H E T E R O B L A S T I C D E V E L O P M E N T I N P L A N T S
143
In later experiments, a delay in heteroblastic development was obtained
using 2-thiouracil and 5-fluorouracil (White, 1963) or 8-azaguanine
(Allsopp and Rao, unpublished observations).
e. Effects of Growth Substances. A considerable range of experiments
was carried out using various auxins, gibberellic acid, and kinetin. In
early investigations (Allsopp, 1952), it was found that the auxins
indoleacetic acid, naphthaleneacetic acid, and 2,4-dichlorophenoxyacetic
acid had little effect on leaf segmentation, unless present at the higher
range of concentrations which also resulted in a general inhibition of
growth. At such concentrations there was a retardation of heteroblastic
development, which was referred to growth inhibition, especially of the
shoot apex, rather than to any specific effect on leaf segmentation. In
later experiments with indoleacetonitrile (Allsopp, 1954b, 1956) some
reduction in the rate of heteroblastic development was obtained at the
optimal concentration of 10 mg/liter, which induced considerable extension of internodes and petioles. This result is not inconsistent with the
view that heteroblastic development in Marsilea is determined by the
strengthening of the shoot apex, for the active growth of internodes and
petioles might well compete with the shoot apex for the available
nutrients.
In contrast to the auxins, gibberellic acid, when present at 10 or
100 mg/liter, induced some increase in the rate of the heteroblastic leaf
segmentation (Allsopp, 1962).
The cytokinin kinetin produced remarkable leaf abnormalities at the
relatively low concentrations of 0.02 and 0.01 mg/liter, but had no apparent effect on the heteroblastic segmentation of normal leaves (Allsopp
and Szweykowska, 1960).
/. Other Morphological Changes. In addition to the effects on leaf
segmentation outlined above, changes in the conditions of culture had
pronounced effects on another aspect of morphogenesis in Marsilea,
which is also relevant in the present connection. As an amphibious
plant, Marsilea can occur in nature both in aquatic and terrestrial
habitats, and as in other such plants, the resulting land water forms
have characteristic morphological differences (Allsopp, 1965c). It is of
interest that similar differences may be evoked by changes in the
composition even of liquid culture media, as well as by other changes
in the conditions of culture.
g. Effects of Sugar Concentration. At the lower concentrations of the
various sugars permitting appreciable growth, all sporelings had the
morphological characteristics of water forms, but at a certain critical
concentration the sporelings, although submerged, developed the characteristic features of typical land forms. The differences between the
two types of sporelings extended to virtually every part of the plant,
143
In later experiments, a delay in heteroblastic development was obtained
using 2-thiouracil and 5-fluorouracil (White, 1963) or 8-azaguanine
(Allsopp and Rao, unpublished observations).
e. Effects of Growth Substances. A considerable range of experiments
was carried out using various auxins, gibberellic acid, and kinetin. In
early investigations (Allsopp, 1952), it was found that the auxins
indoleacetic acid, naphthaleneacetic acid, and 2,4-dichlorophenoxyacetic
acid had little effect on leaf segmentation, unless present at the higher
range of concentrations which also resulted in a general inhibition of
growth. At such concentrations there was a retardation of heteroblastic
development, which was referred to growth inhibition, especially of the
shoot apex, rather than to any specific effect on leaf segmentation. In
later experiments with indoleacetonitrile (Allsopp, 1954b, 1956) some
reduction in the rate of heteroblastic development was obtained at the
optimal concentration of 10 mg/liter, which induced considerable extension of internodes and petioles. This result is not inconsistent with the
view that heteroblastic development in Marsilea is determined by the
strengthening of the shoot apex, for the active growth of internodes and
petioles might well compete with the shoot apex for the available
nutrients.
In contrast to the auxins, gibberellic acid, when present at 10 or
100 mg/liter, induced some increase in the rate of the heteroblastic leaf
segmentation (Allsopp, 1962).
The cytokinin kinetin produced remarkable leaf abnormalities at the
relatively low concentrations of 0.02 and 0.01 mg/liter, but had no apparent effect on the heteroblastic segmentation of normal leaves (Allsopp
and Szweykowska, 1960).
/. Other Morphological Changes. In addition to the effects on leaf
segmentation outlined above, changes in the conditions of culture had
pronounced effects on another aspect of morphogenesis in Marsilea,
which is also relevant in the present connection. As an amphibious
plant, Marsilea can occur in nature both in aquatic and terrestrial
habitats, and as in other such plants, the resulting land water forms
have characteristic morphological differences (Allsopp, 1965c). It is of
interest that similar differences may be evoked by changes in the
composition even of liquid culture media, as well as by other changes
in the conditions of culture.
g. Effects of Sugar Concentration. At the lower concentrations of the
various sugars permitting appreciable growth, all sporelings had the
morphological characteristics of water forms, but at a certain critical
concentration the sporelings, although submerged, developed the characteristic features of typical land forms. The differences between the
two types of sporelings extended to virtually every part of the plant,