234
F.
C. STEWARD AND Η. Y. MOHAN
RAM
undifferentiated and here the vegetative plant body consists only of
thalloid structures.
In the majority of vascular plants however, the activity of well
defined apical growing regions produces the primary tissues of the
plant body. Besides these regions, axillary and adventitious buds, leaf
bases, vascular cambium and phellogen (cork cambium) constitute other
centres of growth, the activity of which must be subject to some form
of regulatory control. It is, therefore, a task for those who would interpret
morphogenesis in chemical terms to understand the stimuli that release
or suppress these activities. Moreover, the pattern of orderly development that flows from the growth of a fertilized egg is not a fixed and
invariable genetic consequence of a 'built-in' capacity to grow, for it
may give rise to different expressions in response to external, morphogenetic, stimuli.
Much recent evidence indicates that fully formed parenchyma cells,
which have passed through many cell generations in culture, may still
retain a degree of totipotency which is comparable with that of the
zygote. Thus solutions of the salient problems of morphogenesis may be
sought in different ways. First, one may attempt to see causal explanations for the events of embryogenesis which lead to the organized
growing regions of shoot and root and, in turn, see how these growing
regions perform the feats of organogenesis. Alternatively, one may
strive to see the nature of the formative substances that may operate in
development by comparing them with those that cause relatively
unorganized cells, or tissue cultures, to grow and re-organize. This has
motivated much recent work. In fact, according to Skoog and Miller
(1957), one could now say that balanced stimuli such as IAA (an auxin)
and kinetin (a so-called kinin) somewhat modified by particular amino
acids (notably tyrosine) will cause callus cells to form shoot-like growths.
In other words, the stimuli which act on cells to cause cell enlargement or cell division may also operate, in the tissue mass, to foster
organization.
This general field of enquiry which regards morphogenesis as the
interaction of organs and organisms endowed with a special or distinctive
'nature' with environments that furnish distinctive 'nurture' will now
be considered.
The primary shoot and root growing regions arise while the embryo is
still subject to the special nurture which its environment provides and
which is superimposed upon the inherent genetic influence provided
at fertilization. By contrast, as already indicated, the embryos of many
saprophytic or semi-parasitic angiosperms are often but poorly developed even at the maturity of the seed (Figs. 1(d) and (e)).
Returning now to the embryo, the earlier it is isolated from the ovule
F.
C. STEWARD AND Η. Y. MOHAN
RAM
undifferentiated and here the vegetative plant body consists only of
thalloid structures.
In the majority of vascular plants however, the activity of well
defined apical growing regions produces the primary tissues of the
plant body. Besides these regions, axillary and adventitious buds, leaf
bases, vascular cambium and phellogen (cork cambium) constitute other
centres of growth, the activity of which must be subject to some form
of regulatory control. It is, therefore, a task for those who would interpret
morphogenesis in chemical terms to understand the stimuli that release
or suppress these activities. Moreover, the pattern of orderly development that flows from the growth of a fertilized egg is not a fixed and
invariable genetic consequence of a 'built-in' capacity to grow, for it
may give rise to different expressions in response to external, morphogenetic, stimuli.
Much recent evidence indicates that fully formed parenchyma cells,
which have passed through many cell generations in culture, may still
retain a degree of totipotency which is comparable with that of the
zygote. Thus solutions of the salient problems of morphogenesis may be
sought in different ways. First, one may attempt to see causal explanations for the events of embryogenesis which lead to the organized
growing regions of shoot and root and, in turn, see how these growing
regions perform the feats of organogenesis. Alternatively, one may
strive to see the nature of the formative substances that may operate in
development by comparing them with those that cause relatively
unorganized cells, or tissue cultures, to grow and re-organize. This has
motivated much recent work. In fact, according to Skoog and Miller
(1957), one could now say that balanced stimuli such as IAA (an auxin)
and kinetin (a so-called kinin) somewhat modified by particular amino
acids (notably tyrosine) will cause callus cells to form shoot-like growths.
In other words, the stimuli which act on cells to cause cell enlargement or cell division may also operate, in the tissue mass, to foster
organization.
This general field of enquiry which regards morphogenesis as the
interaction of organs and organisms endowed with a special or distinctive
'nature' with environments that furnish distinctive 'nurture' will now
be considered.
The primary shoot and root growing regions arise while the embryo is
still subject to the special nurture which its environment provides and
which is superimposed upon the inherent genetic influence provided
at fertilization. By contrast, as already indicated, the embryos of many
saprophytic or semi-parasitic angiosperms are often but poorly developed even at the maturity of the seed (Figs. 1(d) and (e)).
Returning now to the embryo, the earlier it is isolated from the ovule