194
F.
C. STEWARD AND
Η. Y. MOHAN
RAM
other hand, in some plants, like orchids, the seed at maturity is virtually
free of storage substances and its endosperm is very much reduced.
In these cases the embryo requires special organic nutrients which
otherwise might have been supplied by the endosperm, and these must
be obtained from the environment or from the green cells which appear
early after germination of the seed. In those plants which have endosperm it may persist or be abundant to very different degrees. In plants
in which the embryo develops rapidly, the endosperm may be in large
part consumed; in contrast, where the embryo develops slowly, the
endosperm may accumulate due to its relatively rapid growth. Thus, the
extent to which an embryo may develop, in some instances, is regulated
by the quantity of endosperm, and any interference with endosperm
development may present a barrier to the growth of the embryo.
Where the endosperm is consumed during early development of the
embryo, the food material it represents is usually stored up in the
cotyledons (i.e. exalbuminous seeds of a dicotyledonous plant usually
have prominent cotyledons and vice versa), and in turn the cotyledons
nourish the embryo at germination. Growth and the immediate utilization by the embryo of the food deposited in the cotyledons (or endosperm) is prevented by a period of dormancy; this in turn' permits
the seed to function as an organ of perennation. The onset of dormancy
may occur at almost any stage of the development of the embryo. An
extreme case of dormancy in a stage as early as a pro-embryo, comprising only 5 cells but accompanied by endosperm, has been described
for the saprophytic angiosperm Leiphaimos spectabilis (Oehler, 1927)
and, in the case of the coconut, somewhat prolonged embryo dormancy
sets in at a fairly early stage of development. Although the nutritive
stimulus is thus commonly conveyed by the endosperm, cases exist
where the endosperm may be totally lacking as in members of the
Trapaceae (Ram, 1956) and Podostemonaceae (Razi, 1949), or may be
ephemeral as in orchids. In these latter cases, associated with the lack
of endosperm, the embryo is characteristically undifferentiated when
the seed is ready for dispersal. While the last cases cited are exceptional,
the endosperm-embryo relationship is quite characteristic in the majority
of angiosperms. Undifferentiated embryos are also characteristic of
parasitic angiosperms like Orobanche (Fig. 1(e)).
B. Some Problems Posed
Thus the endosperm with its content of special nutrients would be
interesting if only because of the role it plays in the development of the
young embryo. However, in recent years these materials have acquired
an even greater biochemical significance, for they have proved able to
F.
C. STEWARD AND
Η. Y. MOHAN
RAM
other hand, in some plants, like orchids, the seed at maturity is virtually
free of storage substances and its endosperm is very much reduced.
In these cases the embryo requires special organic nutrients which
otherwise might have been supplied by the endosperm, and these must
be obtained from the environment or from the green cells which appear
early after germination of the seed. In those plants which have endosperm it may persist or be abundant to very different degrees. In plants
in which the embryo develops rapidly, the endosperm may be in large
part consumed; in contrast, where the embryo develops slowly, the
endosperm may accumulate due to its relatively rapid growth. Thus, the
extent to which an embryo may develop, in some instances, is regulated
by the quantity of endosperm, and any interference with endosperm
development may present a barrier to the growth of the embryo.
Where the endosperm is consumed during early development of the
embryo, the food material it represents is usually stored up in the
cotyledons (i.e. exalbuminous seeds of a dicotyledonous plant usually
have prominent cotyledons and vice versa), and in turn the cotyledons
nourish the embryo at germination. Growth and the immediate utilization by the embryo of the food deposited in the cotyledons (or endosperm) is prevented by a period of dormancy; this in turn' permits
the seed to function as an organ of perennation. The onset of dormancy
may occur at almost any stage of the development of the embryo. An
extreme case of dormancy in a stage as early as a pro-embryo, comprising only 5 cells but accompanied by endosperm, has been described
for the saprophytic angiosperm Leiphaimos spectabilis (Oehler, 1927)
and, in the case of the coconut, somewhat prolonged embryo dormancy
sets in at a fairly early stage of development. Although the nutritive
stimulus is thus commonly conveyed by the endosperm, cases exist
where the endosperm may be totally lacking as in members of the
Trapaceae (Ram, 1956) and Podostemonaceae (Razi, 1949), or may be
ephemeral as in orchids. In these latter cases, associated with the lack
of endosperm, the embryo is characteristically undifferentiated when
the seed is ready for dispersal. While the last cases cited are exceptional,
the endosperm-embryo relationship is quite characteristic in the majority
of angiosperms. Undifferentiated embryos are also characteristic of
parasitic angiosperms like Orobanche (Fig. 1(e)).
B. Some Problems Posed
Thus the endosperm with its content of special nutrients would be
interesting if only because of the role it plays in the development of the
young embryo. However, in recent years these materials have acquired
an even greater biochemical significance, for they have proved able to