V. DETERMINING FACTORS IN CELL GROWTH
251
how they may lead to growth forms that are strongly reminiscent of
typical filamentous pro-embryos (Fig. 6(a) to (j)).
Having established a multicellular mass, however, whether this is a
heart-shaped embryo or a clump grown from free carrot cells, localized
regions seem to exert specific formative influences. Such regions may be
the embryonic cotyledons or the centre of a nodular mass that arises
within the otherwise unorganized carrot tissue culture. Wetmore (1959)
has pointed out that the formation of vascular tissue deep-seated within
the mass, the origin of roots, the formation of a shoot apex and of leaf
primordia, all represent sequential stages in this progressive development from free cells to plants (c/. Plate 1). Plate 1 summarizes the main
features of the developmental sequence from free carrot cells, suspended
in liquid medium containing coconut milk, to the formation of roots,
shoots, embryo-like forms, and thence to whole plants which later form
normal storage roots and eventually flowers with normal meiotic chromosomes in their microspore mother cells (Steward, Mapes and Smith,
1958; Steward, Mapes and Mears, 1958; Mitra, Mapes and Steward,
1960). Somewhat later, Reinert (1959) described the formation of what
are referred to as 'adventive embryos' in cultures of carrot root phloem.
Once apical shoot growing regions are established in this way, their
regulatory control over the subsequent growth is evident through the
basipetal transmission of stimuli which are most dramatically expressed
by the formation of vascular tissue. As stated before, auxin (indoleacetic
acid) seems to be the main stimulus which emanates from the apex, and
this, with the sugar which is necessary for its activity, stimulates the
formation of vascular strands (see Section III). Recent evidence (Jacobs,
1956) suggests that the quantity of auxin produced in young leaves is
the factor that controls vascular differentiation. Indeed, the formative
effect of auxin upon the development of vascular strands is shown by
grafting a shoot apex on to a callus tissue culture, for then strands may
develop within the culture which was not originally in organic connection
with the bud. But the formative effect of the bud upon the callus tissue
may even be replaced by supplying auxin and sugar (Wetmore and
Sorokin, 1955). One now sees, therefore, that localized sources of stimuli
(whether these are the young cotyledons or leaf primordia in the apex,
or the quiescent regions of shoot and root, or the central portions of a
nest or nodule of carrot cells) can exert their formative influences upon
subjacent or surrounding tissue by differential secretion of growthregulating compounds. The interesting thing is that so far the only
compounds or agents that seem to be effective in this way are those that
operate through their control over growth by cell division or growth by
cell expansion.
Outstanding studies of the factors that may control morphogenesis in
251
how they may lead to growth forms that are strongly reminiscent of
typical filamentous pro-embryos (Fig. 6(a) to (j)).
Having established a multicellular mass, however, whether this is a
heart-shaped embryo or a clump grown from free carrot cells, localized
regions seem to exert specific formative influences. Such regions may be
the embryonic cotyledons or the centre of a nodular mass that arises
within the otherwise unorganized carrot tissue culture. Wetmore (1959)
has pointed out that the formation of vascular tissue deep-seated within
the mass, the origin of roots, the formation of a shoot apex and of leaf
primordia, all represent sequential stages in this progressive development from free cells to plants (c/. Plate 1). Plate 1 summarizes the main
features of the developmental sequence from free carrot cells, suspended
in liquid medium containing coconut milk, to the formation of roots,
shoots, embryo-like forms, and thence to whole plants which later form
normal storage roots and eventually flowers with normal meiotic chromosomes in their microspore mother cells (Steward, Mapes and Smith,
1958; Steward, Mapes and Mears, 1958; Mitra, Mapes and Steward,
1960). Somewhat later, Reinert (1959) described the formation of what
are referred to as 'adventive embryos' in cultures of carrot root phloem.
Once apical shoot growing regions are established in this way, their
regulatory control over the subsequent growth is evident through the
basipetal transmission of stimuli which are most dramatically expressed
by the formation of vascular tissue. As stated before, auxin (indoleacetic
acid) seems to be the main stimulus which emanates from the apex, and
this, with the sugar which is necessary for its activity, stimulates the
formation of vascular strands (see Section III). Recent evidence (Jacobs,
1956) suggests that the quantity of auxin produced in young leaves is
the factor that controls vascular differentiation. Indeed, the formative
effect of auxin upon the development of vascular strands is shown by
grafting a shoot apex on to a callus tissue culture, for then strands may
develop within the culture which was not originally in organic connection
with the bud. But the formative effect of the bud upon the callus tissue
may even be replaced by supplying auxin and sugar (Wetmore and
Sorokin, 1955). One now sees, therefore, that localized sources of stimuli
(whether these are the young cotyledons or leaf primordia in the apex,
or the quiescent regions of shoot and root, or the central portions of a
nest or nodule of carrot cells) can exert their formative influences upon
subjacent or surrounding tissue by differential secretion of growthregulating compounds. The interesting thing is that so far the only
compounds or agents that seem to be effective in this way are those that
operate through their control over growth by cell division or growth by
cell expansion.
Outstanding studies of the factors that may control morphogenesis in