HETEROBLASTIC DEVELOPMENT IN PLANTS
147
nutritional status of the plant a more or less extended period of apical
ontogeny is necessary before the complete leaf form can appear. These
findings and considerations led to the general hypothesis (Allsopp,
1954c, 1963) based on the views of Goebel and Troll, that the kind of
heteroblastic leaf development characteristic of Marsilea, and indeed
of seedlings and sporelings in general, is determined by apical enlargement, the degree of which is dependent on the amount of protein synthesis at the shoot apex, in turn a reflection of the nutritional status of
the entire plant.
Apical strengthening is particularly characteristic of the earlier stages
of ontogeny. It accounts convincingly for increases in size and complexity of the primary leaves, but cannot be invoked to explain certain
changes in leaf shape encountered in later stages of development. Somewhat unexpectedly, however, some of these later changes can be explained on the basis of results obtained in the study of the land-water
form relationship in Marsilea. This work demonstrated that the differences in lobe or leaflet shape and structure between land and water forms
of Marsilea are not correlated with the primary enlargement of the
plant, for they can affect the first sporeling leaves as well as the adult
leaves of the plant. The results indicated rather that it is the carbohydrate balance that is important, an excess of carbohydrate promoting
the more differentiated structure characteristic of land forms.
This explanation originally advanced (Allsopp, 1955) to account for
differences between land and water forms, was later extended (Allsopp,
1963, 1965a) to include various environmental responses and certain
types of heteroblastic development in ordinary terrestrial plants. A
survey of the relevant literature revealed that a more differentiated
structure, frequently associated with differences in leaf shape, had been
obtained by many workers in response to such widely varied factors as
deficient water supply, deficiency of mineral nutrients (especially nitrogen), low temperature, low growth rate, high light intensity, high
transpiration, high carbon dioxide concentration, and high osmotic concentration. The one feature common to all these factors is their tendency
to increase the available concentration of carbohydrates. This relative
increase in carbohydrate balance, although very important, is probably
only one link in the chain of events. Some of the responses, such as cell
wall thickening, may be relatively direct. In other responses, the carbohydrate seems to act as a trigger releasing various morphogenetic processes previously repressed.
This response to sugar concentration in Marsilea has much in common
with the morphological changes encountered in many plants after primary strengthening has largely been completed, and it is probable that
many examples of heteroblastic development affecting the later stages of
147
nutritional status of the plant a more or less extended period of apical
ontogeny is necessary before the complete leaf form can appear. These
findings and considerations led to the general hypothesis (Allsopp,
1954c, 1963) based on the views of Goebel and Troll, that the kind of
heteroblastic leaf development characteristic of Marsilea, and indeed
of seedlings and sporelings in general, is determined by apical enlargement, the degree of which is dependent on the amount of protein synthesis at the shoot apex, in turn a reflection of the nutritional status of
the entire plant.
Apical strengthening is particularly characteristic of the earlier stages
of ontogeny. It accounts convincingly for increases in size and complexity of the primary leaves, but cannot be invoked to explain certain
changes in leaf shape encountered in later stages of development. Somewhat unexpectedly, however, some of these later changes can be explained on the basis of results obtained in the study of the land-water
form relationship in Marsilea. This work demonstrated that the differences in lobe or leaflet shape and structure between land and water forms
of Marsilea are not correlated with the primary enlargement of the
plant, for they can affect the first sporeling leaves as well as the adult
leaves of the plant. The results indicated rather that it is the carbohydrate balance that is important, an excess of carbohydrate promoting
the more differentiated structure characteristic of land forms.
This explanation originally advanced (Allsopp, 1955) to account for
differences between land and water forms, was later extended (Allsopp,
1963, 1965a) to include various environmental responses and certain
types of heteroblastic development in ordinary terrestrial plants. A
survey of the relevant literature revealed that a more differentiated
structure, frequently associated with differences in leaf shape, had been
obtained by many workers in response to such widely varied factors as
deficient water supply, deficiency of mineral nutrients (especially nitrogen), low temperature, low growth rate, high light intensity, high
transpiration, high carbon dioxide concentration, and high osmotic concentration. The one feature common to all these factors is their tendency
to increase the available concentration of carbohydrates. This relative
increase in carbohydrate balance, although very important, is probably
only one link in the chain of events. Some of the responses, such as cell
wall thickening, may be relatively direct. In other responses, the carbohydrate seems to act as a trigger releasing various morphogenetic processes previously repressed.
This response to sugar concentration in Marsilea has much in common
with the morphological changes encountered in many plants after primary strengthening has largely been completed, and it is probable that
many examples of heteroblastic development affecting the later stages of