stratify

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To form, arrange, or deposit in layers.
• intransitive verb To preserve (seeds) by placing them between layers of moist sand or similar material.
• intransitive verb To arrange or separate into castes, classes, or social levels.
• intransitive verb To separate into a sequence of graded status levels.
• intransitive verb To become layered; form strata.
• intransitive verb To develop different levels of caste, class, privilege, or status.

from The Century Dictionary.

• To preserve (tree-seeds) by spreading them in layers alternating with layers of earth or sand.
• To form into a layer or layers, as substances in the earth; lay or arrange in strata.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To form or deposit in strata, or layers, as substances in the earth; to arrange in strata.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb intransitive To become separated out into distinct layers or strata.
• verb transitive To separate out into distinct layers or strata.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb form, arrange, or deposit in layers
• verb develop different social levels, classes, or castes
• verb render fertile and preserve by placing between layers of earth or sand
• verb form layers or strata
• verb divide society into social classes or castes