236
F.
C. STEWARD AND
Η. Y. MOHAN RAM
trophically within the seed or later growing autotrophically in the light,
is self-sufficient for all the needed formative substances required by both
shoot and root apices. But there is a subtle difference here between
shoot and root. It has been noted that roots are heterotrophically
dependent on shoots. When they are excised and grown free from the
shoot, it is apparent that this relationship extends beyond the mere
requirement for sugar, for it includes the need for a variety of Β
vitamins, thiamin, nicotinic acid, etc. Addicott (1939), who followed
the pioneer workers in this field, considered these vitamins to be cell
division factors since, in their absence, mitoses in root meristems were
suppressed. Another feature of the root of many dicotyledons is the
autonomous nature of its meristem. Although they are normally
dependent upon the shoot for energy supply and growth factors, whole
roots can be grown if root tips, a few millimetres in length, are cultured
(Bonner and Devirian, 1939; White, 1943; Reinhard, 1954; Street, 1957).
Extremely small root tips (5 mm or less), which lack any mature cells,
cannot be grown without increasing the concentration of components
of the nutritive medium or of some micronutrients, as demonstrated by
Torrey (1955). But, even so, the dramatic result is the feasibility of the
roots of so many dicotyledons roots being continuously cultured from
such minute tips. Against this result, two other systems stand in
marked contrast.
First there is the case of the attempted continuous, aseptic culture
of roots of monocotyledonous plants. Despite some marked success,
notably by Roberts and Street (1955), and Almestrand (1957), monocotyledons are still largely unresponsive, and their roots fail to grow in
media that suffice for many dicotyledonous roots. Can it be that essential
metabolites are elaborated by the cambial or procambial regions of the
dicotyledonous roots which are lacking in the monocotyledonous roots,
and for which they are irreplaceably dependent upon the shoot?
Second, there is the case of the apical tips of shoots which will be
discussed in more detail below. Suffice it to say here that, although the
normal dependence of the root on the shoot may, in many cases, be
replaced by known nutrients and vitamins, the full growth requirements
of the extreme tips of angiosperm shoots have not been successfully
reproduced. Apical segments of Lupinus and Tropaeolum (0-25-0-5 mm)
with 2 or 3 leaf primordia, subjacent tissue and procambium elements
will regenerate into whole plants when grown on a culture medium
containing sucrose and mineral nutrients (Ball, 1946). The development
of a root in these small shoot apices is a determining factor in their
growth. Fern shoot apices (0-25 mm) smaller than those used by Ball
can also be rooted easily in culture and therefore grown into mature
plants (Wetmore, 1959). But apices containing only the dome, and
F.
C. STEWARD AND
Η. Y. MOHAN RAM
trophically within the seed or later growing autotrophically in the light,
is self-sufficient for all the needed formative substances required by both
shoot and root apices. But there is a subtle difference here between
shoot and root. It has been noted that roots are heterotrophically
dependent on shoots. When they are excised and grown free from the
shoot, it is apparent that this relationship extends beyond the mere
requirement for sugar, for it includes the need for a variety of Β
vitamins, thiamin, nicotinic acid, etc. Addicott (1939), who followed
the pioneer workers in this field, considered these vitamins to be cell
division factors since, in their absence, mitoses in root meristems were
suppressed. Another feature of the root of many dicotyledons is the
autonomous nature of its meristem. Although they are normally
dependent upon the shoot for energy supply and growth factors, whole
roots can be grown if root tips, a few millimetres in length, are cultured
(Bonner and Devirian, 1939; White, 1943; Reinhard, 1954; Street, 1957).
Extremely small root tips (5 mm or less), which lack any mature cells,
cannot be grown without increasing the concentration of components
of the nutritive medium or of some micronutrients, as demonstrated by
Torrey (1955). But, even so, the dramatic result is the feasibility of the
roots of so many dicotyledons roots being continuously cultured from
such minute tips. Against this result, two other systems stand in
marked contrast.
First there is the case of the attempted continuous, aseptic culture
of roots of monocotyledonous plants. Despite some marked success,
notably by Roberts and Street (1955), and Almestrand (1957), monocotyledons are still largely unresponsive, and their roots fail to grow in
media that suffice for many dicotyledonous roots. Can it be that essential
metabolites are elaborated by the cambial or procambial regions of the
dicotyledonous roots which are lacking in the monocotyledonous roots,
and for which they are irreplaceably dependent upon the shoot?
Second, there is the case of the apical tips of shoots which will be
discussed in more detail below. Suffice it to say here that, although the
normal dependence of the root on the shoot may, in many cases, be
replaced by known nutrients and vitamins, the full growth requirements
of the extreme tips of angiosperm shoots have not been successfully
reproduced. Apical segments of Lupinus and Tropaeolum (0-25-0-5 mm)
with 2 or 3 leaf primordia, subjacent tissue and procambium elements
will regenerate into whole plants when grown on a culture medium
containing sucrose and mineral nutrients (Ball, 1946). The development
of a root in these small shoot apices is a determining factor in their
growth. Fern shoot apices (0-25 mm) smaller than those used by Ball
can also be rooted easily in culture and therefore grown into mature
plants (Wetmore, 1959). But apices containing only the dome, and