156
A. ALLSOPP
Since the early work of Zalenski (1904), it has been claimed that the
morphological gradients in the leaves are a result of an increasing
restriction of water supply to the upper leaves. In experiments with
Ipomoea, Ashby (1948b) found that there was a similar gradient in the
cell size of successive leaves in plants grown with an abundant water
supply and in plants grown under dry conditions. These and other results
indicated that water supply is not the major factor in determining the
anatomical and morphological changes associated with increasing level
of leaf insertion. By anology with the results obtained with Marsilea
(Section III,C,l,k), an increase in the carbohydrate level might well
be responsible.
As detailed in Allsopp (1965c), the most pronounced effects of water
supply are seen in the land and water forms of amphibious plants. In
land forms, or even aerial portions of a single plant, submersion is often
followed by an immediate morphological change from the land to the
water form. In this change there is no question of apical enlargement
being involved. As several workers have shown, e.g., Allsopp (1955) for
Marsilea, McCully and Dale (1961) for Hippuris and Bostrack and
Millington (1962) for Ranunculus, an already developing leaf primordium may be affected by the change. The response is clearly initiated
by the sudden hydration of the developing parts and as suggested by
the experimental work on Marsilea is probably the consequence of a
reduction in carbohydrate concentration.
Thus, various environmental factors have their own influence on
heteroblastic development. But in no case has it been demonstrated that
heteroblastic development is determined by the complex of factors
characteristic of the seasonal cycle.
F. The Role of Physiological Aging
Almost all modern authors have interpreted heteroblastic development
in terms of physiological aging, either of the entire plant or of the
apical meristems in particular. It is certain that important physiological
changes do occur during ontogeny and such changes may be referred to
as physiological aging. On the other hand, it is much more debatable
whether there are any major changes, apart from size, in the apical
meristems of the plant, and, if such changes occur, whether they represent an intrinsic aging of the meristem or are imposed by the physiological conditions of the rest of the plant.
At an early date Klebs (1904) supported by Goebel (1908) postulated
that development changes in plants are related to both qualitative and
quantitative nutritional changes. Special emphasis was placed on the
balance between carbohydrates and mineral nutrients. There have been
relatively few later studies of the composition and metabolism of plants
A. ALLSOPP
Since the early work of Zalenski (1904), it has been claimed that the
morphological gradients in the leaves are a result of an increasing
restriction of water supply to the upper leaves. In experiments with
Ipomoea, Ashby (1948b) found that there was a similar gradient in the
cell size of successive leaves in plants grown with an abundant water
supply and in plants grown under dry conditions. These and other results
indicated that water supply is not the major factor in determining the
anatomical and morphological changes associated with increasing level
of leaf insertion. By anology with the results obtained with Marsilea
(Section III,C,l,k), an increase in the carbohydrate level might well
be responsible.
As detailed in Allsopp (1965c), the most pronounced effects of water
supply are seen in the land and water forms of amphibious plants. In
land forms, or even aerial portions of a single plant, submersion is often
followed by an immediate morphological change from the land to the
water form. In this change there is no question of apical enlargement
being involved. As several workers have shown, e.g., Allsopp (1955) for
Marsilea, McCully and Dale (1961) for Hippuris and Bostrack and
Millington (1962) for Ranunculus, an already developing leaf primordium may be affected by the change. The response is clearly initiated
by the sudden hydration of the developing parts and as suggested by
the experimental work on Marsilea is probably the consequence of a
reduction in carbohydrate concentration.
Thus, various environmental factors have their own influence on
heteroblastic development. But in no case has it been demonstrated that
heteroblastic development is determined by the complex of factors
characteristic of the seasonal cycle.
F. The Role of Physiological Aging
Almost all modern authors have interpreted heteroblastic development
in terms of physiological aging, either of the entire plant or of the
apical meristems in particular. It is certain that important physiological
changes do occur during ontogeny and such changes may be referred to
as physiological aging. On the other hand, it is much more debatable
whether there are any major changes, apart from size, in the apical
meristems of the plant, and, if such changes occur, whether they represent an intrinsic aging of the meristem or are imposed by the physiological conditions of the rest of the plant.
At an early date Klebs (1904) supported by Goebel (1908) postulated
that development changes in plants are related to both qualitative and
quantitative nutritional changes. Special emphasis was placed on the
balance between carbohydrates and mineral nutrients. There have been
relatively few later studies of the composition and metabolism of plants