THE FACTORS that control the progre

THE FACTORS that control the progressive and
orderly development of embryos throughout their
ontogeny have attracted the attention of experi-
mental embryologists for some time (Needham,
1942; Brachet, 1950; Wardlaw, 1955). There is
now substantial indirect evidence that growth and
differentiation of plant embryos may result from
an induction by gradients of nutritional substances
and hormonal factors in the immediate environ-
ment of the developing embryos (Rijven, 1952;
Wardlaw, 1955). In the early stages of embryog-
eny, embryos develop at the expense of the
surrounding endosperm tissue, and as far as the
evidence goes, such embryos are highly hetero-
trophic in character. Attempts to culture very
small plant embryos, especially at the morpho-
logical stages designated as "globular" and "heart-
shaped," in chemically defined nutrient media
outside the plant tissues have met with little
success (Van Overbeek, Conklin and Blakeslee,
1942; Rijven, 1952; Norstog, 1961; Swamy, 1961).
It is only after the embryos have developed within
the ovular tissues to the beginning of cotyle-
donary development and the attainment of bi-
lateral symmetry that they become sufficiently
independent and autotrophic to lend themselves
to culture in vitro even in complex nutrient media.
While the nutritional requirements of young em-
bryos are complex, and their responses often
highly variable, progressively older embryos show
less rigorous requirements and more definitive
responses to added nutrients in the medium