Cycadaceae
Pinophyta (Gymnosperms): Cycadophytina (Cycads). The Cycadaceae are woody, unbranched
or sparsely branched, palmlike, dioecious, seed-bearing trees or shrubs with thick, pithy
stems. The leaves are frondlike, pinnately compound, usually stiff, and often with sharply
pointed leaflets that exhibit circinnate vernation. Leaves are alternate, and spirally
arranged in a cluster at the summit of the stem. The ovules and seeds are born naked on
the petioles of reduced leaves that are loosely clustered or more highly modified and
compacted into female or ovulate cones. 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. Male plants produce male or microsporangiate
cones that bear many scales, each with an abundance of microsporangia scattered over the
lower surface. 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. Seeds are typically large, sometimes up to the size of goose eggs.
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 lower left photo shows a single megasporophyll
with naked seeds attached. The lower right 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. |
|
|
Encephalartos sp. This female plant has a very large and compact female cone or
megasporangiate strobilus. The individual megasporophylls are more highly modified and not
at all the leaf-like megasporophylls seen in Cycas. The lower photo shows a
mature, disintegrating female cone with the lower cone scales still intact. Two large
seeds can be seen on each of the scales. |
 |
Zamia furfuracea. These female cones are much smaller than those of the
preceding species. |
 |
Ceratozamia sp. Two microsporangiate strobili are visible on this male plant. |
Non-Flowering Plant Family Access Page
Home Page
