Cycas

Cycas (Class Cycadopsida, Order Cycadales) is widely distributed across the tropical and sub-tropical eastern hemisphere; C. revoluta, C. beddomei, C. circinalis and C. rumphii are common species. The slow-growing, evergreen, xerophytic sporophyte resembles a small palm.

Externally, the columnar, unbranched, woody stem is crowned by spirally arranged pinnately compound leaves and covered in persistent old leaf bases, with adventitious buds at its base. Cycas has two kinds of roots: a persistent primary tap root, and dichotomously branching, coral-like coralloid roots that grow vertically upward just below the soil surface — their cortex hosts the nitrogen-fixing blue-green alga Anabaena. There are also two kinds of leaves: large, pinnately compound foliage (assimilatory) leaves, each with 80–100 pairs of sessile leaflets (single midvein, no lateral veins, circinate vernation, young leaves covered in protective ramenta), and small, brown, triangular, protective scale leaves.

Internally, the primary root shows an epiblema, cortex and a vascular region that is diarch in young roots but tetrarch with age (secondary growth is present); the coralloid root has a similar plan but its middle cortex hosts the Anabaena-containing algal zone, and its xylem is triarch and exarch. The young stem, irregular in outline because of persistent leaf-base armour, shows an epidermis, a broad starch- and mucilage-rich parenchymatous cortex, a ring of collateral, endarch vascular bundles (resembling a dicot stem) around a central pith, with characteristic direct and girdling leaf traces and, after secondary growth, a manoxylic, polyxylic wood. The rachis shows vascular bundles arranged in a distinctive inverted-omega pattern, each bundle diploxylic (xylem on both centripetal and centrifugal faces). The leaflet shows mesophyll differentiated into palisade and spongy tissue, colourless transfusion tissue for lateral water movement, and a mesarch vascular bundle.

Cycas reproduces vegetatively (adventitious buds/bulbils at the stem base) and sexually. It is dioecious and heterosporous: the male cone is borne singly at the stem tip, made of spirally arranged microsporophylls, each bearing thousands of microsporangia grouped into sori on its lower surface (development is eusporangiate); each microspore mother cell undergoes meiosis to give haploid microspores (pollen grains) — rounded, uninucleate cells with a thick outer exine and thin inner intine, representing the male gametophyte. The megasporophylls, by contrast, are not organised into a cone — they occur as a loose spiral near the stem tip of the female plant, each a flat structure bearing ovules laterally. Cycas produces the largest ovule in the plant kingdom: orthotropous, unitegmic, with a thick three-layered integument (fleshy outer sarcotesta, stony middle sclerotesta, and an inner layer fused to the nucellus), a small opening (the micropyle), and a pollen chamber formed where the nucellus dissolves near its tip. A single megaspore mother cell undergoes meiosis to give four haploid megaspores (only the lowest survives), and an enlarged megaspore (embryo sac) sits inside the nucellus with an archegonial chamber of 3–6 archegonia just below the pollen chamber. …