Olacaceae s. str. R. Br.

Genera Included: Dulacia Sleumer, Olax L., and Ptychopetalum Benth.

Habit: Terrestrial, evergreen trees and shrubs, rarely lianas.

Parasitism: root hemiparasites, well documented for Olax (see References). 

Roots: haustorial.  Anatomy of the haustorium of Olax phyllanthii described by Pate et al. (1990).

Stem: without stipules, young branches often zig-zag, longitudinally striate.  Stems sometimes 4-sided and winged in Olax, terete in Ptychopetalum. 

Leaves:  Simple, alternate or alternate-distichous, rarely decussate, glabrous, margin entire. Primary venation pinnate, secondary venation brochidodromous, tertiary venation reticulate, quarternary venation (aerolation) meshes polygonal.  

Leaf anatomy: leaf unicellular hairs absent or present; epidermal cells and guard cells not lignified;  lumina of guard cells narrow; cuticular ledges of guard cells inconsipuous and thick;  paracytic stomata absent or present; cyclocytic and anisocytic stomata absent; schizogenous cavities in leaf absent;  laticiferous channel in leaf absent;  druses in epidermal cells absent;  silicified walls of mesophyll and epidermal cells present;  mesophyll sclereids absent;  leaf cystoliths absent; petiole and median vein slerenchyma fibers and astroslereids absent;  vascularization of the nodes unilacunar or trilacunar; basal and distal petiole vascularization a simple bundle; median vein vascularization a simple bundle.

Wood anatomy: vessel grouping solitary or grouped in multiples;  perforation plate type scalariform; vessel member length over 900 µm;  intervascular pits alternate; vascular tracheids assoc. w/ vessels present; fiber tracheids and libriform fibers present; axillary parenchyma abundant;  axial parenchyma strand width less than 7 cells; ray type heterocellular, one row of erect cells; ray height short, below 1000 µm; wood cystoliths absent; silica bodies in ray cells present.

Inflorescence: axillary, cymose racemes, axis zig-zag (Dulacia, Olax); pedicels subtended by caducous bracts that are usually small but sometimes persist and become foliose. 

Plant Sex: flowers generally bisexual (rarely unisexual in Olax).

Flowers: buds oval in shape; flowers 5-6 merous.

• Calyx: sepals developed but completely fused into a cup, edges often not formed into discrete lobes, sometimes forming a hypanthium; accrescent in fruit in Dulacia and Olax.
• Corolla: connate at the base or to one half the corolla length; petals valvate, linear, white, glabrous on outer surface, pubescent on inner surface, often with uncinate apex; in Olax a petal ligule may be present, i.e. a membranous protruberance between the base of the stamen filament and the petal.
• Nectary: surrounding ovary. May be well-developed (but not accrescent) or not (Ptychopetalum).
• Androecium: in one or two whorls, containing stamens and/or staminodes; generally 3-6 (Dulacia and Olax) or 10 (or less by abortion) in Ptychopetalum;  filaments fused at base to petals; anthers bilocular, dorsifixed or basifixed, introrse, longitudinally dehiscent by slits (rarely apically dehiscent).
• Staminodia: three of the five outer whorl (antesepalous) stamens are staminodes in Ptychopetalum (the other two remain fertile). In some Olax, the outer whorl is usually converted to staminodes and some of these may be lost. Floral variation among Olax species suggests that the floral plan of pentamerous species is pseudodiplostemonous, i.e., derived from a trimerous protype (see Nickrent et al. 2010).
• Pollen: shape breviaxial; symmetry isopolar; mescolpium shape flat or convex.  apocolpium shape convex;  aperture number three to four, zonoaperturate and six, diploporate; ectoaperture shape round (porous, elliptic);  apertural membrane granular or vericose;  ecto- and endoaperture of identical size;  endoaperture granules present, endoaperture smooth;  exine in mesocolpium smooth or with a microperforate tectum;  granules in infratectum present; columella in infratectum absent;  foot layer surface irregular with masses.
• Gynoecium: hypogynous to semi-inferior (embedded in glandular tissue), 1-locular, sometimes 3-locular at base, style short or long, cylindrical; stigma slightly lobed.
• Ovule: from free-central placenta, one per locule, anatropous, tenuinucellate, uni- or ategmic.
• Embryo, etc. Embryological studies have been conducted by Shamanna (1954) and Agarwal (1963), both on Olax.

Fruit: drupes, surrounded by accrescent calyx in Dulacia and some Olax; endocarp coriaceous or crustaceous. 

Seed: with copious oily endosperm, also starch present (Dulacia); embryo tiny.

Chromosomes: Chromosome numbers have been reported for Olax nana (Khosla, 1978; Mehra, 1976): n = 12, 24 and Olax hypoleuca (Carr & McPherson, 1986) with the same numbers.

Alternate Family Names: none.