MORPHOGENESIS OF CROWN
GALL
57
wound edge into division. However, these cells divided at a slower rate
than those closer to the wound edge. The cells of the cambium and the
conductive tissue also divided. This resulted in the formation of smallcelled rows oriented toward the wound cap. The pith was slow to react,
but once the cells started to divide the rate of division was relatively
rapid. All the cells together, but especially those originating in the pith,
formed a callus tissue that burst out from the wound cap as an outgrowth.
The wound reaction in the leaves of the same plant was not as drastic
as in the stem. All the cells close to the wound edge divided, but the
divisions were not as strictly oriented as in the stem.
In plants that normally develop a cambium in the stem or are otherwise very reactive, the cell row formation is often followed by restriction
of the cell divisions to a certain distance from the wound edge. This gives
rise to a so-called wound cambium. When fully developed, it runs
parallel to the wound edge a few cell layers from it. This cambium has
been studied especially in trees, and several hypotheses have been presented to explain its growth pattern. P. M. Warren Wilson and J. Warren
Wilson (1961) refer to the "free surface" hypothesis of Berteand and
Vochting, according to which a cambium is bound to arise beneath and
generally parallel to each natural or artificial free surface, and to the
"cambial ring" hypothesis of Jansen and Snow, according to which the
interruption of the old cambium induces the regeneration of a new one
that extends from the cut edges of the ring in such a way as to form a
closed ring. P. M. Warren Wilson and J. Warren Wilson (1961) have
formulated a "gradient induction" hypothesis according to which in undifferentiated tissue a gradient for some factor tends to arise perpendicular to the exposed surface, and a vascular cambium forms at a
certain position on this gradient where the factor is at an appropriate
level. Of course, fundamentally, none of these hypotheses explains the
cambium formation and/or its location. Zasche (1960) has claimed that
the formation of the cambium at a later stage after wounding can follow
different patterns even in one and the same tree species.
Outside the wound cambium the cells continue to divide for some time
but often they enlarge and form a more or less loosely organized wound
callus (Kupila-Ahvenniemi, 1966). Changes in the cell walls are frequent
(Bloch, 1952). At the wound surface a thickening of the cell walls takes
place. The conductive bundles which are broken by the wounding react
by closing the vessels and the sieve elements. Engleman and Esau (1964)
have observed that sieve plates of Impatiens
sultanii may be plugged
completely within 30 min after wounding.
GALL
57
wound edge into division. However, these cells divided at a slower rate
than those closer to the wound edge. The cells of the cambium and the
conductive tissue also divided. This resulted in the formation of smallcelled rows oriented toward the wound cap. The pith was slow to react,
but once the cells started to divide the rate of division was relatively
rapid. All the cells together, but especially those originating in the pith,
formed a callus tissue that burst out from the wound cap as an outgrowth.
The wound reaction in the leaves of the same plant was not as drastic
as in the stem. All the cells close to the wound edge divided, but the
divisions were not as strictly oriented as in the stem.
In plants that normally develop a cambium in the stem or are otherwise very reactive, the cell row formation is often followed by restriction
of the cell divisions to a certain distance from the wound edge. This gives
rise to a so-called wound cambium. When fully developed, it runs
parallel to the wound edge a few cell layers from it. This cambium has
been studied especially in trees, and several hypotheses have been presented to explain its growth pattern. P. M. Warren Wilson and J. Warren
Wilson (1961) refer to the "free surface" hypothesis of Berteand and
Vochting, according to which a cambium is bound to arise beneath and
generally parallel to each natural or artificial free surface, and to the
"cambial ring" hypothesis of Jansen and Snow, according to which the
interruption of the old cambium induces the regeneration of a new one
that extends from the cut edges of the ring in such a way as to form a
closed ring. P. M. Warren Wilson and J. Warren Wilson (1961) have
formulated a "gradient induction" hypothesis according to which in undifferentiated tissue a gradient for some factor tends to arise perpendicular to the exposed surface, and a vascular cambium forms at a
certain position on this gradient where the factor is at an appropriate
level. Of course, fundamentally, none of these hypotheses explains the
cambium formation and/or its location. Zasche (1960) has claimed that
the formation of the cambium at a later stage after wounding can follow
different patterns even in one and the same tree species.
Outside the wound cambium the cells continue to divide for some time
but often they enlarge and form a more or less loosely organized wound
callus (Kupila-Ahvenniemi, 1966). Changes in the cell walls are frequent
(Bloch, 1952). At the wound surface a thickening of the cell walls takes
place. The conductive bundles which are broken by the wounding react
by closing the vessels and the sieve elements. Engleman and Esau (1964)
have observed that sieve plates of Impatiens
sultanii may be plugged
completely within 30 min after wounding.