Ray cell: xylem cells which are extended radially. In wood, ray cells hold annual growth rings together and allow the products of photosynthesis to move in and out of storage in the xylem tissues. Ray cells contain two populations of cells. Most are short-lived tracheids which are similar to ordinary xylem, except for their growth outwards from the center, rather than along the axis of the stem. A few are long-lived parenchymal cells which presumably form a pool of living tissue to extend or repair the system of radial tracheids. Image from Nakaba et al.
Rhizoid: hair-like filamentous projection for anchorage or absorption.
Rhizome: a (usually) underground stem that is horizontally oriented; rhizomes may appear like roots, but have a definite node and internode architecture. Image from the site: Flora of Roosevelt Monmouth County, New Jersey by Ross Tulloss & Mike Hamilton.
Rhynie Chert: See Devonian Sites and Paleozoic Sites. A critical Pragian site in Scotland with plant tissues preserved to the subcellular level. The Rhynie Chert has recently been dated to 396 ± 12 million years. "The site is interpreted as a series of ephemeral freshwater pools within a hot springs ecosystem. The organisms were rapidly fossilized, perhaps as a result of a silica gel fixing." Taylor et al. (2005).
Rhyniophyte: a paraphyletic group of basal tracheophytes which might be defined as all tracheophytes other than Eutracheophyta.
Root: axial structure of the sporophyte which has all or most of the following characteristics: (1) direction of growth (on average) within 90° of gravitational field; (2) growth away from light (negative phototropism); (3) elongation growth is strictly apical, without bifurcation of the meristem (root cap); (4) possession of a histologically distinct root cap; (5) endodermis; (6) protostele (sometimes with a pith); (7) endogenous origin of lateral roots, i.e., branching by the initiation of an entirely new meristematic growth region from the stele. Raven & Edwards (2001) (parts of this definition are directly quoted from the reference).
S-Type Tracheid: See tracheid types.
Saccate pollen: pollen with a saccus or sacci; characteristic of many conifers. See image at bisaccate.
Saccus: ( pl. sacci) a wing-like or bladder-like extension on a pollen grain. See image at bisaccate.
Scalariform: ladder-like, like the rungs on a ladder. Image from Dr. James Mauseth's Plant Anatomy Laboratory site (U. Texas).
Sclerenchyma: tissue composed of (usually dead) cells having walls thickened with lignin; sclerenchyma tissue functions primarily in strengthening and support. Compare chollenchyma, parenchyma. Image from Michael Muller's Bios 100 Summer 2005 site (Univ. Ill. at Chicago).
Sclerotesta: the middle, fibrous layer of the integument in some seeds. Image from Dunn et al. (2002) showing cross sections of a Rhynchosperma quinii seed from the late Mississippian (Serpukhovian?) of Arkansas.
Secondary growth: growth in width initiated and maintained by the vascular cambium and cork cambium. See cambium. The vascular cambium surrounds the core of the stem, branch, or root. It creates xylem medially and phloem radially. If present, the cork cambium, a second, outer layer of cambium, creates a new outer layer (typically bark) radially. There is a really good explanation of secondary growth at Stem - Secondary Growth.
Secondary phloem: phloem produced by the vascular cambium. See phloem, cambium, secondary growth.
Secondary xylem xylem produced by the vascular cambium. See xylem, cambium, secondary growth.
Seed: a fertilized ovule; megasporangium that contains an embryo enclosed in an integument. A seed has been described as "a baby sporophyte in a jacket with a lunch sack." Gymnosperm Evolution. The advantage of a seed is that it can combine the dispersal functions of a spore with sexual reproduction.
Seed fern: any of a number of extinct seed-bearing plants with fern-like leaves. Image of Neuropteris from the Kentucky Paleontological Society web site.
Seed plants: a monophyletic clade of plants that reproduces by seeds; megagametophyte is retained on the parent sporophyte and enclosed in an integument; microgametophyte is transferred to the megagametophyte.
Sepal: a whorl of a flower; sepals often resemble reduced leaves and function in the protection of the bud; sepals may be modified to function more like a petal.
Seta: This incredibly over-used anatomical term is simply Latin for a bristle or a stiff hair. It is used in a variety of contexts, not only for things that look like bristles (or, to be sure, stiff hairs), but also for things like the stalk of the sporophyte in mosses, which looks nothing like a bristle (or, for that matter, a stiff hair).
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