50
JOHN G. TORREY
source of reduced nitrogen for heart-stage embryos. However, he was unable to get normal development of globular embryos of Capsella in his
most complex synthetic medium. Some success was achieved with early
heart-stage embryos of Datura (Van Overbeek et al., 1942, 1944) in a
nutrient medium including inorganic salts, vitamins, and added coconut
milk (the liquid endosperm of coconut which is a complex natural mixture of inorganic elements, carbohydrates, vitamins, amino acids, hormones, and other organic metabolites).
Increasingly smaller isolated embryos of dicotyledonous plants developed best in vitro in these media if provided with increasing concentrations of sugar, preferably sucrose. Whereas 1-2% sucrose sufficed for
late stages of Capsella embryos, heart-stage embryos grew best with 812% sucrose, and early heart-stage embryos grew be^t with 18% sucrose
in the medium. According to these authors, the sucrose played an osmotic
role: it duplicated the high osmotic concentration of the endosperm
liquid within the embryo sac. Direct measurements of the osmotic concentration of ovular sap of the monocot Haemanthus
and the dicot
Crambe and others by Ryczkowski (1960) do, in fact, confirm the idea
of an increasing osmotic concentration surrounding the developing embryo, followed by a slow decrease toward embryo maturity. These increases can be accounted for in large part by changing concentrations
of reducing and nonreducing sugars (Ryczkowski, 1962).
Although not studied in great detail, other aspects of the environment
seemed to be relatively less important than the chemical environment.
In Datura the carbon dioxide concentration in the gaseous environment
within the ovary was shown to be slightly increased (Rietsema and
Blondel, 1959), presumably favoring embryonic development. Also, there
is reason to believe that embryos in vivo receive low light and perhaps
specially filtered illumination. In general, it appears that high light intensity inhibits normal embryo development in cultured embryos.
Much of the work on embryo culture to date has emphasized the increasing complexity of the nutrient requirements at progressively younger
stages of embryo development as well as the generally increased dependence on the maternal environment in general. This dependence has
been interpreted to mean that the younger embryos lacked biosynthetic
capacities which had to be taken care of by the nutrients in the medium.
Extrapolated back to the zygote, one could argue that many critical
biosynthetic enzyme systems were inoperative—that the zygote is a
highly repressed cell containing as active metabolic systems only those
required to maintain the cell as an intact structural entity, but lacking
activity of those essential to normal development.
Over the past two decades, one finds in reviewing the literature that
JOHN G. TORREY
source of reduced nitrogen for heart-stage embryos. However, he was unable to get normal development of globular embryos of Capsella in his
most complex synthetic medium. Some success was achieved with early
heart-stage embryos of Datura (Van Overbeek et al., 1942, 1944) in a
nutrient medium including inorganic salts, vitamins, and added coconut
milk (the liquid endosperm of coconut which is a complex natural mixture of inorganic elements, carbohydrates, vitamins, amino acids, hormones, and other organic metabolites).
Increasingly smaller isolated embryos of dicotyledonous plants developed best in vitro in these media if provided with increasing concentrations of sugar, preferably sucrose. Whereas 1-2% sucrose sufficed for
late stages of Capsella embryos, heart-stage embryos grew best with 812% sucrose, and early heart-stage embryos grew be^t with 18% sucrose
in the medium. According to these authors, the sucrose played an osmotic
role: it duplicated the high osmotic concentration of the endosperm
liquid within the embryo sac. Direct measurements of the osmotic concentration of ovular sap of the monocot Haemanthus
and the dicot
Crambe and others by Ryczkowski (1960) do, in fact, confirm the idea
of an increasing osmotic concentration surrounding the developing embryo, followed by a slow decrease toward embryo maturity. These increases can be accounted for in large part by changing concentrations
of reducing and nonreducing sugars (Ryczkowski, 1962).
Although not studied in great detail, other aspects of the environment
seemed to be relatively less important than the chemical environment.
In Datura the carbon dioxide concentration in the gaseous environment
within the ovary was shown to be slightly increased (Rietsema and
Blondel, 1959), presumably favoring embryonic development. Also, there
is reason to believe that embryos in vivo receive low light and perhaps
specially filtered illumination. In general, it appears that high light intensity inhibits normal embryo development in cultured embryos.
Much of the work on embryo culture to date has emphasized the increasing complexity of the nutrient requirements at progressively younger
stages of embryo development as well as the generally increased dependence on the maternal environment in general. This dependence has
been interpreted to mean that the younger embryos lacked biosynthetic
capacities which had to be taken care of by the nutrients in the medium.
Extrapolated back to the zygote, one could argue that many critical
biosynthetic enzyme systems were inoperative—that the zygote is a
highly repressed cell containing as active metabolic systems only those
required to maintain the cell as an intact structural entity, but lacking
activity of those essential to normal development.
Over the past two decades, one finds in reviewing the literature that