Shoot Apical Meristems Have Three Developmental Phases

During postembryonic development, the shoot apical meristem passes through three more or less well-defined developmental stages in sequence:

1. The juvenile phase

2. The adult vegetative phase

3. The adult reproductive phase

The transition from one phase to another is called phase change.

The primary distinction between the juvenile and the adult vegetative phases is that the latter has the ability to form reproductive structures: flowers in angiosperms, cones in gymnosperms. However, actual expression of the reproductive competence of the adult phase (i.e., flowering) often depends on specific environmental and developmental signals. Thus the absence of flowering itself is not a reliable indicator of juvenility.

The transition from juvenile to adult is frequently accompanied by changes in vegetative characteristics, such as leaf morphology, phyllotaxy (the arrangement of leaves on the stem), thorniness, rooting capacity, and leaf retention in deciduous plants (Figure 24.9; see also Web Topic 24.1). Such changes are most evident in woody perennials, but they are apparent in many herbaceous species as well. Unlike the abrupt transition from the adult vegetative phase to the

FIGURE 24.9 Juvenile and adult forms of ivy (Hedera helix). The juvenile form has lobed palmate leaves arranged alternately, a climbing growth habit, and no flowers. The adult form (projecting out to the right) has entire ovate leaves arranged in spirals, an upright growth habit, and flowers. (Photo by L. Taiz.)

FIGURE 24.9 Juvenile and adult forms of ivy (Hedera helix). The juvenile form has lobed palmate leaves arranged alternately, a climbing growth habit, and no flowers. The adult form (projecting out to the right) has entire ovate leaves arranged in spirals, an upright growth habit, and flowers. (Photo by L. Taiz.)

reproductive phase, the transition from juvenile to vegetative adult is usually gradual, involving intermediate forms.

Sometimes the transition can be observed in a single leaf. A dramatic example of this is the progressive transformation of juvenile leaves of the leguminous tree Acacia heterophylla into phyllodes, a phenomenon noted by Goethe. Whereas the juvenile pinnately compound leaves consist of rachis (stalk) and leaflets, adult phyllodes are specialized structures representing flattened petioles (Figure 24.10).

Intermediate structures also form during the transition from aquatic to aerial leaf types of aquatic plants such as Hippuris vulgaris (common marestail). As in the case of A. heterophylla, these intermediate forms possess distinct regions with different developmental patterns. To account for intermediate forms during the transition from juvenile to adult in maize (see Web Topic 24.2), a combinatorial model has been proposed (Figure 24.11). According to this model, shoot development can be described as a series of independently regulated, overlapping programs (juvenile, adult, and reproductive) that modulate the expression of a common set of developmental processes.

FIGURE 24.10 Leaves of Acacia heterophylla, showing transitions from pinnately compound leaves (juvenile phase) to phyllodes (adult phase). Note that the previous phase is retained at the top of the leaf in the intermediate forms.

In the transition from juvenile to adult leaves, the intermediate forms indicate that different regions of the same leaf can express different developmental programs. Thus the cells at the tip of the leaf remain committed to the juvenile program, while the cells at the base of the leaf become committed to the adult program. The developmental fates of the two sets of cells in the same leaf are quite different.

Was this article helpful?

0 0
Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment