Agriculture Reference
In-Depth Information
The stem remains as a compressed disc unless flowering occurs; then the
shoot apical meristem develops into an inflorescence. The stem internode
between the last foliage leaf and the leaf-like spathe that surrounds the
inflorescence elongates to form the inflorescence stalk (scape). Nearly always, a
lateral bud develops in the axil between the scape and the last foliage leaf (see
Fig. 2.6). This can continue vegetative growth after flowering has finished.
Bulbs are formed from swollen leaf sheaths. Sometimes the swollen sheaths
are solely those at the base of bladed leaves, as in rakkyo. At the other extreme
the storage tissues of the bulb solely comprise specially modified bladeless leaves
called 'bulb scales', as in garlic. In onion the outer swollen sheaths derive from
bladed leaves and the inner ones are bulb scales (see Figs 2.7 & 2.8).
Having outlined the overall plant structure, each component is now
considered in more detail.
Figure 2.9 shows the development of an onion leaf from a crescent-shaped
ridge on one side of the shoot apical meristem into a structure with a leaf blade
and a short sheath that encloses younger developing leaves. The frequency of
Fig. 2.6.
Scanning electron micrographs of successive stages of inflorescence
differentiation at the onion shoot apex. 1, non-floral; 2, early floral; 3, spathe
begins to overgrow the apical dome; 4, the ridged spathe surrounds the apical
dome and a definite stalk (scape) is visible. A vegetative lateral bud is adjacent to
the scape, which can sometimes compete with the young inflorescence (see
Fig. 4.38) (photographs courtesy of Warwick HRI, UK).
