Apodanthaceae Takhtajan

Genera Included: Apodanthes, Pilostyles. As shown by Bouman and Meijer (1994), the genus Berlinanche differs in no significant way from Pilostyles. Given the variation present in that genus already, I choose to recognize Berlinanche as a member of that genus.

Habit: Achlorophyllous, holoparasitic herbs. Vegetative body consists entirely of endophytic tissues present within the host plant and bracts that subtend the flowers. The only portion of the plant visible outside the host are the tiny flowers (5 mm in length).

Parasitism: Stem and root parasites. Pilostyles parasitic on legumes (Dalea, Daviesia, Mimosa), Apodanthes on Casearia (Flacourtiaceae).

Roots: the endophytic or haustorial system is filamentous, resembling a fungal mycelium inside the host tissues. The uniseriate filaments develop into an anastomosing complex of cortical strands and radial sinkers (Kuijt et al. 1984). The ultrastructure of the endophyte of Pilostyles thurberi has been described by Kuijt et al. 1984). As pointed out by Endriss (1902), Brown (1912), Rutherford (1970) and Kuijt et al. (1984), the parasite tissues can be distinguished from host tissues by the presence of larger nuclei, often with two nucleoli.

Stem: Absent

Leaves: Present as bracts subtending the flower.

Inflorescence: Flower bud primordia are formed endogenously within the host cortex. As the endophyte grows to keep pace with the host (isophasic development), new flowers are formed in regular series (rows) along the host branch. For Pilostyles, flowers of the same developmental stage often occur at uniform distances from the host apex. Flowers emerge singly from the host bark but generally occur in groupings or rows.

Plant Sex: Plants dioecious; flowers unisexual.

Flower Bracts: Each flower is associated with two series of bracts and a perianth (sometimes referred to as three whorls of scales since their anatomy is essentially identical). Each flower arise from a cupule-like structure formed from the host bark. These cupules are pesistent on the host bark after the flowers whither and fall off. In Pilostyles, the 2-6 bracts are free, emerging from the sides of a rounded receptacle (males) or the outside portions of the ovary (females). In Apodanthes, the lower whorl consists of two leathery free bracts, the upper of four larger, rounded, connate bracts. The bracts in Apodanthes are yellow to orange in color which differentiates them from the white perianth. In Pilostyles, the bracts and perianth colors are more similar.

Staminate Flowers
Calyx (perianth): 4-5 sepals, attached to top of a rounded receptacle.
Corolla: Absent.
Nectary: present as a ring of tissue at the base of the column and the perianth.
Androecium: The stamens connate and adnate to a central column. The anthers that appear as several series of protrusions near the apex of the column. In P. holtzii, the monadelphous stamens are free from the central column, thus suggesting the latter is a remnant of the gynoecium. The expanded, flattened apical portion of the column is called the disk. Just below the edge of the disk and above the anthers is a fringe of vescicular, unicellular hairs (the "giant cells" of Rutherford 1970). Each anther has just one chamber which opens by a transverse slit.
Pollen: Pilostyles is triporate and atectate.

Carpellate Flowers: In Pilostyles, often wider and more conical than staminate flowers.
Calyx (perianth): 4-5 sepals, attached near apex of ovary.
Corolla: Absent.
Nectary: as in staminate flowers.
Androecium: absent.
Gynoecium: Epigynous to partly inferior. The central column resembles that seen in the staminate flower, except for the absence of anthers near the apex. The disk is similarly rimmed by vescicular hairs, but here they function as stigmatic hairs. The column is hollow forming a stylar canal which is contiguous with the locule. Ovary unilocular with intrusive parietal placentation. In Apodanthes, 4 placental lobes occur giving a cruciform shape in cross section. Ovules are found only on the placental lobes. For Pilostyles, no intrusive placentae occur, thus the locule is circular in cross section. Ovules occur along the entire inner surface.
Ovules: anatropous, bitegmic, tenuinucellate. As described in Bouman and Meijer (1994), for Apodanthes (Pilostyles is similar):

"The inner integument is two-layered along its whole length. Both layers cound about ten cells in circumference. The inner layer consists of small, densely protoplasmic cells, about 10 um in width and elongated in the length direction of the ovule. The cells bordering the endostome are large and papiillate. The outer layer consists of more or less cubic cells, about 20 um in width, with large vacuoles. Cells above the level of the nucellar apex are radially stretched and help to close the micropyle. A distinct cuticular layer occurs between the nucellus and the inner integument. The outer integument is one-layered, sometimes locally two-layered at the flanks, about 18 cells in circumference. The funicle and adjacent raphe are of very unusual structure. Funicular and raphal tissues are composed of very few cells showing intercellulars and are not bound by a continuous epidermis at the inner side."

Pollination: L. D. Gomez (1983) reports Trigona bees visiting Apodanthes in Costa Rica. In Panama, Croat (1978) reports bees, small butterflies and even mosquitos.

Fruit: An irregularly dehiscent fleshy berry with numerous small seeds. In Pilostyles, little differentiation occurs betwen the mature flower and fruit other than a slight swelling of the ovary and the drying/darkening of the persistent perianth lobes.

Seed: 0.29 to 0.56 mm in length, oval to spherical or pyriform in shape. Testa composed of one layer of thin-walled tanniferous cells. Details of the testa and exotegmin are given in Bouman and Meijer (1994).

Embryo sac Development: Polygonum type.
Embryo: small, undifferentiated (only 8 cells!).
Endosperm: development nuclear; mature endosperm one or two layered surrounding embryo; containing oil and protein bodies.

Fruit/Seed Dispersal: For Pilostyles, rats, mice and ants have been suggested. For Apodanthes (L. D. Gomez 1983), Thraupis birds have been observed eating the fruits. The host specificity (Flacourtiaceae) of Apodanthes suggested to Croat (1978) a more complex dispersal system (e.g. a host-dependent phytophagus insect).

Chromosomes: Pilostyles thurberi, n=30-31 including 0-2 B chromosomes (see Rutherford 1970)

Link to Family Description in Delta

SIUC / College of Science / Parasitic Plant Connection/ Apodanthaceae / Description
URL: http://www.parasiticplants.siu.edu/Apodanthaceae/description.html
Last updated: 17-June-10 / dln