134
A. ALLSOPP
an increase is found in many ferns (Steeves and Wetmore, 1953; Crotty,
1955; Cutter, 1955; Sussex, 1958) and in certain angiosperms, e.g.,
Nuphar and Nymphaea (Cutter, 1957). There are also changes in phyllotaxis, almost invariably from a simpler to a higher system. (For details
see Allsopp, 1965b; Wardlaw, 1965; Cutter, 1965.)
2. Changes in Growth Habit
Changes in growth habit during plant ontogeny are frequent. Most
plants are orthotropic during the seedling or sporeling stage, but many
become plagiotropic on further development, e.g., Pteridium aquilinum,
Hedera helix, and Veronica hederifolia. In some of these plagiotropic
forms, there may even be a subsequent return to the erect habit, as in
Hedera and various climbing Araceae.
Other ontogenetic changes affect the mode of branching. In a considerable number of woody plants, which show differentiation into long and
short shoots in the mature stages, only long shoots are present in the
juvenile stages (e.g., Pinus, Ginkgo). Philipson (1963) has recently discussed "habit-heteroblastism" in New Zealand trees. His account deals
mainly with the transformation from a much-branched divaricate shrub
stage to an adult tree with a single trunk. In the 11 trees described, the
leaf shape as well as the habit is very different in the two stages.
3. Anatomical Changes
The primary strengthening of the stem also involves various anatomical changes. The size-structure correlation was first investigated thoroughly by Bower (1930). He found that a progressive increase in
structural complexity is usually associated with the increase in thickness
of the stem. Changes of this type in the stelar system of pteridophytes
are now well known. In seed plants with distinct vascular bundles there
is often an increase in the number of strands with increasing diameter
of the axis. Changes in phyllotaxis also have an effect on the vascular
system. (For further details see Allsopp, 1965b.)
Although there can be no doubt of a close correlation between the
thickness and structure of the axis, it is certain that not all anatomical
changes in the stem can be explained in this way. Thus, in many plants
there are differences between juvenile and adult phases regardless of size.
Such differences in ivy, Hedera helix, have recently been described by
Goodin (1965). Rumball (1963) compared wood structure in two markedly heteroblastic species, Podocarpus dacryoides, and Elaeocarpus hookerianus, and in two homoblastic species, Podocarpus totara, and Elaeocarpus dentatus, from the same genera. In the heteroblastic species there
was an abrupt change in the transverse characteristics of the wood at a
few centimeters from the pith, and this change corresponded to the time
A. ALLSOPP
an increase is found in many ferns (Steeves and Wetmore, 1953; Crotty,
1955; Cutter, 1955; Sussex, 1958) and in certain angiosperms, e.g.,
Nuphar and Nymphaea (Cutter, 1957). There are also changes in phyllotaxis, almost invariably from a simpler to a higher system. (For details
see Allsopp, 1965b; Wardlaw, 1965; Cutter, 1965.)
2. Changes in Growth Habit
Changes in growth habit during plant ontogeny are frequent. Most
plants are orthotropic during the seedling or sporeling stage, but many
become plagiotropic on further development, e.g., Pteridium aquilinum,
Hedera helix, and Veronica hederifolia. In some of these plagiotropic
forms, there may even be a subsequent return to the erect habit, as in
Hedera and various climbing Araceae.
Other ontogenetic changes affect the mode of branching. In a considerable number of woody plants, which show differentiation into long and
short shoots in the mature stages, only long shoots are present in the
juvenile stages (e.g., Pinus, Ginkgo). Philipson (1963) has recently discussed "habit-heteroblastism" in New Zealand trees. His account deals
mainly with the transformation from a much-branched divaricate shrub
stage to an adult tree with a single trunk. In the 11 trees described, the
leaf shape as well as the habit is very different in the two stages.
3. Anatomical Changes
The primary strengthening of the stem also involves various anatomical changes. The size-structure correlation was first investigated thoroughly by Bower (1930). He found that a progressive increase in
structural complexity is usually associated with the increase in thickness
of the stem. Changes of this type in the stelar system of pteridophytes
are now well known. In seed plants with distinct vascular bundles there
is often an increase in the number of strands with increasing diameter
of the axis. Changes in phyllotaxis also have an effect on the vascular
system. (For further details see Allsopp, 1965b.)
Although there can be no doubt of a close correlation between the
thickness and structure of the axis, it is certain that not all anatomical
changes in the stem can be explained in this way. Thus, in many plants
there are differences between juvenile and adult phases regardless of size.
Such differences in ivy, Hedera helix, have recently been described by
Goodin (1965). Rumball (1963) compared wood structure in two markedly heteroblastic species, Podocarpus dacryoides, and Elaeocarpus hookerianus, and in two homoblastic species, Podocarpus totara, and Elaeocarpus dentatus, from the same genera. In the heteroblastic species there
was an abrupt change in the transverse characteristics of the wood at a
few centimeters from the pith, and this change corresponded to the time