52
JOHN G. TORREY
In Fig. 1 is shown a Capsella embryo at about a 16-cell stage. In
Fig. 2 a Capsella embryo eight times the diameter of that shown in Fig. 1
is still morphologically spherical due to the culture treatment it received.
The similarity in shape and cellular relationships to cell colonies in
culture (Figs. 3, 5, and 6) and to newly initiated endogenous primordia
(Fig. 7) is striking. All of these structures presumably had a single-celled
origin as in Fig. 4. (See p. 70.)
C. Embryogenesis in Tissue Cultures
A second area which has developed rapidly in the last few years and
which has helped change the perspective on the control of embryo development is that of embryo formation in callus tissue cultures. In 1959,
Reinert made the remarkable claim that following a succession of changes
of nutrient media, callus parenchyma tissue derived from the root of
Daucus carota L. and previously subcultured for a prolonged period as
callus underwent differentiation of cells of the callus mass with the
initiation of normal bipolar embryos. He illustrated the process of embryo development as it occurred embedded in the callus tissue in culture.
The changes in nutrient medium were as follows: callus had been maintained on a White's coconut-milk medium containing a high auxin level
(IAA at 10 mg/liter). The tissue was transferred first to a synthetic
medium based on White's medium with added vitamins, purine, amino
acids, and amides and auxin for several months. At the end of this period,
the tissue had become more granular and showed small organized centers
or protuberances on the surface of the tissue. Histological sections of
these callus tissues showed centers of organized development, comparable to globular embryos or later stages of embryonic development.
Transfer of the partially organized tissues was then made to the synthetic
medium lacking auxin but containing added coconut milk. On this last
medium, the embryos developed into whole plants.
In these experiments, the successive changes in nutrient conditions,
notably in the balance of hormone levels, led to internal differentiation of
embryos—not within the complex maternal environment of the embryo
sac, but in the midst of a multicellular mass of parenchyma cells !
Since that finding, a number of reports of embryo formation, the
formation of adventive embryos, of pseudobulbils or other embryo-like
structures from callus tissues have been published. Greatest success has
been achieved with callus derived rather directly from embryos placed
into culture. Considerable work has been done by the group at Delhi
under Maheshwari and Johri. Embryonic propagules have been produced
from callus tissues derived from embryos of the parasitic genera Cuscuta
(Maheshwari and Baldev, 1962), Dendrophthoe
(Johri and Bajaj, 1962,
JOHN G. TORREY
In Fig. 1 is shown a Capsella embryo at about a 16-cell stage. In
Fig. 2 a Capsella embryo eight times the diameter of that shown in Fig. 1
is still morphologically spherical due to the culture treatment it received.
The similarity in shape and cellular relationships to cell colonies in
culture (Figs. 3, 5, and 6) and to newly initiated endogenous primordia
(Fig. 7) is striking. All of these structures presumably had a single-celled
origin as in Fig. 4. (See p. 70.)
C. Embryogenesis in Tissue Cultures
A second area which has developed rapidly in the last few years and
which has helped change the perspective on the control of embryo development is that of embryo formation in callus tissue cultures. In 1959,
Reinert made the remarkable claim that following a succession of changes
of nutrient media, callus parenchyma tissue derived from the root of
Daucus carota L. and previously subcultured for a prolonged period as
callus underwent differentiation of cells of the callus mass with the
initiation of normal bipolar embryos. He illustrated the process of embryo development as it occurred embedded in the callus tissue in culture.
The changes in nutrient medium were as follows: callus had been maintained on a White's coconut-milk medium containing a high auxin level
(IAA at 10 mg/liter). The tissue was transferred first to a synthetic
medium based on White's medium with added vitamins, purine, amino
acids, and amides and auxin for several months. At the end of this period,
the tissue had become more granular and showed small organized centers
or protuberances on the surface of the tissue. Histological sections of
these callus tissues showed centers of organized development, comparable to globular embryos or later stages of embryonic development.
Transfer of the partially organized tissues was then made to the synthetic
medium lacking auxin but containing added coconut milk. On this last
medium, the embryos developed into whole plants.
In these experiments, the successive changes in nutrient conditions,
notably in the balance of hormone levels, led to internal differentiation of
embryos—not within the complex maternal environment of the embryo
sac, but in the midst of a multicellular mass of parenchyma cells !
Since that finding, a number of reports of embryo formation, the
formation of adventive embryos, of pseudobulbils or other embryo-like
structures from callus tissues have been published. Greatest success has
been achieved with callus derived rather directly from embryos placed
into culture. Considerable work has been done by the group at Delhi
under Maheshwari and Johri. Embryonic propagules have been produced
from callus tissues derived from embryos of the parasitic genera Cuscuta
(Maheshwari and Baldev, 1962), Dendrophthoe
(Johri and Bajaj, 1962,