HETEROBLASTIC DEVELOPMENT IN PLANTS
151
Investigations on the effects of carbohydrate supply on isolated leaf
primordia are considered in Section III,D,2.
D. The Ontogeny of Leaf Primordia
1. Factors Affecting Leaf Development
The work discussed in previous sections has demonstrated that there
is clearly a close correlation between primary strengthening and leaf
development. On the other hand, other factors can evidently affect the
developing leaf directly, as in the land-water form relationship in
Marsilea. It is now pertinent to consider the relative contribution of
these various factors and correlations in leaf development.
The observed correlation between apical size and leaf development is
understandable. It might be expected and is indeed found, e.g., in
Acrostichum (Crotty, 1955), that a larger apical meristem will bear
primordia of greater meristematic mass than those borne by a smaller
apex. Such an initially greater meristematic capital could well account
for the increases in size and complexity of many adult leaves, although
the subsequent development of the primordium would still be open to
modification. During the continued development of the primordium the
leaf lamina arises by the activity of a marginal meristem, and it is
largely the duration and localization of this activity that determines the
final shape of the leaf. In a recent careful study of leaf ontogeny in the
fern genus Adiantum, Hagemann (1965) has likened the development
of the individual primordium to the strengthening of the shoot axis. The
enlargement of the primordium results in an extension of the marginal
meristem, together with a limited thickening of the submarginal meristem.
The pinnae of the compound leaf can only develop when the marginal
meristem is of sufficient extent. Hagemann (1965) also found that the
primordia of primary leaves are not only smaller, but develop into
permanent tissue much more rapidly, completing their growth within
a few days as opposed to the several months required by adult leaves.
2. Growth of Excised Primordia
Recent investigations on the growth in aseptic culture of isolated leaf
primordia have added considerably to our knowledge of factors affecting
the development of the individual primordium (summarized by Steeves,
1962; Cutter, 1965). It was first established that excised externally
undifferentiated primordia of both ferns (Sussex and Steeves, 1953, 1958;
Steeves and Sussex, 1957) and angiosperme (Steeves et al., 1957) can
continue their development to produce small but otherwise normal dorsiventral leaves. It was also found, using the fern Osmunda cinnamomea,
(Sussex and Clutter, 1960) that on low concentrations of sucrose the
primordia, although derived from adult plants, formed leaves resembling
151
Investigations on the effects of carbohydrate supply on isolated leaf
primordia are considered in Section III,D,2.
D. The Ontogeny of Leaf Primordia
1. Factors Affecting Leaf Development
The work discussed in previous sections has demonstrated that there
is clearly a close correlation between primary strengthening and leaf
development. On the other hand, other factors can evidently affect the
developing leaf directly, as in the land-water form relationship in
Marsilea. It is now pertinent to consider the relative contribution of
these various factors and correlations in leaf development.
The observed correlation between apical size and leaf development is
understandable. It might be expected and is indeed found, e.g., in
Acrostichum (Crotty, 1955), that a larger apical meristem will bear
primordia of greater meristematic mass than those borne by a smaller
apex. Such an initially greater meristematic capital could well account
for the increases in size and complexity of many adult leaves, although
the subsequent development of the primordium would still be open to
modification. During the continued development of the primordium the
leaf lamina arises by the activity of a marginal meristem, and it is
largely the duration and localization of this activity that determines the
final shape of the leaf. In a recent careful study of leaf ontogeny in the
fern genus Adiantum, Hagemann (1965) has likened the development
of the individual primordium to the strengthening of the shoot axis. The
enlargement of the primordium results in an extension of the marginal
meristem, together with a limited thickening of the submarginal meristem.
The pinnae of the compound leaf can only develop when the marginal
meristem is of sufficient extent. Hagemann (1965) also found that the
primordia of primary leaves are not only smaller, but develop into
permanent tissue much more rapidly, completing their growth within
a few days as opposed to the several months required by adult leaves.
2. Growth of Excised Primordia
Recent investigations on the growth in aseptic culture of isolated leaf
primordia have added considerably to our knowledge of factors affecting
the development of the individual primordium (summarized by Steeves,
1962; Cutter, 1965). It was first established that excised externally
undifferentiated primordia of both ferns (Sussex and Steeves, 1953, 1958;
Steeves and Sussex, 1957) and angiosperme (Steeves et al., 1957) can
continue their development to produce small but otherwise normal dorsiventral leaves. It was also found, using the fern Osmunda cinnamomea,
(Sussex and Clutter, 1960) that on low concentrations of sucrose the
primordia, although derived from adult plants, formed leaves resembling