositely arranged leaves, joined at their bases, occur and 5 cm in diameter, toA. douglasii, a small species at each node (fig. 10.lA-B). This decussate phyllotaxis with shoots 3 cm tall and 0.3 cm in diameter, the characterizes the entire genus. anatomical features are consistent. The mature, paired leaves form a boat-shaped We have studied diverse species of Arceuthobium, structure that encircles the main stem and its branches including large primitive species and highly reduced at the node (fig. 10.lB-C). The merged leaf bases typi­ specialized species. For this account, however, we cally extend some distance into the internode below concentrated on the shoots of A. globosum and A. tsu­ the attachment point (fig. 10.lD). This contributes to gense. We chose these species for several reasons: the much larger diameter of the internode below the (1) material on developmental stages was available; point of attachment than above (fig. 10.lB). Generally, (2) A. globosum presumably represents a primitive the mean diameter of the stem 1 to 2 mm below the member within the genus, whereas A. tsugense repre­ node measures almost twice the diameter an equal dis­ sents intermediate specialization; and (3) these species tance above. The merger of leaf structure into the main represent the two geographical areas of greatest speci­ axis results in a stem morphology in which internodes ation, central Mexico and northern California. Where widen in an acropetal direction, particularly in the appropriate, we compare features of Arceuthobium upper third of the internode. The functions of the with the related genera Korthalsella, Phoradendron, widened upper portions of internodes are discussed and Viscum. These three genera share features with more fully in a later section on the epidermis. Arceuthobium, and Korthalsella had been proposed Several different branching patterns occur in as a sister genus to Arceuthobium (Wiens and Barlow Arceuthobium (fig. 2.1). The A. tsugense shoot (fig. 1971, but see chapter 15). 10.lB) displays decussate branching, a pattern com­ In this chapter, we focus on leaf, stem, and fruit mon for shoots with decussate phyllotaxis. The evolu­ structure, and, wherever possible, attempt to relate tionary and systematic importance of these patterns structure to function. General morphology presents was established by Kuijt (1970) and further refined by the best starting point for discussing the shoot system Hawksworth and Wiens (1972) and by Mark and of Arceuthobium. Hawksworth (1981). • Depa~ment of In~egrative Biol?gy, University of California, Berkeley and Department of Biology, Portland State University, Portland, OR, respectively; contnbuted as EnVironmental SCiences and Resources Program Publication No. 242. Anatomy ofthe DwarfMistletoe Shoot System 95 Chapter 10 c D Figure 10.1 -Surface (A-B) and transectional views (C-D) of Arceuthobium tsugense shoots. A: shoot tip showing opposite, paired leaves and decussate phyllotaxis, x60. B: nodal region showing stem (st), mature leaves (I) and axillary branches (ab), x25. C: leaf 300 ~m above point of attachement to stem, unlabeled arrows at points where leaf margins join, x31. D: stem at point of leaf base fusion, x31. 96 --------------------------- Anatomy ofthe DwarfMistletoe Shoot System Chapter 10 Shoot Apical Organization Sclerified parenchyma cells abut directly against the tracheary elements (fig. 10.2E). No sieve elements In median longitudinal section, a shoot tip of were seen in any of the leaves we examined. In older Arceuthobium (fig. 10.2A) shows a highly stratified leaves, sclerified parenchyma forms a mostly continu­ apex. Two stratified layers were present in all shoot ous plate of tissue that continues into the leaf base. apices that we examined. Although we did not ana­ This sclerified tissue is absent in the internode beneath lyze a large enough sample (Gifford 1954) to state the leaf base. Some of the cells contiguous with this authoritatively the number of tunica layers present, sclerenchymatous layer contain rhomboidal (prisma­ our observations suggest that the tunica is biseriate. tic) crystals (fig. 10.2E). The sclerified parenchyma in A biseriate tunica has also been reported for A. globosum veins generally extends acropetallya Phoradendron (Cutter 1955). In the apices we exam­ greater distance than the tracheary elements; but in ined, axillary buds were visible in the axils of the third some cases, the two tissues appear to terminate at leaf pair and were well developed in the axils of the about the same level. Some tracheary elements at vein fifth leaf pair. Studying several parasitic and "sapro­ endings appeared to be tracheids. In more proximal phytic" angiosperms, Cutter (1955) found no anomaly positions, however, vessel members were present. of shoot apical organization associated with these nutritional modes. Mauseth and others (1985) exam­ The ground tissue in leaves of Arceuthobium tsu­ ined shoots of the mistletoe Tristerix aphyllus (Loran­ gense consists mainly of parenchyma, and only an thaceae) and arrived at the same conclusion. Through occasional sclereid is present. As viewed basipetally in our observations of Arceuthobium (fig. 10.2A-B), we developing leaves (figs. 10.lC-D), chlorenchyma tis­ concur. sue begins as small groups of large cells in the midvein region and fused margins of the leaf. Moving basi­ petally, chlorenchyma becomes more abundant at the margins where the leaves are joined (at unlabeled Leaf Anatomy arrows in fig. 10.lC). At a lower level where the leaf Although Arceuthobium is described as "leafless," joins the stem, chlorenchyma occurs all the way shoots are in fact squamate; that is, they bear simple, around the sheathing leaf base (fig. 10.lD). This scale-like leaves. Leaves are initiated at the periphery chlorenchyma layer is 3 to 5 cells thick around the of the shoot apex by periclinal divisions in the subsur­ entire stem (fig. 10.lD). No differentiation of the mes­ face layer (fig. 10.2B). When leaves are initiated, pri­ ophyll into palisade and spongy tissue occurred in any mordia are more or less circular in transverse section, of the species we examined, and intercellular space and each primordium is independent. As develop­ was minimal. ment continues, however, meristematic activity is lim­ The leaf epidermis contains ordinary epidermal ited to the lower leaf zone, so that by the third leaf pair cells and stomatal complexes consisting of guard cells the leaf bases are visible as a Single unit due to con­ and subsidiary cells. No trichomes are present. genital fusion. Leaf bases continue to expand and are Stomata are abundant in the abaxial epidermis but are tubular at maturity; but the upper leaf zone, which was sparse or absent in the adaxial epidermis. The abaxial prominent in early development (fig. 10.lA), becomes stomata density on fully developed leaves reaches 38 almost indistinguishable (fig. 10.lB). per mm2 (table 10.1). This value compares to those Three stages of leaf development in Arceuthobium reported for leaves of numerous nonparasitic angio­ tsugense are shown in transverse sections in figure sperms (Meyer and Anderson 1952). As reported for 10.2C-E. In figure 10.2C, a more or less continuous many other members of Santalales (Butterfass 1987), plate of procambial tissue occupies the center of the stomata have a transverse orientation with respect to leaf, and one mature tracheary element is visible (at the plant axis. As viewed in transverse (fig. 10.3A) and arrow). No other cells have matured from procambi­ longitudinal (fig. 10.3B) sections, guard cells are urn at this level. At a slightly later developmental stage recessed beneath over-arching subsidiary cells. The (fig. 10.2D), one mature tracheary element (at arrow), subsidiary cells project above the surface and form a plus several adjacent cells in which secondary wall small crypt (fig. 10.3C) at the bottom of which occurs deposition is beginning, can be seen. In figure 10.2E, the stomatal aperture (fig. 10.3B). A small substomatal an older developmental stage, all procambial deriva­ chamber is present (fig. 10.3B); but generally, cells of tives have matured. Several tracheary elements are the stomatal complex have wall contacts with cells in present (at unlabeled arrows), but the majority of the subepidermal layer (figs. 10.3A-B). derivatives have matured as sclerified parenchyma Epidermal cells are covered by a thick cuticular cells, each with numerous, large, simple pits.