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C. STEWARD AND Η. Y. MOHAN RAM
relatively rigid. Thus a factor or factors which could keep the surfaces
plastic and fluid in the terminal part of the pro-embryo, while permitting
them to become solid and more rigid in the suspensor could go far to
explain these events. By analogy with the cases cited above, it would
seem that the thin walls of the pro-embryo would be random cellulose
fibrils in a homogeneous matrix (like the first formed wall on a Valonia
sporeling), whereas the walls of the suspensor may be expected to be
spirally wound around its length and therefore to be elastic, not plastic.
So far as the authors are aware, no examination of this point has
been made, but this is clearly an area badly in need of investigation
which could profitably be undertaken with the electron microscope. The
extent to which the suspensor becomes attached or anchored, could
alone cause asymmetry in its growth (cf. the sporeling of V. ventricosa),
whereas the terminal pro-embryo cells, destined to form the embryo
proper, are more free to reflect uniform external stimuli and to respond
to nutrients with which they are bathed.
Although the zygote in the angiosperm grows in a more or less
unrestricted space, this is not so in the ferns. In the latter situations,
mechanical constraints seem to impose their effect upon the way the
embryo develops. Following out this train of thought, Ward and
Wetmore (1954) have used surgical techniques to modify the mechanical
environment and have studied the consequential effects upon embryo
development. After partially releasing (the zygote could not be isolated
without injuring it) the restraining influences of the venter and jacket
cells of the calyptra that surround a fertilized egg in the fern Phlebodium
aureum, these authors showed that such embryos exhibited: (1) no
regular orientation of cell walls in early embryogeny, (2) postponement
of organ formation, and (3) ultimate development of a normal plant.
The physical environment, therefore, does interfere with the timing and
orderly development of events. In extreme cases, the embryo developed
into a tuber-like undifferentiated mass of parenchyma, from which,
ultimately, shoots developed. Recently, in a private communication,
Professor Wetmore has disclosed that in Todea barbara, a fern of the
osmundaceae, fertilized eggs, before they embark on their first division,
have been isolated without injury and cultured successfully on a
nutrient medium. By randomly oriented divisions, an isolated zygote
produces an undifferentiated mass which, after attaining a critical size,
will first produce a leaf and later a root. These observations recall the
work on carrot cultures where, after proliferation, a root organizes
within a mass which has achieved a certain minimum or critical size,
but the two cases differ in the first formation of a leaf in the fern and of
a root in the carrot cultures.
At this point it is interesting to recall that Sachs early drew attention
F.
C. STEWARD AND Η. Y. MOHAN RAM
relatively rigid. Thus a factor or factors which could keep the surfaces
plastic and fluid in the terminal part of the pro-embryo, while permitting
them to become solid and more rigid in the suspensor could go far to
explain these events. By analogy with the cases cited above, it would
seem that the thin walls of the pro-embryo would be random cellulose
fibrils in a homogeneous matrix (like the first formed wall on a Valonia
sporeling), whereas the walls of the suspensor may be expected to be
spirally wound around its length and therefore to be elastic, not plastic.
So far as the authors are aware, no examination of this point has
been made, but this is clearly an area badly in need of investigation
which could profitably be undertaken with the electron microscope. The
extent to which the suspensor becomes attached or anchored, could
alone cause asymmetry in its growth (cf. the sporeling of V. ventricosa),
whereas the terminal pro-embryo cells, destined to form the embryo
proper, are more free to reflect uniform external stimuli and to respond
to nutrients with which they are bathed.
Although the zygote in the angiosperm grows in a more or less
unrestricted space, this is not so in the ferns. In the latter situations,
mechanical constraints seem to impose their effect upon the way the
embryo develops. Following out this train of thought, Ward and
Wetmore (1954) have used surgical techniques to modify the mechanical
environment and have studied the consequential effects upon embryo
development. After partially releasing (the zygote could not be isolated
without injuring it) the restraining influences of the venter and jacket
cells of the calyptra that surround a fertilized egg in the fern Phlebodium
aureum, these authors showed that such embryos exhibited: (1) no
regular orientation of cell walls in early embryogeny, (2) postponement
of organ formation, and (3) ultimate development of a normal plant.
The physical environment, therefore, does interfere with the timing and
orderly development of events. In extreme cases, the embryo developed
into a tuber-like undifferentiated mass of parenchyma, from which,
ultimately, shoots developed. Recently, in a private communication,
Professor Wetmore has disclosed that in Todea barbara, a fern of the
osmundaceae, fertilized eggs, before they embark on their first division,
have been isolated without injury and cultured successfully on a
nutrient medium. By randomly oriented divisions, an isolated zygote
produces an undifferentiated mass which, after attaining a critical size,
will first produce a leaf and later a root. These observations recall the
work on carrot cultures where, after proliferation, a root organizes
within a mass which has achieved a certain minimum or critical size,
but the two cases differ in the first formation of a leaf in the fern and of
a root in the carrot cultures.
At this point it is interesting to recall that Sachs early drew attention