114
LUISE STANGE
branch growth occurred, indicating that apical dominance decreased
with increasing age.
In his studies on the developmental physiology of the water-fern
Marsilea, Allsopp (1953) showed that the lateral buds, one at the base of
each leaf, normally remain dormant unless the main rhizome apex is
damaged or removed. Under such conditions, growth activity begins in
nearly all the lateral buds but the one nearest to the apex quickly gains
the ascendency and the others are then suppressed. However, when
the rhizome is cut into segments, each containing one node with its
dormant lateral bud ('excised nodes'), a new plant is usually produced
from the lateral bud of each segment.
Investigations on the problem of apical dominance in higher plants
have repeatedly shown (at first by Thimann and Skoog, 1934) that the
influence of the apical bud can be replaced by application of indoleacetic
acid to the apical cut surface. This result has led to the widely held view
that auxin produced in the apical growing bud inhibits the growth of
lower buds. To explain these observations different hypotheses have
been proposed, which attribute to the auxin either a direct or an indirect
role. It is necessary to give here a short summary of the different possibilities visualized for the mechanism underlying the inhibition of lateral
buds by auxin. Thimann (1937, 1952) assumed that auxin itself is the
major inhibiting influence and its influence on different organs is represented by different optimum curves, optimal concentrations promoting
growth and higher concentrations inhibiting it. The hypotheses proposing an indirect action are of a different kind. Snow (1937, 1940) presented evidence to support his hypothesis that auxin in migrating
through stem tissue is either converted into an inhibiting substance or
stimulates the production of an inhibitor. Went (1936, 1939) emphasized the view that auxin produced in actively growing regions directs
the transport of nutrients of specific substances to these regions. Van
Overbeek (1938) finally attributed the inactivity of lateral buds to the
impediment of transport of nutrients due to incomplete vascular connexion of the lateral buds. High auxin concentrations prevent the
formation of vascular connexions between axillary buds and the main
shoot. This view has recently been supported by results of Gregory and
Veale (1957). These last hypotheses are similar to the conception of
earlier botanists such as Goebel (1908, 1928) and Loeb (1924) that the
main apex is an 'attraction centre' diverting the nutrients from the
lateral buds; competition for nutrients was assumed to be the main
factor in correlative inhibition. More recently it has been pointed out
that growth or inhibition of lateral buds is determined by interactions
between different growth substances (van Overbeek, 1959; Thimann,
1963),
LUISE STANGE
branch growth occurred, indicating that apical dominance decreased
with increasing age.
In his studies on the developmental physiology of the water-fern
Marsilea, Allsopp (1953) showed that the lateral buds, one at the base of
each leaf, normally remain dormant unless the main rhizome apex is
damaged or removed. Under such conditions, growth activity begins in
nearly all the lateral buds but the one nearest to the apex quickly gains
the ascendency and the others are then suppressed. However, when
the rhizome is cut into segments, each containing one node with its
dormant lateral bud ('excised nodes'), a new plant is usually produced
from the lateral bud of each segment.
Investigations on the problem of apical dominance in higher plants
have repeatedly shown (at first by Thimann and Skoog, 1934) that the
influence of the apical bud can be replaced by application of indoleacetic
acid to the apical cut surface. This result has led to the widely held view
that auxin produced in the apical growing bud inhibits the growth of
lower buds. To explain these observations different hypotheses have
been proposed, which attribute to the auxin either a direct or an indirect
role. It is necessary to give here a short summary of the different possibilities visualized for the mechanism underlying the inhibition of lateral
buds by auxin. Thimann (1937, 1952) assumed that auxin itself is the
major inhibiting influence and its influence on different organs is represented by different optimum curves, optimal concentrations promoting
growth and higher concentrations inhibiting it. The hypotheses proposing an indirect action are of a different kind. Snow (1937, 1940) presented evidence to support his hypothesis that auxin in migrating
through stem tissue is either converted into an inhibiting substance or
stimulates the production of an inhibitor. Went (1936, 1939) emphasized the view that auxin produced in actively growing regions directs
the transport of nutrients of specific substances to these regions. Van
Overbeek (1938) finally attributed the inactivity of lateral buds to the
impediment of transport of nutrients due to incomplete vascular connexion of the lateral buds. High auxin concentrations prevent the
formation of vascular connexions between axillary buds and the main
shoot. This view has recently been supported by results of Gregory and
Veale (1957). These last hypotheses are similar to the conception of
earlier botanists such as Goebel (1908, 1928) and Loeb (1924) that the
main apex is an 'attraction centre' diverting the nutrients from the
lateral buds; competition for nutrients was assumed to be the main
factor in correlative inhibition. More recently it has been pointed out
that growth or inhibition of lateral buds is determined by interactions
between different growth substances (van Overbeek, 1959; Thimann,
1963),