V. DETERMINING
FACTORS IN
CELL GROWTH
235
the greater is the difficulty which is encountered in its separate culture.
Whereas angiosperm embryos in the later stages of development are
readily cultured, those which have not advanced beyond the globular
stage do not normally grow when isolated (Rappaport, 1954). Thus
there is associated with the morphological development which occurs at
the heart-shaped stage a corresponding biochemical self-sufficiency, and
this must, in some way, be associated with the differentiation of the
primordial cotyledons. It will be recalled that it is through cotyledons
(Figs. 2(6) and (c); Fig. 1(c)) that the special nutrients of endosperm are
made available to the emlbryo. The appearance of the cotyledons heralds,
concomitantly, the organization of the growing regions in the embryo—
first the shoot apex in the notch between the cotyledons—and later the
radicle, or root apex, opposite the plumule. Although the cotyledons
act primarily in absorbing and storing the nutriment for the embryonal
axis, their appearance in an otherwise globular embryo signifies a
cardinal morphogenetic and biochemical change. It is hard to conceive
the role of the cotyledons solely as that of absorbing organs for ordinary
nutrients, for it is at this level of development of the embryo that the
special role of the endosperm, with its content of growth factors, seems
to be paramount.
As stated before, the familiar 'endoscopic' polarity of angiosperm
embryos seems to follow from the attachment, at one end, of the
filamentous suspensor which carries, at the other end, an embryonal
head. The growth of the filament pushes the embryonal head into the
mass of special nutritive materials by which it is nurtured.
The primary root normally originates from the hypophysis cell. The
hypophysis is situated, opposite the plumule, at the point of attachment
of the suspensor with the globular embryo and is hardly different, in any
essential or genetic respect, from the rest of the embryonal or suspensor
cells. It may be, however, and presumably is subject to different
asymmetric stimuli by virtue of its position. It is hard to visualize
whether these stimuli emanate from the embryonic 'head' or from the
suspensor, or both. It would, however, be more credible if the initiation
of the primary root requires substances or stimuli transmitted by the
globular embryonal 'head', with its rudimentary cotyledons, which
absorb or re-work the nutrients present in the endosperm. If this were
so, the dependence of root upon shoot would be evident from its inception. [Several features are of interest here. First, the regeneration of
roots on shoots is far more common than the reverse (Priestley and
Swingle, 1929).] In some water plants like Trapa, a primary root apex
is lacking, but in such plants roots arise adventitiously after germination
of the seeds.
From a certain point the embryo, whether still growing hetero-
FACTORS IN
CELL GROWTH
235
the greater is the difficulty which is encountered in its separate culture.
Whereas angiosperm embryos in the later stages of development are
readily cultured, those which have not advanced beyond the globular
stage do not normally grow when isolated (Rappaport, 1954). Thus
there is associated with the morphological development which occurs at
the heart-shaped stage a corresponding biochemical self-sufficiency, and
this must, in some way, be associated with the differentiation of the
primordial cotyledons. It will be recalled that it is through cotyledons
(Figs. 2(6) and (c); Fig. 1(c)) that the special nutrients of endosperm are
made available to the emlbryo. The appearance of the cotyledons heralds,
concomitantly, the organization of the growing regions in the embryo—
first the shoot apex in the notch between the cotyledons—and later the
radicle, or root apex, opposite the plumule. Although the cotyledons
act primarily in absorbing and storing the nutriment for the embryonal
axis, their appearance in an otherwise globular embryo signifies a
cardinal morphogenetic and biochemical change. It is hard to conceive
the role of the cotyledons solely as that of absorbing organs for ordinary
nutrients, for it is at this level of development of the embryo that the
special role of the endosperm, with its content of growth factors, seems
to be paramount.
As stated before, the familiar 'endoscopic' polarity of angiosperm
embryos seems to follow from the attachment, at one end, of the
filamentous suspensor which carries, at the other end, an embryonal
head. The growth of the filament pushes the embryonal head into the
mass of special nutritive materials by which it is nurtured.
The primary root normally originates from the hypophysis cell. The
hypophysis is situated, opposite the plumule, at the point of attachment
of the suspensor with the globular embryo and is hardly different, in any
essential or genetic respect, from the rest of the embryonal or suspensor
cells. It may be, however, and presumably is subject to different
asymmetric stimuli by virtue of its position. It is hard to visualize
whether these stimuli emanate from the embryonic 'head' or from the
suspensor, or both. It would, however, be more credible if the initiation
of the primary root requires substances or stimuli transmitted by the
globular embryonal 'head', with its rudimentary cotyledons, which
absorb or re-work the nutrients present in the endosperm. If this were
so, the dependence of root upon shoot would be evident from its inception. [Several features are of interest here. First, the regeneration of
roots on shoots is far more common than the reverse (Priestley and
Swingle, 1929).] In some water plants like Trapa, a primary root apex
is lacking, but in such plants roots arise adventitiously after germination
of the seeds.
From a certain point the embryo, whether still growing hetero-