HETEROBLASTIC DEVELOPMENT IN PLANTS
157
at different developmental stages, but in general the results of Gregory
and his co-workers, summarized and discussed by Heath (1957), support
the view that there is an excess of carbohydrate in the later stages of
development. Other workers, e.g., Farkas and Rajâthy (1955) have
described gradients in other physiological properties, such as water
content water saturation deficit, succulence level, chlorophyll content,
etc., in the successive leaves of various herbaceous plants.
Thus, there is evidence of ontogenetic alterations in the metabolic
status of the plant. Such changes might well have a direct effect on the
plant meristems, and the ready reversibility of the morphological modifications in herbaceous plants is indicative of such a direct effect. But
the most convincing evidence of a response of the apical meristem to a
stimulus from the rest of the plant is provided by the onset of flowering,
which in some photoperiodic plants can be initiated by a single exposure
to the inductive day-length.
1. Stable Modifications in Woody Plants
In contrast to the developmental stages of herbaceous plants, the
juvenile and adult stages of many woody plants are remarkably résistent
to environmental changes, suggesting that the apical meristems have
undergone permanent or semipermanent modification. The most evident
distinction between the two stages lies in the restriction of the production
of reproductive organs to the adult stage, and on this basis the juvenile
stage of different woody species may persist for from less than 1 to 50
years or more (Sax, 1962). There are other marked differences, notably
in the greater rooting capacity of cuttings from juvenile shoots. In some
deciduous trees, e.g., Fagus sylvatica, Quercus robur and Q. petraea the
juvenile stages retain their leaves during winter. I t is of considerable
interest in the present connection that even in older trees, the basal
regions of the main trunk and branches may still display this and other
juvenile characters in contrast to the distal regions. Further details of
differences between juvenile and adult stages of woody plants are given
in the articles by Robbins (1957a), Schaffalitzky de Muckadell (1959),
Wareing (1959), Brink (1962), and Doorenbos (1965).
> a. Cuttings and Grafts. Experiments with cuttings and grafts have
been very informative regarding the degree of stability of the juvenileadult transformation. As would be expected, cuttings from juvenile plants
or juvenile zones of trees retain juvenile characteristics for a time, but
eventually develop into normal adult forms. At one time, it was widely
believed that "fixed" juvenile forms of conifers could be produced by
taking cuttings from juvenile parts, but Woycicki (1954) demonstrated
that even in conifers adult forms are normally produced from juvenile
cuttings. Cuttings from adult stages, however, show great stability. For
157
at different developmental stages, but in general the results of Gregory
and his co-workers, summarized and discussed by Heath (1957), support
the view that there is an excess of carbohydrate in the later stages of
development. Other workers, e.g., Farkas and Rajâthy (1955) have
described gradients in other physiological properties, such as water
content water saturation deficit, succulence level, chlorophyll content,
etc., in the successive leaves of various herbaceous plants.
Thus, there is evidence of ontogenetic alterations in the metabolic
status of the plant. Such changes might well have a direct effect on the
plant meristems, and the ready reversibility of the morphological modifications in herbaceous plants is indicative of such a direct effect. But
the most convincing evidence of a response of the apical meristem to a
stimulus from the rest of the plant is provided by the onset of flowering,
which in some photoperiodic plants can be initiated by a single exposure
to the inductive day-length.
1. Stable Modifications in Woody Plants
In contrast to the developmental stages of herbaceous plants, the
juvenile and adult stages of many woody plants are remarkably résistent
to environmental changes, suggesting that the apical meristems have
undergone permanent or semipermanent modification. The most evident
distinction between the two stages lies in the restriction of the production
of reproductive organs to the adult stage, and on this basis the juvenile
stage of different woody species may persist for from less than 1 to 50
years or more (Sax, 1962). There are other marked differences, notably
in the greater rooting capacity of cuttings from juvenile shoots. In some
deciduous trees, e.g., Fagus sylvatica, Quercus robur and Q. petraea the
juvenile stages retain their leaves during winter. I t is of considerable
interest in the present connection that even in older trees, the basal
regions of the main trunk and branches may still display this and other
juvenile characters in contrast to the distal regions. Further details of
differences between juvenile and adult stages of woody plants are given
in the articles by Robbins (1957a), Schaffalitzky de Muckadell (1959),
Wareing (1959), Brink (1962), and Doorenbos (1965).
> a. Cuttings and Grafts. Experiments with cuttings and grafts have
been very informative regarding the degree of stability of the juvenileadult transformation. As would be expected, cuttings from juvenile plants
or juvenile zones of trees retain juvenile characteristics for a time, but
eventually develop into normal adult forms. At one time, it was widely
believed that "fixed" juvenile forms of conifers could be produced by
taking cuttings from juvenile parts, but Woycicki (1954) demonstrated
that even in conifers adult forms are normally produced from juvenile
cuttings. Cuttings from adult stages, however, show great stability. For