148
A. ALLSOPP
ontogeny may be explained in terms of carbohydrate balance. It seems
likely that in many plants there will be an increase in the carbohydrate
supply with increasing age, both as a direct result of an increase in the
photosynthetic surface, and indirectly as a sequel to increasing difficulty
in maintaining an adequate supply of mineral nutrients to the multiplying meristematic regions.
Thus, the nutrition of a plant can have at least two effects on heteroblastic development. First by its role in primary enlargement, then by
changes in the carbohydrate balance. In general the former will exert
its main effect in the primary stages of ontogeny, the latter chiefly after
the primary stages have been completed. But the two effects are not
necessarily sequential. Primary strengthening may be long continued
and may overlap with the carbohydrate response, which as shown in
Marsilea can affect even the first leaf of the sporeling. Experiments with
other plants, which may be interpreted along these lines, are discussed
in later sections.
I. Other Recent Investigations. Several recent studies have extended
our knowledge of morphogenesis in Marsilea, but have not necessitated
any changes in the views expressed above. In an investigation on the
effects of mineral nutrition, Sossountzov (1961) found that deficiency of
calcium, magnesium or sulfate, or an excess of calcium, resulted in a
reversion to primary leaf forms, usually accompanied by a reduction
of the rhizome stele from a solenostele to a protostele. The formation of
simpler leaves and vascular tissue was always associated with a considerable reduction in size of the meristematic regions of the rhizome apex.
Experiments on the effects of indoleacetonitrile (Sossountzov, 1959) in
general confirmed previous results (Allsopp, 1954b, 1956), but also
showed that prolonged exposure to the nitrile was followed by reversion
to the primary condition of leaf and rhizome, again accompanied by a
reduction in the size of the apical meristem.
Gaudet (1964a,b) has recently paid particular attention to the ontogeny of the individual leaf in submerged and land forms of both
primary and adult leaves of Marsilea vestita. Since "the leaf primordia
of the land and submerged leaves are similar until a late stage in their
development," Gaudet questioned the role of the shoot apex in determining the adult leaf form of Marsilea. He considers that "the differences between the adult leaves seem more likely due to the activity of
the marginal meristem."
These views are virtually identical to those previously advanced by
the present author (Allsopp, 1955) to account for the differences between
land and water leaves. Such differences arise relatively late in the
development of the leaf, as opposed to the segmentation of the leaf,
which, as demonstrated by previous authors, is determined at an un-
A. ALLSOPP
ontogeny may be explained in terms of carbohydrate balance. It seems
likely that in many plants there will be an increase in the carbohydrate
supply with increasing age, both as a direct result of an increase in the
photosynthetic surface, and indirectly as a sequel to increasing difficulty
in maintaining an adequate supply of mineral nutrients to the multiplying meristematic regions.
Thus, the nutrition of a plant can have at least two effects on heteroblastic development. First by its role in primary enlargement, then by
changes in the carbohydrate balance. In general the former will exert
its main effect in the primary stages of ontogeny, the latter chiefly after
the primary stages have been completed. But the two effects are not
necessarily sequential. Primary strengthening may be long continued
and may overlap with the carbohydrate response, which as shown in
Marsilea can affect even the first leaf of the sporeling. Experiments with
other plants, which may be interpreted along these lines, are discussed
in later sections.
I. Other Recent Investigations. Several recent studies have extended
our knowledge of morphogenesis in Marsilea, but have not necessitated
any changes in the views expressed above. In an investigation on the
effects of mineral nutrition, Sossountzov (1961) found that deficiency of
calcium, magnesium or sulfate, or an excess of calcium, resulted in a
reversion to primary leaf forms, usually accompanied by a reduction
of the rhizome stele from a solenostele to a protostele. The formation of
simpler leaves and vascular tissue was always associated with a considerable reduction in size of the meristematic regions of the rhizome apex.
Experiments on the effects of indoleacetonitrile (Sossountzov, 1959) in
general confirmed previous results (Allsopp, 1954b, 1956), but also
showed that prolonged exposure to the nitrile was followed by reversion
to the primary condition of leaf and rhizome, again accompanied by a
reduction in the size of the apical meristem.
Gaudet (1964a,b) has recently paid particular attention to the ontogeny of the individual leaf in submerged and land forms of both
primary and adult leaves of Marsilea vestita. Since "the leaf primordia
of the land and submerged leaves are similar until a late stage in their
development," Gaudet questioned the role of the shoot apex in determining the adult leaf form of Marsilea. He considers that "the differences between the adult leaves seem more likely due to the activity of
the marginal meristem."
These views are virtually identical to those previously advanced by
the present author (Allsopp, 1955) to account for the differences between
land and water leaves. Such differences arise relatively late in the
development of the leaf, as opposed to the segmentation of the leaf,
which, as demonstrated by previous authors, is determined at an un-