ORGANIZED DEVELOPMENT IN PLANTS
45
biochemical and physiological mechanisms at work. The material is difficult to work with, to fix, to orient, and to interpret, but the technique
offers an intriguing new approach to the questions posed by morphogenesis.
How does one study the initial processes whereby a single-celled
zygote buried in maternal tissue begins to organize into a multicellular
system? The description of the process should tell us the cellular
events, the timing of cell divisions, and the differential directions of
oriented divisions and of cell enlargement. Such parameters have been
detailed for a limited number of genera—for example, Capsella (Rijven,
1952), Datura (Rietsema and Blondel, 1959), and Gossypium
(Jensen,
1964). Precise timing of embryo development, cell numbers, cell and
embryo size, and cell arrangement and internal differentiation are part
of the analysis which serves as a useful background for more experimental approaches. Such details are available for too few genera; this
leads to a situation where we know quite a bit about a very few genera
and tend to generalize from the behavior of these few. Thus, Capsella,
Datura, and Gossypium are most studied from this point of view. These
studies fit together well with related anatomical studies of a wider group
of plants. Cellular patterns and developmental stages have been worked
out for many species [for example, for Phlox by Miller and Wetmore
(1945)], but without precise timing, so that systems suitable for experimental work are few.
Two major points which come from a comparison of the many anatomical studies of embryogenesis may be made. Both points have been
emphasized in recent years by those interested in understanding the
mechanisms. The first point is the striking similarity of the earliest stages
of embryogenesis in many plants—the typical first unequal division of
the zygote, which usually sets off the cell which will form the embryo
proper from that which gives rise to the cells of a supporting structure or
suspensor. As has been well illustrated by Wardlaw (1955), these early
stages in embryogenesis seem to be comparable in many large taxonomic
groups in the plant kingdom. This basic behavior of zygotes remains to
be clearly understood in terms of its developmental significance. The
second point which has recently been restated by Jensen (1964) is that
the botanist should not seek to define precise patterns of cellular lineages
in embryongenesis. To do so is to formalize what in nature is not so
stereotyped and, further, can be misleading in trying to understand the
processes of development. Thus, Gossypium
embryos reach a rather
large globular stage with apparently random divisions before oriented
divisions begin to define the organization of cotyledons, the future radicle,
and epicotyl. In Capsella where the globular embryo has fewer cells than
45
biochemical and physiological mechanisms at work. The material is difficult to work with, to fix, to orient, and to interpret, but the technique
offers an intriguing new approach to the questions posed by morphogenesis.
How does one study the initial processes whereby a single-celled
zygote buried in maternal tissue begins to organize into a multicellular
system? The description of the process should tell us the cellular
events, the timing of cell divisions, and the differential directions of
oriented divisions and of cell enlargement. Such parameters have been
detailed for a limited number of genera—for example, Capsella (Rijven,
1952), Datura (Rietsema and Blondel, 1959), and Gossypium
(Jensen,
1964). Precise timing of embryo development, cell numbers, cell and
embryo size, and cell arrangement and internal differentiation are part
of the analysis which serves as a useful background for more experimental approaches. Such details are available for too few genera; this
leads to a situation where we know quite a bit about a very few genera
and tend to generalize from the behavior of these few. Thus, Capsella,
Datura, and Gossypium are most studied from this point of view. These
studies fit together well with related anatomical studies of a wider group
of plants. Cellular patterns and developmental stages have been worked
out for many species [for example, for Phlox by Miller and Wetmore
(1945)], but without precise timing, so that systems suitable for experimental work are few.
Two major points which come from a comparison of the many anatomical studies of embryogenesis may be made. Both points have been
emphasized in recent years by those interested in understanding the
mechanisms. The first point is the striking similarity of the earliest stages
of embryogenesis in many plants—the typical first unequal division of
the zygote, which usually sets off the cell which will form the embryo
proper from that which gives rise to the cells of a supporting structure or
suspensor. As has been well illustrated by Wardlaw (1955), these early
stages in embryogenesis seem to be comparable in many large taxonomic
groups in the plant kingdom. This basic behavior of zygotes remains to
be clearly understood in terms of its developmental significance. The
second point which has recently been restated by Jensen (1964) is that
the botanist should not seek to define precise patterns of cellular lineages
in embryongenesis. To do so is to formalize what in nature is not so
stereotyped and, further, can be misleading in trying to understand the
processes of development. Thus, Gossypium
embryos reach a rather
large globular stage with apparently random divisions before oriented
divisions begin to define the organization of cotyledons, the future radicle,
and epicotyl. In Capsella where the globular embryo has fewer cells than