Agriculture Reference
In-Depth Information
bulb at maturity has two dry skins enclosing four swollen sheaths from bladed
leaves. These in turn enclose three or four swollen, bladeless bulb scales, and
finally at the centre are found five leaf initials with blades (see Fig. 2.7). These
inner bladed leaf initials emerge when the bulb sprouts (see Fig. 7.5), and
hence are termed the 'sprout leaves'. Usually, one or more lateral shoots occur
towards the centre of the bulb, consisting of a few swollen, bladeless bulb scales
enclosing several sprout leaves or, nearer the centre, just sprout leaves.
The reserve tissue in garlic bulbs consists solely of bladeless storage leaves,
which constitute about 75% of the weight of trimmed bulbs and form the bulk
of the familiar garlic clove. They arise as buds within the axils of the sheaths of
foliage leaves, on the side of the axis below the leaf blade. Each clove
primordium may divide as the axil in which it develops expands, so that up to
six or seven cloves can be found in the axil of a single foliage leaf (see Fig. 2.11).
In a typical mature bulb, the number of cloves per leaf axil increases from
one or two in the innermost leaf axil to six or seven in the fourth youngest axil
and then decreases again in the axils of older leaves. Ultimately, the apical bud
may also develop into a clove. In plants grown from very small cloves, the
terminal clove may constitute the only storage leaf; such single-cloved bulbs
are termed 'rounds'. The sheaths of the foliage leaves remain non-swollen, but
dry to form a papery protective envelope surrounding the cloves within. When
the bulb is fully ripened, the original stem and leaves are dead, and merely act
as dry container for the dormant cloves. As bulbs ripen, parenchymal cells
degenerate and collapse on the inner side of the leaf sheaths that form the
pseudostem. This leaves alive just the outer epidermis, the conducting vessels
and some surrounding cells. As a result the neck of the bulb softens and the
foliage collapses, as in onion (Mann, 1952).
Individual cloves consist of a tough, outer, dry protective leaf, a swollen
storage leaf that constitutes the bulk of the weight and which contains
30-40% dry matter, a sprout leaf and three or four foliage leaf primordia
surrounding the apical meristem (see Fig. 2.3A). The protective, storage and
sprout leaves have only vestigial leaf blades, but the foliage leaf primordia have
blades. As with onion bulbs, two or so additional foliage leaf primordia
differentiate at the clove apical meristem during storage. During sprouting, the
sprout leaf elongates and emerges from the pore at the tip of the storage leaf,
and then the first foliage leaf pushes through the pore near the tip of the sprout
leaf (see Fig. 2.3B). In development, the protective leaf forms first and this
ensheaths the developing storage leaf (see Fig. 2.11).
Ideally, ripe garlic bulbs should be near circular in transverse section and
covered by several outer skins; such bulbs are termed 'smooth'. In some
conditions (see Fig. 4.47), lateral buds may initiate in the axils of the outer leaf
blades and give rise to green side shoots, which themselves go on to develop
cloves, resulting in bulbs made up of several sub-clusters of cloves. Such bulbs
have an irregular outer surface and are termed 'rough'. This is a common
defect in garlic production (see Fig. 2.12).
