Flor is a protective layer of naturally occurring yeast that grows, under certain circumstances, in the barrels after fortification. Its presence (or absence) is what determines the style of a given Sherry. When it does grow, it protects the wine from oxidation while feasting on the available glycerin - the resultant Sherries are thus dry and very pale in color (called Finos or Manzanillas). Sometimes the layer forms early on, but then falls into the wine and disintegrates - these Sherries thus become more full-bodied and slightly oxidized (called Amontillados). When no flor grows at all, the Sherries become very dark and highly oxidized (called Olorosos). The development of flor is based on a number of climatic factors (temperature, humidity, etc), but is most easily controlled by the degree of fortification of the wines. Yeasts can only grow in environments that contain up to about 16% alcohol - a higher percentage inhibits their ability to develop.