The Cycadaceae are woody, unbranched or sparsely branched, palmlike, dioecious, seed-bearing trees or shrubs with thick, pithy stems. The leaves are alternate, spirally arranged in a cluster at the summit of the stem, frondlike, pinnately compound, usually stiff, often with sharply pointed leaflets that have a single midvein (without laterals) and exhibit circinnate vernation. The ovules and seeds (2-8) are born naked on the petioles of reduced leaves that are loosely clustered at the stem apex of female plants. Male plants produce male or microsporangiate cones that bear many scales, each with an abundance of microsporangia scattered over the lower surface. Seeds are typically large.
[Details of reproduction of cycads: one of four haploid megaspores in each ovule potentially produces a highly multicellular but dependent female gametophyte bearing 1 or more archegonia, each containing an egg cell. Motile sperm cells are eventually produced by the pollen grains or microgametophytes that develop from the microspores. Once pollen grains are dispersed to the vicinity of the micropyle of the ovule, a pollen tube delivers the sperm in close proximity to the archegonium where the motile sperm may swim through a droplet of nectarlike fluid to reach the egg. The developing embryo derives its nutrients from the female gametophyte.]
Each "thumbnail" image below is linked to a larger photograph.
|Several cycads are visible in this photo. Note the somewhat tufted, pinnately compound leaves.|
|Cycas circinalis, sago palm. This female plant is producing a cluster of modified leaves (megasporophylls) that bear naked ovules along the petiole margins.|
|Cycas revoluta, sago palm. In this species the fertile leaves of the female are a little more highly modified and the collection of megasporophylls is a little more like a cone but is still loosely organized. The middle photo shows a single megasporophyll with naked seeds attached. The third photo shows a male or microsporangiate strobilus or cone of a male plant. The scales are smaller and more numerous than in female cones. Though not visible here, each scale has many microsporangia scattered over the lower surface.|
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