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Open cultural system Any system for growing plants in which nutrient solution is allowed to pass though the root zone and out into the environment. (Nelson, 2003)

Opportunity cost The value of other opportunities (alternatives) given up in order to produce or consume any good; that which must be forfeited when alternative A is abandoned in order t pursue alternative B. (Nelson, 2003)

Organelle One of several types of small structures within plant or animal cells that is bounded by a membrane. The chloroplast is an organelle in which photosynthesis occurs. (Nelson, 2003)

Ovipositor A prominent structure projecting from the posterior end of females of some insects that is used to deposit eggs. (Nelson, 2003)