154
A. ALLSOPP
fied by environmental changes, and studies on the effects of such changes
may contribute towards an understanding of plant ontogeny.
1. The Effects of Light
Various workers have observed that changes in light intensity or photoperiod can affect leaf shape and the rate of heteroblastic development.
Earlier work in this field was summarized by Allsopp (1965b). Of relatively recent work, mention may be made of the investigations of Ashby
(1950) and Njoku (1956a) on Ipomoea caerulea, a species which shows
a progression from entire to 3-lobed leaves, with increasing depth of
lobing at successive nodes. Ashby had obtained pronounced differences
in heteroblastic development between plants grown in 16- and 8-hr days.
Njoku later concluded, however, that during the vegetative phase daylength itself has no direct effect on leaf shape, but the onset of flowering
inhibits leaf lobing. Njoku found that intensity of light is a more potent
factor than day-length. In an investigation in which plants were grown
in normal daylight and under screens giving light intensities of 0.74,
0.56, 0.28, and 0.23 of the full daylight he obtained a marked increase
in lobing. Plants already forming lobed leaves reverted to the production
of entire leaves when transferred to deep shade.
Montaldi et al. (1963a) studied the heteroblastic leaf development
of Passiflora caerulea in which the leaf form ranges from the entire
primary leaves to the 5-lobed or occasionally 6- or 7-lobed adult leaves
(Fig. 1). In plants grown under artificial light of relatively low intensity
only the 3-lobed leaf form was attained. In plants which had already
produced 5-lobed leaves there was a reversion to 3-lobed leaves. The
similarity to Njoku's results on Ipomoea extended to defoliation experiments, in which continued removal of leaves from plants at the 3-lobed
leaf stage prevented the development of 5-lobed leaves.
Both Njoku and Montaldi et al concluded that leaf lobing is probably
determined by some substance synthesized in the leaves and exerting its
action on the meristems. It was considered that this substance is still
unknown, but both sets of results are clearly compatible with the view
that the effects of light are a consequence of changes in the carbohydrate
supply, as already postulated for Marsilea (see Section III,C,l,k).
As pointed out by Millener (1961), there have been few investigations
of photoperiod as such on vegetative development. Some of the results
previously reported may be explained simply as a response to increased
carbohydrate production in the longer days rather than to day length
itself. In his own work on Ulex europaeus, Millener made a comparison
of seedling development in long and short days, 16 and 8 hr, with full
light intensity of 800 foot candles, as well as with 8 hr of full light + 8
hours of low intensity light. It is well known that Ulex europaeus has
A. ALLSOPP
fied by environmental changes, and studies on the effects of such changes
may contribute towards an understanding of plant ontogeny.
1. The Effects of Light
Various workers have observed that changes in light intensity or photoperiod can affect leaf shape and the rate of heteroblastic development.
Earlier work in this field was summarized by Allsopp (1965b). Of relatively recent work, mention may be made of the investigations of Ashby
(1950) and Njoku (1956a) on Ipomoea caerulea, a species which shows
a progression from entire to 3-lobed leaves, with increasing depth of
lobing at successive nodes. Ashby had obtained pronounced differences
in heteroblastic development between plants grown in 16- and 8-hr days.
Njoku later concluded, however, that during the vegetative phase daylength itself has no direct effect on leaf shape, but the onset of flowering
inhibits leaf lobing. Njoku found that intensity of light is a more potent
factor than day-length. In an investigation in which plants were grown
in normal daylight and under screens giving light intensities of 0.74,
0.56, 0.28, and 0.23 of the full daylight he obtained a marked increase
in lobing. Plants already forming lobed leaves reverted to the production
of entire leaves when transferred to deep shade.
Montaldi et al. (1963a) studied the heteroblastic leaf development
of Passiflora caerulea in which the leaf form ranges from the entire
primary leaves to the 5-lobed or occasionally 6- or 7-lobed adult leaves
(Fig. 1). In plants grown under artificial light of relatively low intensity
only the 3-lobed leaf form was attained. In plants which had already
produced 5-lobed leaves there was a reversion to 3-lobed leaves. The
similarity to Njoku's results on Ipomoea extended to defoliation experiments, in which continued removal of leaves from plants at the 3-lobed
leaf stage prevented the development of 5-lobed leaves.
Both Njoku and Montaldi et al concluded that leaf lobing is probably
determined by some substance synthesized in the leaves and exerting its
action on the meristems. It was considered that this substance is still
unknown, but both sets of results are clearly compatible with the view
that the effects of light are a consequence of changes in the carbohydrate
supply, as already postulated for Marsilea (see Section III,C,l,k).
As pointed out by Millener (1961), there have been few investigations
of photoperiod as such on vegetative development. Some of the results
previously reported may be explained simply as a response to increased
carbohydrate production in the longer days rather than to day length
itself. In his own work on Ulex europaeus, Millener made a comparison
of seedling development in long and short days, 16 and 8 hr, with full
light intensity of 800 foot candles, as well as with 8 hr of full light + 8
hours of low intensity light. It is well known that Ulex europaeus has