The anatomical and ultrastructural development of the haustorium of the Cuscuta japonica, a holoparasitic angiosperm, growing on the host plant Impatiens balsamina was studied. After the shoot tips of light-grown parasite seedlings contacted the host, the upper haustorium (external to the host organ) developed through three main successive stages of the haustorial initials, the meristem, and the endophyte primoridium (EP) within the middle layer of the cortex of the parasite stem. The haustorial initial cells were characterized by abundant starch-bearing amyloplasts and mitochondria with an expanded intermembrane space. The meristem cells had numerous large chloroplasts with well-developed thylakoids, reflecting the capability for photosynthesis. Commonly, all three stages of haustorial cells contained conspicuous, large nuclei with enlarged nucleoli and dense cytoplasm including many other organelles, indicating a very active metabolism. In the final stage of upper haustorium development, the meristem cells differentiated into the EP, a host-penetrating tissue. The primordium had smaller file cells at the proximal end and elongate digitate cells at the distal end. The file cells divided actively, while the digitate cells contained abundant chloroplasts, dictyosomes, rough endoplasmic reticulum, and other organelles, suggesting that the EP was cytohistologically well organized for penetration into the host tissue.

The structure and development of the haustorium of a parasitic angiosperm Cuscuta australis R. Brown growing on the host plant Trifolium repens L. was studied with light and electron microscopy. The upper haustorium, which lies external to the host organ, initiates endogenously from cortical cells of the middle layers of the parasite stem. The initial cells develop into a group of meristematic cells. As haustorial maturation progresses, the meristematic cells develop into an endophyte primordium that penetrates the host tissue. The endophyte primordium consists of three cell types: (i) remarkably enlarged elongate cells (digitate cells) with very dense cytoplasm and large nuclei at the central region; (ii) smaller file cells with prominent nuclei proximal to the digitate cells; and (iii) highly compressed cells distal to the digitate cells. Evidence suggests that the digitate cells are metabolically very active. The endophyte, which lies internal to the host tissue, consists of parenchymatous axial cells and elongate tip cells with dense cytoplasm and conspicuous nuclei. The axial and tip cells of endophyte are interpreted as originating from the file and digitate cells of the endophyte primordium, respectively. The tip cells independently penetrate the host tissues and transform into the filamentous hyphae. The hyphae reach the host vascular tissues and eventually differentiate into xylary or phloic conductive hyphae.

Morphological and anatomical features of mature embryos and seedlings were observed at different growth stages in the parasitic angiosperm Cuscutajaponica Choisy. The spirally coiled embryos from scarified seeds had no cotyledons but possessed blunt radicles. Seeds germinated at 30~ in the dark. Although most embryo cells incubated for 16 h did not have starch grains, the shoot cells of three-day-old seedlings possessed numerous starch grains. After these seedlings were transferred to a lightened growth chamber, all the shoot apical regions of seedlings grown for 6, 8, and 10 days became greenish and hooked. Most of the shoot cells, including the green apical parts, contained abundant starch grains. The hooks opened only when one seedling made contact with another seedling, This suggested that the green and hooked shoot apical regions played an important role in searching for and twining about their host plants. In some two-day-old seedlings, the massive roots were circular or semi-circular. This enabled the shoot axes to stand erect on some substratum. It would assist the shoots in making contact with the host plant. In eight-day-old seedlings, the green apical regions also were hooked and the roots were considerably degraded.Keywords: Anatomy, Cuscuta japonica, Embryo, Morphology, Parasitic angiosperm, SeedlingThe seedlings of Cuscuta have chlorophyll that enables photosynthesize (Zimmermann, 1962;Pattee et al., 1965). Although all the Cuscuta species still contain chlorophyll a and b while exploiting their host plants (Macleod, 1963;Ismail and Obeid, 1976), the rate of photosynthesis is decreased (Pattee et al., 1965). The excised stem tips of C. campestris, grown in vitro, produce a considerable amount of chlorophyll. This probably plays an important role in maintaining the growth of the culture (Loo, 1946). Fujita (1964) has characterized the mature embryo of C. japonica as a spirally coiled, yellow cylindrical structure, with a tapering apex, no cotyledon, and having a blunt radicle without a root cap. Truscott (1966) has suggested that in embryo culture of C.gronovii, the loss of both cotyledons and roots was a single embryonic event in the evolution of Cuscuta from its autotrophic ancestors. The morphology and anatomy of early seedlings in C. pedicellata has been studied by Lyshede (1985), who has also observed the fine structures of the tuberous radicular end (Lyshede, 1986) and the one-week-old seedlings in the same species (Lyshede, 1989). However, the function of organs during their autotrophic growth *Corresponding author; fax +82-62-230-7363 e-mail leekb@mail.chosun.ac.kr period is not yet clearly understood. The present study attempts to describe some morphological and anatomical features of C. japonica embryos and seedlings at various developmental stages, and to evaluate these features in view of their parasitic roles. MATERIALS AND METHODS Seed GerminationWe scarified mature, dormant seeds of C. japonicaChoisy with concentrated sulfuric acid for 25, 35, or 45 min, then rinsed them in tap water a...

The ultrastructural features of embryos were studied from mature dry and soaked seeds of the parasitic angiosperm Cuscuta japonica. Outer tangential walls in the protoderm cells were thickened and covered by a thin cuticle layer. These walls could play important roles in preventing water loss from the Cuscuta seedling surfaces after germination and in strengthening the surfaces against various environmental stresses. In the protoderm cells of soaked embryos, lipid materials were released into the thick outer walls through the fusion of lipid bodies with the plasma membrane. In the dry embryos were stored a large number of protein bodies with globoid crystals and lipid bodies. Numerous lipid bodies also were aligned under the plasma membrane. In both dry and soaked embryos, protein bodies were digested and transformed into small vacuoles. The degraded reserves of the lipid and protein bodies could then be mobilized to nourish subsequent germination and seedling growth. Proplastids in the soaked embryo cells contained a few thylakoids and electron-dense plastoglobuli, and crystallized phytoferritin. The phytoferritin, an iron-protein complex, would also be utilized in chloroplast development for autotrophic seedling growth.

The endophyte, that is, the haustorial part within the tissues of the host plant Impatiens balsamina, of the parasitic angiosperm Cuscuta japonica was studied with light and electron microscopy. The endophyte consisted mainly of vacuolated parenchymatous axial cells and elongate, superficial (epidermal) cells. Then the elongate, epidermal cells separated from each other and transformed into filamentous cells, called searching hyphae. The hyphae grew independently either intercellularly or intracellularly in the host parenchyma. The apical end of the hyphal cells was characterized by conspicuous, large nuclei with enlarged nucleoli and very dense cytoplasm with abundant organelles, suggesting that the hyphal cells penetrating host tissue were metabolically very active. Numerous osmiophilic particles and chloroplasts were noted in the hyphae. The osmiophilic particles were assumed to be associated with elongation of the growing hyphe. Plasmodemata connections between the searching hyphal cells of the parasite and the host parenchyma cells were not detected. Hyphal cells that reached the host xylem differentiated into water-conducting xylic hyphae by thickening of the secondary walls. A xylem bridge connecting the parasite and the host was confirmed from serial sections. Some hyphal cells that reached the host phloem differentiated into nutrient-conducting phloic hyphae. Phloic hyphae had a thin layer of peripheral cytoplasm with typical features of sieve-tube members in autotrophic angiosperms, i.e., parallel arrays of smooth endoplasmic reticulum, mitochondria, and plastids with starch granules. Interspecific open connections via the sieve pores of the host sieve elements and plasmodesmata of the parasite phloic hyphae were very rarely observed, indicating that the symplastic translocation of assimilate to the parasite from the host occurred.