REGENERATION IN LOWER PLANTS
137
creases equally in neighbouring cells. But then neighbouring cells
differentiate into dividing cells, rhizoid initials and cells with nucleoli
decreasing again in size. This differentiation must be due to the inhibiting correlations in question. In this case, the inhibiting correlations
become effective during the process of embryonization. Embryonization
is initiated in all cells but continues to completion only in a portion of
the cells. The correlations responsible for the inhibition of regeneration
from the marginal cells in the fragment become effective previous to any
increase in nucleolar size in the marginal zone. Measurements of nucleolar diameter in the different zones of rectangular fragments extending
over the whole width of the wing (Fig. 4) showed that the fastest
increase in nucleolar size takes place in the cells of the adaxial zone,
while no increase can be observed in the cells of the marginal zone. In
the middle zone, there is a clear increase in nucleolar size, but it progresses more slowly than in the adaxial zone and does not reach the
same end value. However, when the marginal zone is isolated from the
adjacent cells by another cut parallel to the edge, a clear increase in
nucleolar size in its cells can be observed some time before nuclear
division occurs in the non-subdivided control fragments. As to the
behaviour of marginal cells in the fragments not further subdivided, it
must be assumed that either no reaction at all occurred in these cells
after isolation of the tissue, since their relations to the adjacent adaxial
cells were undisturbed, or the marginal cells did react to isolation but
this reaction did not reach the stage of visible increase in nucleolar size.
However, as can be shown in the same material, correlative inhibition
between the cells of the fragment can become effective also in later
stages of regeneration. Not all the dividing cells in the fragment continue to divide ; many of these cells cease to regenerate further after the
first or after several divisions (cf. Section III, A, 1). As Bünning (1953)
has pointed out, it is a general principle in plant development that a
place of embryonic activity does not tolerate another one in its vicinity.
When in Riella a rectangular fragment is cut out of the wing a short
distance below the apical meristem, the cauloid-regenerates which are
formed in the adaxial zone, always arise near the apical cut surface,
while rhizoids appear in the same zone near the basal edge of the fragment. If the basal part of such a fragment is separated off by another
cut, its cells also produce cauloids. It has been shown that in the wing of
the whole plant apico-basal gradients exist for different metabolic
activities (Stange, 1957) and these gradients are specially steep in the
subapical region. Obviously, in this case the different regenerative
behaviour of the cells in the fragment can be attributed to differences
between the cells already present before isolation. Some cells are superior
to others in respect to the prerequisites for division. They inhibit divi-
137
creases equally in neighbouring cells. But then neighbouring cells
differentiate into dividing cells, rhizoid initials and cells with nucleoli
decreasing again in size. This differentiation must be due to the inhibiting correlations in question. In this case, the inhibiting correlations
become effective during the process of embryonization. Embryonization
is initiated in all cells but continues to completion only in a portion of
the cells. The correlations responsible for the inhibition of regeneration
from the marginal cells in the fragment become effective previous to any
increase in nucleolar size in the marginal zone. Measurements of nucleolar diameter in the different zones of rectangular fragments extending
over the whole width of the wing (Fig. 4) showed that the fastest
increase in nucleolar size takes place in the cells of the adaxial zone,
while no increase can be observed in the cells of the marginal zone. In
the middle zone, there is a clear increase in nucleolar size, but it progresses more slowly than in the adaxial zone and does not reach the
same end value. However, when the marginal zone is isolated from the
adjacent cells by another cut parallel to the edge, a clear increase in
nucleolar size in its cells can be observed some time before nuclear
division occurs in the non-subdivided control fragments. As to the
behaviour of marginal cells in the fragments not further subdivided, it
must be assumed that either no reaction at all occurred in these cells
after isolation of the tissue, since their relations to the adjacent adaxial
cells were undisturbed, or the marginal cells did react to isolation but
this reaction did not reach the stage of visible increase in nucleolar size.
However, as can be shown in the same material, correlative inhibition
between the cells of the fragment can become effective also in later
stages of regeneration. Not all the dividing cells in the fragment continue to divide ; many of these cells cease to regenerate further after the
first or after several divisions (cf. Section III, A, 1). As Bünning (1953)
has pointed out, it is a general principle in plant development that a
place of embryonic activity does not tolerate another one in its vicinity.
When in Riella a rectangular fragment is cut out of the wing a short
distance below the apical meristem, the cauloid-regenerates which are
formed in the adaxial zone, always arise near the apical cut surface,
while rhizoids appear in the same zone near the basal edge of the fragment. If the basal part of such a fragment is separated off by another
cut, its cells also produce cauloids. It has been shown that in the wing of
the whole plant apico-basal gradients exist for different metabolic
activities (Stange, 1957) and these gradients are specially steep in the
subapical region. Obviously, in this case the different regenerative
behaviour of the cells in the fragment can be attributed to differences
between the cells already present before isolation. Some cells are superior
to others in respect to the prerequisites for division. They inhibit divi-