HETEROBLASTIC DEVELOPMENT IN PLANTS
153
hastened the elongation of the following leaves. But application of auxin
to the cut stump replaced the effect of the excised leaf. Similar effects
of defoliation and the application of auxin were obtained by Arney
(1955a,b) in strawberry, Fragaria vesca. In the fern Leptopteris hymenophylloides, Sussex (1958) was able to induce precocious expansion of
immature leaves of the apical bud by removal of developing fronds in
spring and summer, but not by excision of mature fronds in late winter.
A different level of correlation was demonstrated by Neville (1964)
between different parts of individual leaves in the markedly heteroblastic species Gleditschia triacanthos, which normally forms pinnate
leaves with an increasing number of pinnae during the first year of
growth and bipinnate leaves later. When the apical 50-200 μ was
removed from young primordia of 100-300 μ in length, the terminal
regions of the resulting leaves were almost always bipinnate, whereas
in control plants of the same age only pinnate leaves were formed. If the
operation were carried out on larger primordia, only pinnate leaves were
obtained, with about 13 pairs of leaflets as compared with the 15-18
pairs of the control plants. These results are comparable in one respect
to the normal production of the bipinnate leaves, during which the number of the main pinnae is reduced to a maximum of 8 pinna pairs.
There is also evidence of an inhibitory effect of the apex on the development of the basal pinnae of the primordium.
In a hypothesis advanced to account for apical changes and heteroblastic development in Acrostichum, Crotty (1955) attached considerable
importance to correlations with both developing and expanded leaves.
He considered that the plant regulates its own development by a kind
of feed back mechanism. The first-formed leaves, as a source of auxin
and nutrients, are believed to promote the meristematic growth and
retard the maturation processes of the succeeding leaves. In consequence,
the latter are larger and better developed than the earlier leaves, and
thus produce more nutrients and hormones, which in turn promote a
more sustained meristematic growth, until finally a more or less stable
condition is attained in the larger specimens. This interpretation is
clearly in accord with many of the results previously discussed.
E. The Effects of Environment
Although some authors have related the progressive morphological
differences characteristic of heteroblastic development to seasonal or
other environmental changes, it is abundantly evident that heteroblastic
development is an intrinsic feature of the ontogeny of a majority of
plants and will proceed in any environment capable of sustaining
adequate growth. Nevertheless, heteroblastic development can be modi-
153
hastened the elongation of the following leaves. But application of auxin
to the cut stump replaced the effect of the excised leaf. Similar effects
of defoliation and the application of auxin were obtained by Arney
(1955a,b) in strawberry, Fragaria vesca. In the fern Leptopteris hymenophylloides, Sussex (1958) was able to induce precocious expansion of
immature leaves of the apical bud by removal of developing fronds in
spring and summer, but not by excision of mature fronds in late winter.
A different level of correlation was demonstrated by Neville (1964)
between different parts of individual leaves in the markedly heteroblastic species Gleditschia triacanthos, which normally forms pinnate
leaves with an increasing number of pinnae during the first year of
growth and bipinnate leaves later. When the apical 50-200 μ was
removed from young primordia of 100-300 μ in length, the terminal
regions of the resulting leaves were almost always bipinnate, whereas
in control plants of the same age only pinnate leaves were formed. If the
operation were carried out on larger primordia, only pinnate leaves were
obtained, with about 13 pairs of leaflets as compared with the 15-18
pairs of the control plants. These results are comparable in one respect
to the normal production of the bipinnate leaves, during which the number of the main pinnae is reduced to a maximum of 8 pinna pairs.
There is also evidence of an inhibitory effect of the apex on the development of the basal pinnae of the primordium.
In a hypothesis advanced to account for apical changes and heteroblastic development in Acrostichum, Crotty (1955) attached considerable
importance to correlations with both developing and expanded leaves.
He considered that the plant regulates its own development by a kind
of feed back mechanism. The first-formed leaves, as a source of auxin
and nutrients, are believed to promote the meristematic growth and
retard the maturation processes of the succeeding leaves. In consequence,
the latter are larger and better developed than the earlier leaves, and
thus produce more nutrients and hormones, which in turn promote a
more sustained meristematic growth, until finally a more or less stable
condition is attained in the larger specimens. This interpretation is
clearly in accord with many of the results previously discussed.
E. The Effects of Environment
Although some authors have related the progressive morphological
differences characteristic of heteroblastic development to seasonal or
other environmental changes, it is abundantly evident that heteroblastic
development is an intrinsic feature of the ontogeny of a majority of
plants and will proceed in any environment capable of sustaining
adequate growth. Nevertheless, heteroblastic development can be modi-