Bulb, Bulbil
- the basic anatomy
A bulb can be regarded as an entire plant - stem, roots, leaves and flower bud - telescoped into a single, compact structure. In this form it can over-winter, ready to grow rapidly into a fully functioning plant come the spring, without having to wait for seeds to germinate or buds to swell. It is no coincidence that most of our early spring flowering plants in the garden are either from bulbs or corms (which perform a similar function).
The culinary onion (Allium cepa) shown here, is a textbook example of a bulb: the bulk of it consists of concentric layers of what, anatomically, are leaf bases, sometimes referred to as 'scales'. These don't develop into leaves proper, but serve as a store of metabolic reserve to nurture the plant when active growth starts up in the spring.
Like most bulbs, the onion has an outer protective layer - the tunic. In most species (including most varieties of onion) this is a pale brown colour, but the example shown here is a 'redskin' variety: i.e. the tunic is red (Fig. a), as are the epidermes (the surface layers) of each of the thickened leaf bases, which makes them easier to see (Figs. b & c).
The basal plate represents the stem of the plant, and it is from this that the roots and the shoots develop. The shoots - there are three in this bulb (Fig. c) - develop during the preceding season, before the bulb goes dormant. They contain the 'true' leaves and the flower buds. It is also around the basal plate that new daughter bulbs (bulbils) develop during the growing season. The mother bulb itself may last just one season or persist for many.
