Agriculture Reference
In-Depth Information
Fig. 2.12.
Rough and smooth garlic. Top row: bulbs planted 21 October at Davis,
California from planting stock stored at 5°C for 9 weeks; all are rough. Note that the
small side bulbs have separate tops, the remains of leafy axillary shoots visible in
green, immature plants. Bottom row: bulbs planted 30 September from planting
stock stored at 20°C for 6 weeks; most are smooth (from Mann and Minges, 1958.
Courtesy of
Hilgardia
).
The bulb structure of great-headed garlic is similar to that of garlic. It
frequently forms a single terminal clove or 'round'. In the following season a
round usually develops an inflorescence surrounded by multiple axillary cloves,
like normal garlic except that the clove size is usually larger (van der Meer and
Hanelt, 1990). Frequently, additional smaller cloves or bulblets form in the
outer leaf axils of the bulbs, a feature that is occasionally observed in leeks.
The structure of rakkyo, onion and garlic bulbs is diagrammatically
summarized in Fig. 2.8.
Studies on onion leaf anatomy have shown the tissues typical of most green
leaves (de Mason, 1990). There is an outer epidermal layer coated with a waxy
cuticle that contains many sunken stomata. Below this are three or four layers
of columnar palisade cells. Within the palisade layer are numerous inter-
connecting elongated cells called laticifers. These contain the milky fluid that
oozes when an onion leaf is cut across. This fluid is rich in sulfur-containing
flavour compounds (Hayward, 1938). Below the palisade cells are about two
layers of larger and rounder cells surrounded by much intercellular air space.
Many chloroplasts occur within these cells, close to the cell walls adjacent to the
air spaces. Below these chlorophyll-dense layers lie the vascular bundles
surrounded by large parenchymatous cells containing fewer chloroplasts than
the outer layers. The leaf cavity is lined by senescent cells lacking in protoplasm.
