Axillary Floral and Inflorescence Shoot Meristems Are Variants of the Vegetative Meristem

Several different types of shoot meristems can be distinguished on the basis of their developmental origin, the types of lateral organs they generate, and whether they are determinate (having a genetically programmed limit to their growth) or indeterminate (showing no predetermined limit to growth; growth continues so long as resources permit).

The vegetative shoot apical meristem usually is indeterminate in its development. It repetitively forms phy-tomeres as long as environmental conditions favor growth but do not generate a flowering stimulus. A phytomere is a developmental unit consisting of one or more leaves, the node to which the leaves are attached, the internode below the node, and one or more axillary buds (Figure 16.14). Axillary buds are secondary meristems; if they are also vegetative meristems, they will have a structure and developmental potential similar to that of the apical meristem.

Leaf Node Internode Bud

FIGURE 16.14 The shoot apical meristem repetitively forms units known as phytomeres. Each phytomere consists of one or more leaves, the node at which the leaves are attached, the internode immediately below the leaves, and one or more buds in the axils of the leaves.

Leaf Node Internode Bud

> Phytomere

FIGURE 16.14 The shoot apical meristem repetitively forms units known as phytomeres. Each phytomere consists of one or more leaves, the node at which the leaves are attached, the internode immediately below the leaves, and one or more buds in the axils of the leaves.

Vegetative meristems may be converted directly into floral meristems when the plant is induced to flower (see Chapter 24). Floral meristems differ from vegetative meristems in that instead of leaves they produce floral organs: sepals, petals, stamens, and carpels. In addition, floral meristems are determinate: All meristematic activity stops after the last floral organs are produced.

In many cases, vegetative meristems are not directly converted to floral meristems. Instead, the vegetative meristem is first transformed into an inflorescence meristem. The types of lateral organs produced by an inflorescence meristem are different from the types produced by a floral meristem. The inflorescence meristem produces bracts and floral meristems in the axils of the bracts, instead of the sepals, petals, stamens, and ovules produced by floral meristems. Inflorescence meristems may be determinate or indeterminate, depending on the species.

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