Juglandaceae

Growth Form and Habit

Deciduous trees, occasionally large shrubs. Often possess stout twigs with distinct pith. Bark typically becomes furrowed with age, sometimes exfoliating in plates or long strips (Shagbark Hickory - Carya ovata). Tissues, especially leaves and husks, are often aromatic when crushed due to glandular hairs.

Leaves

Leaves are large, arranged alternately (rarely opposite), and characteristically odd-pinnately compound (sometimes even-pinnate). Leaflets are numerous (ranging from 3 to 20+), typically sessile or short-stalked, often with serrated margins, and frequently aromatic when crushed. Stipules are absent (exstipulate).

Typical large, alternate, odd-pinnately compound leaf (e.g., Hickory).

Inflorescence and Flowers

Plants are typically monoecious, with unisexual, apetalous (lacking petals), wind-pollinated flowers appearing in spring, usually with or before the leaves.

• Male (Staminate) Flowers: Borne in long, slender, pendulous catkins (aments). These usually arise laterally from buds on the previous year's wood or sometimes at the base of the current year's growth. Each flower is small, subtended by a bract, and typically consists of a rudimentary calyx (perianth) and a variable number of stamens (3 to many).
• Female (Pistillate) Flowers: Borne solitary or, more commonly, in few-flowered clusters or short spikes, typically terminal on the current year's growth. Each flower is subtended by an involucre of fused bracts (which develops into the husk). The perianth (calyx) is small, usually 4-lobed, and fused to the top of the inferior ovary.
• Gynoecium (Female Flowers): Ovary is inferior, composed of usually 2 (sometimes 3 or 4) fused carpels. It is typically unilocular (single chamber) below but may become partitioned above by intrusions from the ovary wall. Contains a single, large, basal, orthotropous ovule. Styles usually 2, short, bearing large, plumose or fleshy stigmas adapted for catching windborne pollen.

Male flowers in long pendulous catkins (left); Female flowers in a small terminal cluster, showing inferior ovary and large stigmas (right).

Fruits (Nut in Husk)

The fruit is large and distinctive, consisting of a nut (botanically sometimes considered a drupe-like nut or tryma) enclosed within a husk. The husk develops from the accrescent (enlarging after flowering) involucre of bracts (and sometimes perianth remnants) that surrounded the female flower. The nature of the husk is key for distinguishing genera:

• Juglans (Walnuts): Husk is fleshy or semi-fleshy, typically indehiscent (does not split open naturally), and remains attached to the rough, sculptured nut shell. Pith of twigs is distinctively chambered.
• Carya (Hickories, Pecan): Husk is typically woody or leathery and dehiscent, splitting open along 4 sutures (valves) at maturity to release the relatively smooth or angled nut shell. Pith of twigs is solid and homogenous (though sometimes angled).
• Pterocarya (Wingnuts): Fruit is a small nut with two prominent wings derived from bracts, borne in long pendulous chains.

The seed inside the nut is large, usually 2-4 lobed, lacks endosperm at maturity, and has large, oily, often convoluted cotyledons.

Juglans: Nut in indehiscent husk (left); Chambered twig pith (right).

Carya: Nut released from dehiscent 4-valved husk (left); Solid twig pith (right).

Chemical Characteristics

Tissues often contain tannins and aromatic compounds (terpenoids, naphthoquinones like juglone). Juglone, produced particularly by Juglans nigra, is known for its allelopathic properties, inhibiting the growth of certain other plant species nearby. Seeds are rich in oils and fats. Plants commonly form ectomycorrhizal associations.