ORGANIZED DEVELOPMENT IN PLANTS
47
contents. No sudden switches in chemical processes associated with
changes in form have been reported.
Attempts have been made to find the sequential activation of enzymes
which one might predict in a system exhibiting such coordinated control
(Pardee, 1959). No clear evidence that progressive enzyme activation
plays a role in plant embryo development has been given, although the
search has been quite limited. There are several reports that immature
embryos can be induced to form specific enzymes in response to substrate (Rij ven, 1958, 1961; Raghavan and Torrey, 1964). In the case of
the orchid embryos studied by Raghavan and Torrey, it was quite clear
that the capacity to induce the enzyme nitrate reductase was not present
at all morphological stages of embryo development, but was restricted to
the later seedling stages when leaves and roots were beginning to develop.
As will be discussed later, the nutritional studies from isolated embryo
culture have suggested that progressive enzyme activation might be part
of the basic developmental course in embryogenesis. There is no direct
evidence in support of this view, but it is one which bears further study.
Analyses of physiological activities in addition to specific enzyme
reactions have been made. Because of the very tiny size of the early
embryo stages, special techniques have been used. Thus, studies of gas
exchange were made on immature and mature embryos with Cartesiandiver techniques in Prunus (Β. M. Pollock and Olney, 1959) and in
Gossypium
(Jensen, 1964). Based on whole embryo activity, oxygen
consumption increased with embryo size, which is not unexpected. However, when expressed on a cell basis, cotton embryos showed a relatively high oxygen consumption at the globular stage, a decrease at the
torpedo stage to only one-third the initial value, and then a slight increase
per cell as the mature embryo size was reached. In mature embryos of
Prunus, Pollock and Olney (1959) found changes in the respiratory activity which they associated with the resting condition of the seed. Their
studies did not include the early stages of embryogenesis. These data are
inevitably based on average cell activities and cannot give us a detailed
view of changing cell activities as embryonic cells change during development. Nevertheless, they do demonstrate that critical changes in
metabolism occur during embryo development which may be related to
changing organization. For detailed information at the cellular level,
other techniques have proved useful. For example, recent progress has
been made in histochemical studies of embryo development. Here again
data are quite limited, and we have little more than promise of useful
techniques in studying microscopically small cellular systems. Of fundamental interest are studies on the nucleic acid metabolism in embryogenesis. The amounts of DNA in the egg and in the sperm nuclei are
47
contents. No sudden switches in chemical processes associated with
changes in form have been reported.
Attempts have been made to find the sequential activation of enzymes
which one might predict in a system exhibiting such coordinated control
(Pardee, 1959). No clear evidence that progressive enzyme activation
plays a role in plant embryo development has been given, although the
search has been quite limited. There are several reports that immature
embryos can be induced to form specific enzymes in response to substrate (Rij ven, 1958, 1961; Raghavan and Torrey, 1964). In the case of
the orchid embryos studied by Raghavan and Torrey, it was quite clear
that the capacity to induce the enzyme nitrate reductase was not present
at all morphological stages of embryo development, but was restricted to
the later seedling stages when leaves and roots were beginning to develop.
As will be discussed later, the nutritional studies from isolated embryo
culture have suggested that progressive enzyme activation might be part
of the basic developmental course in embryogenesis. There is no direct
evidence in support of this view, but it is one which bears further study.
Analyses of physiological activities in addition to specific enzyme
reactions have been made. Because of the very tiny size of the early
embryo stages, special techniques have been used. Thus, studies of gas
exchange were made on immature and mature embryos with Cartesiandiver techniques in Prunus (Β. M. Pollock and Olney, 1959) and in
Gossypium
(Jensen, 1964). Based on whole embryo activity, oxygen
consumption increased with embryo size, which is not unexpected. However, when expressed on a cell basis, cotton embryos showed a relatively high oxygen consumption at the globular stage, a decrease at the
torpedo stage to only one-third the initial value, and then a slight increase
per cell as the mature embryo size was reached. In mature embryos of
Prunus, Pollock and Olney (1959) found changes in the respiratory activity which they associated with the resting condition of the seed. Their
studies did not include the early stages of embryogenesis. These data are
inevitably based on average cell activities and cannot give us a detailed
view of changing cell activities as embryonic cells change during development. Nevertheless, they do demonstrate that critical changes in
metabolism occur during embryo development which may be related to
changing organization. For detailed information at the cellular level,
other techniques have proved useful. For example, recent progress has
been made in histochemical studies of embryo development. Here again
data are quite limited, and we have little more than promise of useful
techniques in studying microscopically small cellular systems. Of fundamental interest are studies on the nucleic acid metabolism in embryogenesis. The amounts of DNA in the egg and in the sperm nuclei are