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JOHN G. TORREY
in cotton, a smaller range of patterns of cell division is to be seen, but
even these patterns vary and yet lead to the same final structure. Thus
embryogenesis in the angiosperms assumes a range of patterns of cell
division which one cannot understand as a single-line course; rather
embryo development follows a wide path with patterns of cell division
and enlargement varying, but held within broad limits which result in
recognizable morphological types. Such a view allows one to think of the
control of morphogenesis in terms of a constantly changing and adjusting
system where external forces of the physical and chemical environment
influence and modify the behavior of a plastic cellular population whose
basic organization proceeds under the drive of internally controlled forces
which ultimately are activated at the level of the genes.
B. Quantitative Analysis of Embryo Development
One can treat embryo development as a process subject to analysis
more or less independent of its cellular basis, that is, a process related
to increase in mass per unit time which has a fundamental chemicophysical basis. Although this approach is too simple, it allows one to
look at parameters of change with a view to exploring developmental phenomena which proceed more or less independent of
morphology. Such analyses are of considerable interest. As has been
shown in the case of Datura (Rietsema and Blondel, 1959), one can plot
with the same curve increasing fresh or dry weight, increase in volume,
increase in protein nitrogen, or increase in cell number on essentially the
same graph: only the values of the coordinates need be changed. The
logarithmic increase in fresh weight or volume proceeds with time at a
constant rate, quite without regard to the changing shape of the embryo
from early globular stage to the mature embryo. During this entire
period, the average cell size changes very little. Thus one must expect
a constant rate of cell division during this period of development. What
is surprising is that, in the face of this constant increase in volume
and a constant rate of cell division, the progressive morphological
changes do occur. The basic question of morphogenesis in embryogeny
centers around this puzzle: "How is form superimposed on these apparently constant cellular processes?"
Chemical analyses are not easily performed on all stages of plant
embryos. Early embryos are manifestly difficult to isolate in numbers
amenable to direct extraction and chemical analysis. Analyses of later
stages have not contributed much to our general thinking about developmental processes. In later stages, depending on the embryo, storage
materials become an increasing part of the total mass. Carbohydrates
and proteins increase commensurate with increases in cell walls and cell
JOHN G. TORREY
in cotton, a smaller range of patterns of cell division is to be seen, but
even these patterns vary and yet lead to the same final structure. Thus
embryogenesis in the angiosperms assumes a range of patterns of cell
division which one cannot understand as a single-line course; rather
embryo development follows a wide path with patterns of cell division
and enlargement varying, but held within broad limits which result in
recognizable morphological types. Such a view allows one to think of the
control of morphogenesis in terms of a constantly changing and adjusting
system where external forces of the physical and chemical environment
influence and modify the behavior of a plastic cellular population whose
basic organization proceeds under the drive of internally controlled forces
which ultimately are activated at the level of the genes.
B. Quantitative Analysis of Embryo Development
One can treat embryo development as a process subject to analysis
more or less independent of its cellular basis, that is, a process related
to increase in mass per unit time which has a fundamental chemicophysical basis. Although this approach is too simple, it allows one to
look at parameters of change with a view to exploring developmental phenomena which proceed more or less independent of
morphology. Such analyses are of considerable interest. As has been
shown in the case of Datura (Rietsema and Blondel, 1959), one can plot
with the same curve increasing fresh or dry weight, increase in volume,
increase in protein nitrogen, or increase in cell number on essentially the
same graph: only the values of the coordinates need be changed. The
logarithmic increase in fresh weight or volume proceeds with time at a
constant rate, quite without regard to the changing shape of the embryo
from early globular stage to the mature embryo. During this entire
period, the average cell size changes very little. Thus one must expect
a constant rate of cell division during this period of development. What
is surprising is that, in the face of this constant increase in volume
and a constant rate of cell division, the progressive morphological
changes do occur. The basic question of morphogenesis in embryogeny
centers around this puzzle: "How is form superimposed on these apparently constant cellular processes?"
Chemical analyses are not easily performed on all stages of plant
embryos. Early embryos are manifestly difficult to isolate in numbers
amenable to direct extraction and chemical analysis. Analyses of later
stages have not contributed much to our general thinking about developmental processes. In later stages, depending on the embryo, storage
materials become an increasing part of the total mass. Carbohydrates
and proteins increase commensurate with increases in cell walls and cell