Agriculture Reference
In-Depth Information
Fig. 2.7. An onion bulb dissected to show the dry outer protective skin (SK); the
fleshy, swollen sheaths derived from bladed leaf bases (SH); the swollen bulb scales
without leaf blades (SC); and, towards the centre, the sprout leaves (SP) with
successively increasing proportions of leaf blade, which will elongate and emerge
when the bulb sprouts (photograph courtesy of Warwick HRI, UK).
branching at the shoot apex varies with species, cultivar and growing con-
ditions. For example, shoots of chives initiate a lateral every two or three leaves,
thereby forming a clump of shoots (Poulsen, 1990). Onion cultivars bred to
produce large, single bulbs will often produce 12 or more leaves before
initiating a lateral shoot. The same is true of leeks and Japanese bunching
onion, which are bred to produce large, unsplit pseudostems. On the other
hand, shallots (Krontal et al. , 1998), multiplier onions and Japanese bunching
onions grown for green shoots start to branch at lower nodes, and continue to
branch more freely than the single-centred types. In onion, branching and
splitting is more frequent under high temperatures and light levels.
STEM STRUCTURE
The morphology and anatomy and the development of the garlic stem (Mann,
1952) and the onion stem (de Mason, 1990) are very similar. Despite the
compressed and flattened shape of the stem, some of the tissues found in a
typical elongated stem can be distinguished. Figure 2.5 shows a diagrammatic
longitudinal section of a stem and Fig. 2.10 a photograph of such a section
from onion.
 
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