44
JOHN G. TORREY
fusing with the egg nucleus to form the zygote, the other sperm nucleus
fusing with two or several nuclei of the embryo sac to form the endosperm nucleus. The zygote is initially arrested in its development, for a
few days to a week or more, whereas the endosperm nucleus rapidly
proliferates by mitotic divisions and fills the embryo sac with a multinucleate coenocytic fluid which later becomes cellular. The endosperm
provides the elaborate nutrient materials which the embryo draws upon
for its development (see review by Steward and Shantz, 1959, and
Steward and Mohan Ram, 1961). Thus, the single-celled zygote, embedded in an elaborate medium upon which it draws for inorganic and
organic nutrients as a heterotrophic nonphotosynthetic organism, is perhaps the epitome of a meristemoid. The evidence suggests that the
embryo depends also upon this environment for the external stimuli
which lead to mitotic activity and cell division—essential processes in
the initiation of the organized structures of the embryo. The zygote
contains the potential in its genetic constitution for all the ultimate
developmental expressions of the adult. Yet it is a highly repressed cell
capable of expressing these potentialities only in a multicellular progeny.
Once started, cell divisions and accompanying cell enlargement produce a multicellular embryo which passes rapidly from a single cell to a
multicellular structure of hundreds and then thousands of cells, with
distinctive cellular patterns which can be described in terms of specific
morphological stages: globular, heart, torpedo, etc. In many angiosperme, the mature embryo develops in the ovule, absorbing and using
up the entire endosperm and surrounding tissues of the ovule; when the
embryo is mature, it occupies the entire interior of the ovule, surrounded
only by investing layers of the ovule integuments, which are termed the
seed coats at this stage. By now the mature embryo is highly organized
and possesses the organized primordia of the shoot apex, the root apex,
even sometimes the youngest embryonic leaves, and usually as storage
structures one or more cotyledons. The mature seed may then enter a
period of developmental inactivity and only later, perhaps weeks,
months, or many years, become active again, germinate, and develop
into the seedling and then adult plant. In some plants, such as certain
species of the orchids, the seed is shed from the maternal plant when
the embryo is quite immature or only beginning to develop. In such
cases, development proceeds very slowly with rather heavy dependence
upon the external environment.
Descriptions of the embryo sac and early embryogeny in angiosperms
are now being pushed to the ultrastructural level (Jensen, 1964). Here
we may expect new insights into the early stages of development and,
perhaps, answers to morphological questions which will make clear the
JOHN G. TORREY
fusing with the egg nucleus to form the zygote, the other sperm nucleus
fusing with two or several nuclei of the embryo sac to form the endosperm nucleus. The zygote is initially arrested in its development, for a
few days to a week or more, whereas the endosperm nucleus rapidly
proliferates by mitotic divisions and fills the embryo sac with a multinucleate coenocytic fluid which later becomes cellular. The endosperm
provides the elaborate nutrient materials which the embryo draws upon
for its development (see review by Steward and Shantz, 1959, and
Steward and Mohan Ram, 1961). Thus, the single-celled zygote, embedded in an elaborate medium upon which it draws for inorganic and
organic nutrients as a heterotrophic nonphotosynthetic organism, is perhaps the epitome of a meristemoid. The evidence suggests that the
embryo depends also upon this environment for the external stimuli
which lead to mitotic activity and cell division—essential processes in
the initiation of the organized structures of the embryo. The zygote
contains the potential in its genetic constitution for all the ultimate
developmental expressions of the adult. Yet it is a highly repressed cell
capable of expressing these potentialities only in a multicellular progeny.
Once started, cell divisions and accompanying cell enlargement produce a multicellular embryo which passes rapidly from a single cell to a
multicellular structure of hundreds and then thousands of cells, with
distinctive cellular patterns which can be described in terms of specific
morphological stages: globular, heart, torpedo, etc. In many angiosperme, the mature embryo develops in the ovule, absorbing and using
up the entire endosperm and surrounding tissues of the ovule; when the
embryo is mature, it occupies the entire interior of the ovule, surrounded
only by investing layers of the ovule integuments, which are termed the
seed coats at this stage. By now the mature embryo is highly organized
and possesses the organized primordia of the shoot apex, the root apex,
even sometimes the youngest embryonic leaves, and usually as storage
structures one or more cotyledons. The mature seed may then enter a
period of developmental inactivity and only later, perhaps weeks,
months, or many years, become active again, germinate, and develop
into the seedling and then adult plant. In some plants, such as certain
species of the orchids, the seed is shed from the maternal plant when
the embryo is quite immature or only beginning to develop. In such
cases, development proceeds very slowly with rather heavy dependence
upon the external environment.
Descriptions of the embryo sac and early embryogeny in angiosperms
are now being pushed to the ultrastructural level (Jensen, 1964). Here
we may expect new insights into the early stages of development and,
perhaps, answers to morphological questions which will make clear the