60
JOHN G. TORREY
tiation or organized development. Haccius and Reichert (1964) reported
that X-irradiation of immature embryos of Eranthis at levels that damaged tissues of the embryo led to the production of new meristematic
centers which formed viable adventive embryos. Their interpretation
centered on the idea of physiological isolation.
In his account of adventive embryo formation in carrot callus tissue,
Reinert (1959, 1964) emphasized that in the earliest stage of initiation
small cell groups and even single cells could be identified within the callus tissue mass. These cells were distinguished from other callus parenchyma by virtue of their dense cytoplasmic contents and the presence of
a heavy wall, which apparently cut them off in some way from their
neighboring cells. No evidence for physiological isolation was given other
than that they followed a course of differentiation quite unlike other
adjacent callus cells in the tissue. Halperin and Wetherell (1964)
stressed that in a similar system with wild carrot callus tissue they found
early embryos quite intimately embedded in callus parenchyma and that
they were unable to observe evidence of the necessity for physical isolation of cells which were to differentiate as embryos.
Embryonic Development and Apomixis in the Ferns and Mosses
Much interest has centered on the early embryology of these groups
which illustrates an embryonic development of complexity intermediate
between the elaborate supporting structures seen in the angiosperms and
the extreme simplicity seen in some of the algae, where the zygote is free
in the environment and at the outset the single-celled stage is autotrophic
and independent. In the mosses and ferns, the single-celled zygote is
formed within an archegonium (a specialized reproductive structure of
the gametophyte) borne at or near the surface of the vegetative tissues
of the plant. The egg itself forms in the base of the archegonium and is
surrounded by a series of cells which form the venter. Immediately prior
to fertilization, the central cells of the elongate neck of the archegonium
break down; then the motile sperms enter the neck canal and reach the
egg where fertilization takes place. After a few days, the zygote begins to
divide, and the embryonic sporophyte develops within the venter of the
archegonium, still attached to the parent plant. Embryonic development
is believed to occur with nutritional dependence upon the photosynthetic
gametophytic tissues. In late embryogeny, a succession of green leaves
and roots is formed, and the young sporophyte becomes photosynthetic
and independent of the parent prothallus.
In these groups, another single-celled stage in the development of the
vegetative plants is the spore, the product of meiosis, which, upon germination, gives rise typically to the gametophyte plant. Here again, from
JOHN G. TORREY
tiation or organized development. Haccius and Reichert (1964) reported
that X-irradiation of immature embryos of Eranthis at levels that damaged tissues of the embryo led to the production of new meristematic
centers which formed viable adventive embryos. Their interpretation
centered on the idea of physiological isolation.
In his account of adventive embryo formation in carrot callus tissue,
Reinert (1959, 1964) emphasized that in the earliest stage of initiation
small cell groups and even single cells could be identified within the callus tissue mass. These cells were distinguished from other callus parenchyma by virtue of their dense cytoplasmic contents and the presence of
a heavy wall, which apparently cut them off in some way from their
neighboring cells. No evidence for physiological isolation was given other
than that they followed a course of differentiation quite unlike other
adjacent callus cells in the tissue. Halperin and Wetherell (1964)
stressed that in a similar system with wild carrot callus tissue they found
early embryos quite intimately embedded in callus parenchyma and that
they were unable to observe evidence of the necessity for physical isolation of cells which were to differentiate as embryos.
Embryonic Development and Apomixis in the Ferns and Mosses
Much interest has centered on the early embryology of these groups
which illustrates an embryonic development of complexity intermediate
between the elaborate supporting structures seen in the angiosperms and
the extreme simplicity seen in some of the algae, where the zygote is free
in the environment and at the outset the single-celled stage is autotrophic
and independent. In the mosses and ferns, the single-celled zygote is
formed within an archegonium (a specialized reproductive structure of
the gametophyte) borne at or near the surface of the vegetative tissues
of the plant. The egg itself forms in the base of the archegonium and is
surrounded by a series of cells which form the venter. Immediately prior
to fertilization, the central cells of the elongate neck of the archegonium
break down; then the motile sperms enter the neck canal and reach the
egg where fertilization takes place. After a few days, the zygote begins to
divide, and the embryonic sporophyte develops within the venter of the
archegonium, still attached to the parent plant. Embryonic development
is believed to occur with nutritional dependence upon the photosynthetic
gametophytic tissues. In late embryogeny, a succession of green leaves
and roots is formed, and the young sporophyte becomes photosynthetic
and independent of the parent prothallus.
In these groups, another single-celled stage in the development of the
vegetative plants is the spore, the product of meiosis, which, upon germination, gives rise typically to the gametophyte plant. Here again, from