Shoot apical meristems of Picea abies seedlings can be cultured on a relatively simple, defined, basal medium. Dome‐like explants initially about 200 μ tall, without externally obvious primordia, and having dry weights of about 3 μg, usually initiate 5–10 new primordia within a week. They typically show 10‐ to 30‐fold dry weight increases in three weeks. None of the 5,000 meristems cultured has produced any basal callus. Growth is strongly influenced by both the type and concentration of agar used to gel the medium. Dry weight yield increases as agar concentration decreases. This is probably partly due to increased diffusion rates of enzymes or other large molecules through more dilute agar gels but possibly also partly ascribable to unknown agarborne inhibitors. About half of the agar concentration effect can be eliminated by substituting glucose and fructose for sucrose in the medium. This suggests that diffusion of invertase through the agar gel in this medium may be a growth limiting factor. Growth of cultures is also promoted by autoclaving sucrose in the presence of the agar. The basis of this effect is not yet understood.
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