48
JOHN G. TORREY
presumed to be at the haploid level, although detailed measurement of
these values by microspectrophotometric measurements of stained nuclei
seem not to have been published. (E. G. Pollock and Jensen, 1962, have
reported making such measurements.) The egg nucleus is notoriously difficult to stain with the usual DNA stains, and the DNA is presumed to
be exceedingly diffuse partly because of the large size of the nucleus.
Some question has been raised as to whether extranuclear DNA is to be
found in the plant egg cell or in early stages of embryonic development,
but the matter is not clear. Jensen (1964) reported the incorporation of
H
3 -thymidine into the cytoplasm of early embryos of cotton. Combined
cytological and histochemical information could go far in permitting the
formulation of hypotheses and experimental approaches to early
embryogenesis. Of particular interest would be detailed data on changes
in nuclear and cytoplasmic RNA and nucleoprotein during embryo
development.
The distribution of carbohydrates in early embryos is rather interesting. Halperin and Wetherell (1964) were able to distinguish early
embryological stages in cultured carrot tissues by the prevalence of
large amounts of starch, which was lacking from nonembryonic cells in
culture (see Section III,C). The presence of starch was reported in the
earlier literature on carrot embryos (Borthwick, 1931). In his electronmicroscopic studies of cotton embryos, Jensen (1964) reported a high
content of plastids containing starch grains in the zygote of cotton before
it entered into rapid cell division. With the first division of the zygote,
the suspensor cell contained fewer starch-containing plastids than the cell
of the embryo proper. During subsequent divisions of the embryo, the
starch content per cell decreased, associated with a decrease in cell size
and a decrease in oxygen consumption per cell. These changes in
carbohydrate storage and utilization which have been followed in light
and electron microscopy need to be correlated in experimental systems
which will provide respiratory data, and also information on nutritional
requirements.
III. Embryo Culture
It is unfortunately true that the earliest stages of embryo development
have thus far proved unamenable to isolation from the ovule and study
in vitro. Success has been achieved in the physical isolation from the
ovule of embryos as early as the 4-cell stage, excluding the suspensor,
but no one has successfully developed to maturity isolated embryos
smaller than globular embryos of the approximately 50-cell stage or
about 50 /ι in diameter. Thus, the contribution of embryo culture work
to understanding the early stages of embryo organization is limited. Yet,
JOHN G. TORREY
presumed to be at the haploid level, although detailed measurement of
these values by microspectrophotometric measurements of stained nuclei
seem not to have been published. (E. G. Pollock and Jensen, 1962, have
reported making such measurements.) The egg nucleus is notoriously difficult to stain with the usual DNA stains, and the DNA is presumed to
be exceedingly diffuse partly because of the large size of the nucleus.
Some question has been raised as to whether extranuclear DNA is to be
found in the plant egg cell or in early stages of embryonic development,
but the matter is not clear. Jensen (1964) reported the incorporation of
H
3 -thymidine into the cytoplasm of early embryos of cotton. Combined
cytological and histochemical information could go far in permitting the
formulation of hypotheses and experimental approaches to early
embryogenesis. Of particular interest would be detailed data on changes
in nuclear and cytoplasmic RNA and nucleoprotein during embryo
development.
The distribution of carbohydrates in early embryos is rather interesting. Halperin and Wetherell (1964) were able to distinguish early
embryological stages in cultured carrot tissues by the prevalence of
large amounts of starch, which was lacking from nonembryonic cells in
culture (see Section III,C). The presence of starch was reported in the
earlier literature on carrot embryos (Borthwick, 1931). In his electronmicroscopic studies of cotton embryos, Jensen (1964) reported a high
content of plastids containing starch grains in the zygote of cotton before
it entered into rapid cell division. With the first division of the zygote,
the suspensor cell contained fewer starch-containing plastids than the cell
of the embryo proper. During subsequent divisions of the embryo, the
starch content per cell decreased, associated with a decrease in cell size
and a decrease in oxygen consumption per cell. These changes in
carbohydrate storage and utilization which have been followed in light
and electron microscopy need to be correlated in experimental systems
which will provide respiratory data, and also information on nutritional
requirements.
III. Embryo Culture
It is unfortunately true that the earliest stages of embryo development
have thus far proved unamenable to isolation from the ovule and study
in vitro. Success has been achieved in the physical isolation from the
ovule of embryos as early as the 4-cell stage, excluding the suspensor,
but no one has successfully developed to maturity isolated embryos
smaller than globular embryos of the approximately 50-cell stage or
about 50 /ι in diameter. Thus, the contribution of embryo culture work
to understanding the early stages of embryo organization is limited. Yet,