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Materials For Growth: Cells require a basic supply of raw materials for growth; when any of them is used up, growth stops, regardless of how much of the other nutrients remain. Second, the environment becomes too toxic to permit further growth. Cells pour out waste materials as they respire and synthesize needed components. At low population densities the level of these materials remains insignificant, but as the density increases, the environment becomes so toxic that further growth is impossible, regardless of the food supply.
Although all of these techniques are being ir proved and refined constantly, none yields pr cise dates. Only dendrochronology, which allov the scientist to date wooden materials by cor paring their growth rings with the known pa tern of growth rings in trees dating back to aboi 3,000 years, is exact, and then only within i narrow limits.
Two significant advances in materials during the past year have been (1) the growth of much better gallium arsenide and gallium phosphide single crystals through the use of liquid epitaxy techniques, and (2) the synthesis of new single-crystalline materials such as lithium tantalate and barium sodium niobate. The gallium arsenide and gallium phosphide crystals have been used in constructing improved solid-state lasers, electroluminescent lamps, and Gunn oscillators, while the latter materials have been used in nonlinear optical systems.
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