Cycadaceae

Growth Form and Habit

Members are typically palm-like evergreen shrubs or small trees with stout, woody, often unbranched trunks (pachycaul). Trunks can be subterranean or aerial and are usually covered in persistent leaf bases. Growth is generally very slow.

Leaves

Leaves are large, pinnately compound, resembling fern fronds or palm leaves, arranged spirally in a dense terminal crown. The leaflets (pinnae) are typically numerous, leathery, linear or lance-shaped, and possess a characteristic single, prominent midrib with no other visible parallel veins (a key feature distinguishing Cycas from Zamiaceae). Young leaves often exhibit circinate vernation (coiling like a fern fiddlehead). Leaves lack stipules.

Reproductive Structures (No Flowers)

Cycadaceae plants are dioecious (separate male and female individuals). They reproduce via specialized structures, not flowers.

• Male Cones (Microstrobili): Male plants produce large, erect, cone-like structures in the center of the leaf crown. These consist of tightly packed, modified leaves (microsporophylls), each bearing numerous pollen sacs (microsporangia) on its lower surface, which release pollen when mature.
• Female Structures (Megasporophylls): Female Cycas plants do not form a true compact cone like other cycads (Zamiaceae). Instead, they produce successive whorls of individual megasporophylls in the apical crown, typically appearing after a flush of vegetative leaves. Each megasporophyll is a somewhat leaf-like, often hairy structure, bearing several large, exposed ovules along its margins.

Pollination is typically by wind or insects (often specific beetles). Fertilization involves motile sperm cells, an ancient trait shared with Ginkgo and some lower plants.

Seeds (No Fruits)

Following fertilization, the ovules develop into large naked seeds (not enclosed in a fruit). The seeds often have a brightly colored (red, orange, yellow), fleshy outer layer (sarcotesta) surrounding a hard inner stony layer (sclerotesta), making them resemble drupes superficially. Seeds contain toxic compounds like cycasin.

Roots

In addition to a primary taproot system, Cycadaceae possess specialized coralloid roots. These grow near the soil surface, are branched and nodular (resembling coral), and host symbiotic nitrogen-fixing cyanobacteria (blue-green algae, e.g., Nostoc), providing nutrients to the plant.

Chemical Characteristics

All parts of the plant, especially seeds, contain potent neurotoxins, primarily cycasin and related compounds, as well as BMAA (β-Methylamino-L-alanine). These serve as defenses against herbivores but make the plants dangerous if ingested without proper processing.