138
A. ALLSOPP
capable of continuing their development for a longer period, thus producing a more mature leaf form. This hypothesis has the merit of explaining certain observations which cannot be accounted for directly in
terms of the nutritional status of the plant. It explains why primary
leaf forms are frequently produced at the base of lateral branches, even
in well-grown plants, and in general why such leaves occur in any parts
where attenuated stems have arisen, e.g., in floral regions and on various
kinds of adventitious shoots. There can indeed be little doubt that there
is some measure of correlation between apical strengthening and leaf
development, although there have been few detailed studies of the connection between a particular leaf shape and the size of the shoot apex.
In an investigation by Röbbelen (1957), however, it was demonstrated
for Arabidopsis thaliana that the type of leaf produced is dependent on
the size of the apex, which must attain a diameter of 80-90 μ before
the adult leaf form can develop. Seidlovâ et al. (1964) have recently
studied changes in the anatomical structure of the shoot apex of groundsel, Senecio vulgaris, in relation to the formation of leaves. It was found
that the complete development of the zonation characteristic of the
vegetative shoot apex coincided with the initiation of the primordia of
the leaves which attained the largest size.
C. The Role of Nutrition
Since changes in the primary stages of ontogeny are evidently correlated with the progressive strengthening of the plant, it is of interest
to consider the effects of nutrition on this aspect of development. Numerous experiments were carried out by Goebel and by various other workers
(summarized by Allsopp, 1965b) in the attempt to elucidate the role
of nutrition, but like most other early experimental studies these suffered
from the drawback that the environmental conditions were not adequately controlled, so that several factors, such as light, temperature
and humidity, could all vary simultaneously. Even in modern investigations, with carefully regulated environments, it is sometimes difficult to
differentiate between specific morphogenetic effects of a factor, e.g.,
light, and its effect on the nutritional status of the plant. These difficulties can be overcome, however, by the supply of selected nutrients to
plants growing in aseptic culture under closely controlled conditions.
1. Experiments with Marsilea
Some 15 years ago, it was discovered (Allsopp, 1951, 1952) that the
water fern, Marsilea drummondii, can be grown heterotrophically in
various aseptic culture media. Since the sporeling of Marsilea is markedly heteroblastic (Fig. 3), the use of aseptic cultures permitted the
A. ALLSOPP
capable of continuing their development for a longer period, thus producing a more mature leaf form. This hypothesis has the merit of explaining certain observations which cannot be accounted for directly in
terms of the nutritional status of the plant. It explains why primary
leaf forms are frequently produced at the base of lateral branches, even
in well-grown plants, and in general why such leaves occur in any parts
where attenuated stems have arisen, e.g., in floral regions and on various
kinds of adventitious shoots. There can indeed be little doubt that there
is some measure of correlation between apical strengthening and leaf
development, although there have been few detailed studies of the connection between a particular leaf shape and the size of the shoot apex.
In an investigation by Röbbelen (1957), however, it was demonstrated
for Arabidopsis thaliana that the type of leaf produced is dependent on
the size of the apex, which must attain a diameter of 80-90 μ before
the adult leaf form can develop. Seidlovâ et al. (1964) have recently
studied changes in the anatomical structure of the shoot apex of groundsel, Senecio vulgaris, in relation to the formation of leaves. It was found
that the complete development of the zonation characteristic of the
vegetative shoot apex coincided with the initiation of the primordia of
the leaves which attained the largest size.
C. The Role of Nutrition
Since changes in the primary stages of ontogeny are evidently correlated with the progressive strengthening of the plant, it is of interest
to consider the effects of nutrition on this aspect of development. Numerous experiments were carried out by Goebel and by various other workers
(summarized by Allsopp, 1965b) in the attempt to elucidate the role
of nutrition, but like most other early experimental studies these suffered
from the drawback that the environmental conditions were not adequately controlled, so that several factors, such as light, temperature
and humidity, could all vary simultaneously. Even in modern investigations, with carefully regulated environments, it is sometimes difficult to
differentiate between specific morphogenetic effects of a factor, e.g.,
light, and its effect on the nutritional status of the plant. These difficulties can be overcome, however, by the supply of selected nutrients to
plants growing in aseptic culture under closely controlled conditions.
1. Experiments with Marsilea
Some 15 years ago, it was discovered (Allsopp, 1951, 1952) that the
water fern, Marsilea drummondii, can be grown heterotrophically in
various aseptic culture media. Since the sporeling of Marsilea is markedly heteroblastic (Fig. 3), the use of aseptic cultures permitted the