82
JOHN G. TORREY
part of the expiant. The isolated apical dome itself without any leaf
primordia will not develop in vitro, even though provided with a variety
of nutrients and added growth factors (Ball, 1960). This result is in
contrast to studies of the isolated culture of the apical meristem in ferns
and in other lower vascular plants (such as Selaginella, Lycopodium,
or
Equisetum)
where isolated apices lacking leaf primordia were grown to
whole plants (Wetmore, 1954). In the case of the isolated apical dome
of Lupinus, Ball (1960) was able to produce shoots with up to nine leaf
primordia following treatment with GA and coconut milk. The apices
failed, however, to develop into whole plants. No root formation occurred
in these explants. On the same medium, isolated apical domes excised
from Tropaeolum and Lycopersicum
esculentum failed to develop.
These results with the apical meristem per se are of considerable interest when thought of in relation to the evidence discussed above concerning the developmental expression of single cells or small cell clumps
from callus tissues grown in culture. In the angiosperms the cells of the
shoot apical meristem itself are quite specialized and appear to resist
change. They do not easily respond to externally supplied chemical
stimuli which might be expected to induce cell divisions or callus-like
proliferation. According to Ball (1960), the cells of the apical meristem
in the angiosperms have undergone biochemical differentiation with a
resultant loss of biosynthetic capacity. Another way to express the idea
would be that cells in their site in the apical meristem are, like the
zygote, highly repressed cells. Whether cultural conditions can be found
that will allow such cells either to become free of repression or to regain
their biosynthetic capacity remains to be seen. The assumption is made
that tissues associated with and including adjacent leaf primordia initially excised with the apical dome do retain an essential capacity which
allows the development of the whole organism. Why the isolated apical
dome, which can form leaves if properly nurtured, cannot develop into a
whole plant is not easily explained. It is of interest that the apical dome
itself does not readily give rise to a callus, although peripheral tissues
of the apex on a suitable medium will form a callus tissue. Both in the
organized shoot apical meristem and in the root apical meristem, there
appears to reside a self-contained and continuing integrity which is not
easily upset or modified. Although the evidence for this idea is not very
great, it is a reasonable guess based on our present knowledge that these
meristems are themselves centers of hormone synthesis and as such are
self-determining in some respects. Thus the biochemical differentiation
suggested by Ball might be an increased capacity to synthesize one or
more plant hormones.
A related topic which should be mentioned for completeness is the
JOHN G. TORREY
part of the expiant. The isolated apical dome itself without any leaf
primordia will not develop in vitro, even though provided with a variety
of nutrients and added growth factors (Ball, 1960). This result is in
contrast to studies of the isolated culture of the apical meristem in ferns
and in other lower vascular plants (such as Selaginella, Lycopodium,
or
Equisetum)
where isolated apices lacking leaf primordia were grown to
whole plants (Wetmore, 1954). In the case of the isolated apical dome
of Lupinus, Ball (1960) was able to produce shoots with up to nine leaf
primordia following treatment with GA and coconut milk. The apices
failed, however, to develop into whole plants. No root formation occurred
in these explants. On the same medium, isolated apical domes excised
from Tropaeolum and Lycopersicum
esculentum failed to develop.
These results with the apical meristem per se are of considerable interest when thought of in relation to the evidence discussed above concerning the developmental expression of single cells or small cell clumps
from callus tissues grown in culture. In the angiosperms the cells of the
shoot apical meristem itself are quite specialized and appear to resist
change. They do not easily respond to externally supplied chemical
stimuli which might be expected to induce cell divisions or callus-like
proliferation. According to Ball (1960), the cells of the apical meristem
in the angiosperms have undergone biochemical differentiation with a
resultant loss of biosynthetic capacity. Another way to express the idea
would be that cells in their site in the apical meristem are, like the
zygote, highly repressed cells. Whether cultural conditions can be found
that will allow such cells either to become free of repression or to regain
their biosynthetic capacity remains to be seen. The assumption is made
that tissues associated with and including adjacent leaf primordia initially excised with the apical dome do retain an essential capacity which
allows the development of the whole organism. Why the isolated apical
dome, which can form leaves if properly nurtured, cannot develop into a
whole plant is not easily explained. It is of interest that the apical dome
itself does not readily give rise to a callus, although peripheral tissues
of the apex on a suitable medium will form a callus tissue. Both in the
organized shoot apical meristem and in the root apical meristem, there
appears to reside a self-contained and continuing integrity which is not
easily upset or modified. Although the evidence for this idea is not very
great, it is a reasonable guess based on our present knowledge that these
meristems are themselves centers of hormone synthesis and as such are
self-determining in some respects. Thus the biochemical differentiation
suggested by Ball might be an increased capacity to synthesize one or
more plant hormones.
A related topic which should be mentioned for completeness is the