H E T E R O B L A S T I C D E V E L O P M E N T I N P L A N T S
149
usually early stage in leaf ontogeny (Schneider, 1913; Tournay, 1951;
Sossountzov, 1965). It is considered that the results discussed above
establish without much doubt that leaf segmentation in Marsilea is
correlated with apical ontogeny. It is true that no author has made
detailed anatomical studies of the ontogeny of the apex in this plant,
but external observations indicate that there is a substantial enlargement of the apical regions during sporeling development and conversely
an attenuation during starvation or other unfavorable conditions (Fig.
6). The results of Sossountzov, summarized above, have demonstrated
unequivocally that the apical meristem itself is reduced in size when
conditions result in reversion to the production of primary leaves. Anatomical studies (Allsopp, 1953a) have revealed a gradual reduction in
the size of the lateral meristems during progressive starvation. Furthermore, as in all other plants investigated from this standpoint, the young
sporeling apex of Marsilea is very small when compared with the apex
of a mature plant (Allsopp, unpublished observations).
Gaudet (1963, 1965a,b) has also studied the effects of various environmental factors on the land-water form relationship in Marsilea vestita.
He obtained the rather surprising result that an exposure to either
darkness or far-red light induced the development of land forms from
water forms when sporelings were subsequently placed in continuous
light. As in the previously described work on M. drummondii, addition
of appropriate concentrations of indoleacetonitrile or gibberellic acid
inhibited the formation of land forms. It is not known how darkness or
far-red light exert this effect, but, as in the experiments with M. drummondii, changes in the carbohydrate balance might well be involved.
2. Experiments with Other Plants
Apart ftrom the work on Marsilea there have been relatively few
direct studies on the effects of nutrition on heteroblastic development.
The effects of various environmental factors have been studied more
frequently and many responses to such factors have almost certainly a
nutritional basis. These results are considered later.
a. Effects of Mineral Nutrients. It is well known that changes in
mineral nutrition can have considerable morphological effects. Heteroblastic development has rarely been investigated from this standpoint,
but results with Marsilea (Edwards and Allsopp, 1956) indicated that
differences would be expected. In his detailed investigations of the
heteroblastic development of Ipomoea, Njoku (1957) studied the role
of the three elements, potassium, phosphorus, and nitrogen. The nitrogen
supply had the greatest effect. On a nodal basis, the leaves of plants
grown with a high nitrogen supply were less deeply lobed than those
of the control plants. It was shown that the reduction in lobing is asso-
149
usually early stage in leaf ontogeny (Schneider, 1913; Tournay, 1951;
Sossountzov, 1965). It is considered that the results discussed above
establish without much doubt that leaf segmentation in Marsilea is
correlated with apical ontogeny. It is true that no author has made
detailed anatomical studies of the ontogeny of the apex in this plant,
but external observations indicate that there is a substantial enlargement of the apical regions during sporeling development and conversely
an attenuation during starvation or other unfavorable conditions (Fig.
6). The results of Sossountzov, summarized above, have demonstrated
unequivocally that the apical meristem itself is reduced in size when
conditions result in reversion to the production of primary leaves. Anatomical studies (Allsopp, 1953a) have revealed a gradual reduction in
the size of the lateral meristems during progressive starvation. Furthermore, as in all other plants investigated from this standpoint, the young
sporeling apex of Marsilea is very small when compared with the apex
of a mature plant (Allsopp, unpublished observations).
Gaudet (1963, 1965a,b) has also studied the effects of various environmental factors on the land-water form relationship in Marsilea vestita.
He obtained the rather surprising result that an exposure to either
darkness or far-red light induced the development of land forms from
water forms when sporelings were subsequently placed in continuous
light. As in the previously described work on M. drummondii, addition
of appropriate concentrations of indoleacetonitrile or gibberellic acid
inhibited the formation of land forms. It is not known how darkness or
far-red light exert this effect, but, as in the experiments with M. drummondii, changes in the carbohydrate balance might well be involved.
2. Experiments with Other Plants
Apart ftrom the work on Marsilea there have been relatively few
direct studies on the effects of nutrition on heteroblastic development.
The effects of various environmental factors have been studied more
frequently and many responses to such factors have almost certainly a
nutritional basis. These results are considered later.
a. Effects of Mineral Nutrients. It is well known that changes in
mineral nutrition can have considerable morphological effects. Heteroblastic development has rarely been investigated from this standpoint,
but results with Marsilea (Edwards and Allsopp, 1956) indicated that
differences would be expected. In his detailed investigations of the
heteroblastic development of Ipomoea, Njoku (1957) studied the role
of the three elements, potassium, phosphorus, and nitrogen. The nitrogen
supply had the greatest effect. On a nodal basis, the leaves of plants
grown with a high nitrogen supply were less deeply lobed than those
of the control plants. It was shown that the reduction in lobing is asso-