Agriculture Reference
In-Depth Information
continuously expand and develop on the periphery of the PTM. This is paralleled
by leaf initiation in the apical meristem and the subsequent expansion growth of
the associated leaf bases in concentric rings away from the apex. The stem
expands radially to accommodate the continuous production of roots and
leaves. As this occurs, the bases of old leaves and old roots are pushed
progressively further from the apex, ultimately down the side and to the bottom
of the stem. After 1-2 months' growth, the thickening caused by the PTM
causes the apical meristem to become somewhat sunk below the shoulders of
the stem disc, resulting in a heart-shaped longitudinal section (see Fig. 2.5).
Ultimately, the old leaf bases split and degenerate and are sloughed off. The
underside of the stem may also start to decay, so that it becomes flattened.
The stems of the other edible allium crops are similar in general appearance to
those of onion and garlic, except for Chinese chives. In this species the stem is a
branched rhizome which forms a thickened, underground storage structure that
connects several shoots (Jones and Mann, 1963). The rhizome is coated by a
fibrous brown mat formed from the remains of decayed leaf bases. Thick, persistent
roots grow from the underside of the rhizome. In Japanese bunching onion and
chives, tillering results in short horizontal rhizomes connecting adjacent shoots,
but the older portions die off, leaving the clusters of mostly unconnected shoots.
The bulbs of rakkyo, onion and shallot and garlic show a gradation of increas-
ing complexity (see Fig. 2.8). Bulbs in the true sense do not form in Japanese
bunching onion, chives or Chinese chives, although the thickened basal
sheaths may act as food reserves during winter dormancy in the former two
species. In
Allium ampeloprasum
, great-headed garlic, pearl onions and most
wild types form bulbs but, with leek and kurrat, bulbs form only in some
specimens after bolting or in unnaturally long photoperiods (see Chapter 4).
Rakkyo bulbs form as a result of swelling of the foliage leaf sheaths, and
these appear as concentric swollen rings in transverse section. There is some
growth of leaf blades as the sheaths thicken, resulting in oval-shaped bulbs.
The leaf blades decay at the end of bulbing. Many lateral buds form in the axils
between sheaths, and these develop into the following season's shoots. The
apical buds of the bulb-forming shoots differentiate into inflorescences during
bulb formation and dormancy.
The early stages of bulb formation in onion and shallot are similar to those in
rakkyo, involving the swelling of leaf sheaths. The thickening of the sheaths is
frequently preceded by their sudden elongation. Thickening occurs as a result of
lateral expansion of cells in the lower third of the sheaths and does not involve cell
division. During bulbing the young developing leaves cease to form blades but
develop into swollen, bladeless 'bulb scales'. In all but these leaves the length from
the base of the sheath to the pore where the next leaf emerges, and which marks
