II.

Varieties

Apple trees have been cultivated for their fruit for many centuries. Early apple growers selected superior strains from wild seedlings and propagated them by grafting. Currently, many apple varieties are developed by controlled crossing of desirable parents. Beneficial mutation in standard varieties is also a source for new varieties. It is generally thought that modern apple varieties resulted from natural cross-pollination involving several species because modern varieties are heterozygous—that is, they do not reproduce true to type.

Apple trees are best adapted to areas in which the average temperature approaches or reaches freezing during at least two months. The exact chilling requirement varies somewhat with each variety. The trees withstand temperatures down to -40°C (-40°F).