Seafloor spreading
The pulling apart of crustal plates to permit the rise of deep-seated magma to the earth's surface in midocean areas.

Secondary root
A root that extends from a primary root.

Sedimentary basins
Depressions in the surface of the earth that are bounded by elevated topography and contain geologic material laid bown by the action of wind or water.

Senescence
The process of plant degeneration that generally occurs at the end of the growing season.  It is typically characterized by increasing respiration, decreasing growth rates, chlorophyll breakdown, and mobilization of nitrogen out of leaves and into other plant organs.

Shade-intolerant species
Plants that typically grow in places that receive lots of direct sunlight.  They generally have high relative growth rates, highly-regulated stomata and thin leaves.

Shade-tolerant species
Plants that typically grow in places that receive less than full sunlight, such as the lower levels of a forest.  They generally have low relative growth rates, open stomata and thick densely-packed leaves.

Ship tracks
Narrow lines of perturbed regions in marine stratiform clouds, caused by moving ships, which appear brighter than the clouds around them in satellite imagery.  They can also appear as narrow lines of clouds in an otherwise cloud-free sky.

Silviculture
The industry that deals with the care and development of forests.

Sink
A part of a plant that is actively growing and requires large amounts of photosynthetic sugars to support its development.  In many plants, reproductive structures such as flowers and fruits are large sinks for photosynthetic products.

Soluble protein
A protein that is soluble in aqueous solution and that lacks strong associations with cellular membranes.

Sorbitol
A sugar alcohol that is synthesized and transported to sink tissues in apples.  This is different from most C3 plants, which synthesize and transport sucrose as the primary carbon form to sink tissues.

Source
In the parlance of plant physiology, source refers to leaves and/or stems where carbon is acquired by photosynthesis.  The sugars produced at these sites are subsequently transported to areas that are actively involved in growth and development (sinks), where the demand for sugars is high.

SPAR units
Growth chambers that function as Soil-Plant-Atmosphere-Research units.  Each unit has a steel basin for holding soil to accommodate root growth and a Plexiglas chamber for aboveground plant development.

Spores
Reproductive cells produced by seedless vascular plants.  Before seeds evolved, spores were the primary carriers of the plant genetic material used to replicate plant species.

Stomata
Small pores in plant leaves through which water vapor and carbon dioxide diffuse during transpiration and photosynthesis (carbon fixation), respectively.

Stomatal conductance
A plant property related to the ease with which water vapor escapes from plant leaves through small pores in the leaves know as stomata.

Stomatal density
A measure of the number of stomatal pores per unit area of leaf.

Stomatal index
The proportion of stomata to epidermal cells.

Stratosphere
The layer of the atmosphere typically located between 15 and 50 kilometers above sea level that is characterized by stable, less dense dry air.

Sunfleck
A spot of high-intensity direct sunlight that impinges on a leaf for a short period of time.  Sunflecks are responsible for providing up to half of the daily photosynthetically active radiation received by understory plants growing beneath the canopies of forests.

Superoxide dismutase
An antioxidizing enzyme that detoxifies very reactive superoxide molecules.

Symbiont
An organism involved in a mutually beneficial relationship between other organisms that live in close association with it.

Symbiosis
A mutually beneficial relationship between organisms that live in close association with one another.