152
A. ALLSOPP
primary leaves, with only a small number of pinnae, whereas at higher
concentrations progressively more pairs of pinnae were produced. Although no comparison was made between primordia from sporeling and
adult plants, the importance of primordium size was revealed in comparisons between younger and older primordia from the same plant, in
which the latter produced a considerably greater number of pinna pairs.
When tips of larger primordia were excised so as to equal the size of
smaller primordia their development was similar to that of the latter.
It was concluded that while the leaf apex can grow when supplied with
simple nutrient materials, the amount of tip growth is highly sensitive
to the metabolic status of the plant (Sussex, 1964).
3. Other Correlations
Apart from the evident correlations discussed above, various experiments have demonstrated that the development of leaf primordia can
be considerably affected by other developing or mature organs. A pronounced example of such correlations is provided by the conversion of
prospective cataphylls (scale leaves) of Osmunda cinnamomea into
ordinary foliage leaves by the removal of the expanded fronds produced
during the current year (Steeves and Wetmore, 1953). Sun (1960) also
obtained conversion of cataphylls into foliage leaves in the aroid Arisaema consanguineum following defoliation or removal of the central
bud. In Pseudotsuga menziesii removal of the upper half or all of the
leaves of the developing annual shoots led to premature expansion of
next year's terminal bud, with leaves developing in part from presumptive bud-scale primordia (Al-Talib and Torrey, 1961).
It has also been found that defoliation has effects on the development
of normal foliage leaves. As long ago as 1904, Zalenski removed mature
leaves from Coleus plants and discovered, as he had expected, that the
newly expanded leaves then resembled the lower ones in their less dense
venation and larger cell size. Of other investigations on defoliation, those
of Njoku (1956b) on Ipomoea are of particular interest in the present
context. It was found that continued defoliation led to a reduction of leaf
lobing, finally resulting in a reversion to simple juvenile leaves. Similarly,
Dostâl (1960) obtained juvenile leaves in various woody species
(Aesculus hippocastanum, Fraxinus excelsior, Tilia platyphylla, Syringa
vulgaris) by defoliating young shoots of the current year's growth.
It seems likely that the results of the drastic defoliations described
in the previous paragraph can be ascribed to a fall in the carbohydrate
level following great reduction in photosynthetic surface. On the other
hand, the effects of more limited defoliation suggest that hormonal effects
might also be involved. Thus, Goodwin (1937) found in Solidago sempervirens that removal of a rapidly expanding leaf in the basal rosette
A. ALLSOPP
primary leaves, with only a small number of pinnae, whereas at higher
concentrations progressively more pairs of pinnae were produced. Although no comparison was made between primordia from sporeling and
adult plants, the importance of primordium size was revealed in comparisons between younger and older primordia from the same plant, in
which the latter produced a considerably greater number of pinna pairs.
When tips of larger primordia were excised so as to equal the size of
smaller primordia their development was similar to that of the latter.
It was concluded that while the leaf apex can grow when supplied with
simple nutrient materials, the amount of tip growth is highly sensitive
to the metabolic status of the plant (Sussex, 1964).
3. Other Correlations
Apart from the evident correlations discussed above, various experiments have demonstrated that the development of leaf primordia can
be considerably affected by other developing or mature organs. A pronounced example of such correlations is provided by the conversion of
prospective cataphylls (scale leaves) of Osmunda cinnamomea into
ordinary foliage leaves by the removal of the expanded fronds produced
during the current year (Steeves and Wetmore, 1953). Sun (1960) also
obtained conversion of cataphylls into foliage leaves in the aroid Arisaema consanguineum following defoliation or removal of the central
bud. In Pseudotsuga menziesii removal of the upper half or all of the
leaves of the developing annual shoots led to premature expansion of
next year's terminal bud, with leaves developing in part from presumptive bud-scale primordia (Al-Talib and Torrey, 1961).
It has also been found that defoliation has effects on the development
of normal foliage leaves. As long ago as 1904, Zalenski removed mature
leaves from Coleus plants and discovered, as he had expected, that the
newly expanded leaves then resembled the lower ones in their less dense
venation and larger cell size. Of other investigations on defoliation, those
of Njoku (1956b) on Ipomoea are of particular interest in the present
context. It was found that continued defoliation led to a reduction of leaf
lobing, finally resulting in a reversion to simple juvenile leaves. Similarly,
Dostâl (1960) obtained juvenile leaves in various woody species
(Aesculus hippocastanum, Fraxinus excelsior, Tilia platyphylla, Syringa
vulgaris) by defoliating young shoots of the current year's growth.
It seems likely that the results of the drastic defoliations described
in the previous paragraph can be ascribed to a fall in the carbohydrate
level following great reduction in photosynthetic surface. On the other
hand, the effects of more limited defoliation suggest that hormonal effects
might also be involved. Thus, Goodwin (1937) found in Solidago sempervirens that removal of a rapidly expanding leaf in the basal rosette