Do You Know Your Bulbs?

By: L.M. Collicutt

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This is a short overview of the visual world beneath the soil looking at bulb structures of plants. Pull your chair up for a short botany-anatomy lesson.

Bulbs are produced by monocotyledonous plants such as the lily and onion. They are underground organs, which serve for both food storage and vegetative propagation.

The easiest way to understand a bulb is to think of a plant stem, then visually compress or squash it into a roundish "bulb" Bulbs actually consist of a short stem surrounded by thick fleshy leaves. The stem axis is called a basal plate and the fleshy leaves are termed scales. The main growing pint is located in the centre of the bulb and expands upward into the current seasons shoots(s). The scales function as food storage for the plant. Small bulbs known as bulblets can develop in the axil of these scales in lilies, or from the basal plate as in other bulbs. In lilies, these bulblets can grow and develop into what are called daughter bulbs. There are two types of bulbs - 'tunicate' and 'non-tunicate'. Tunicate bulbs((figure 1) have outer bulb scales which are dry and membranous, like onions and tulips. This 'tunic' protects the bulbs from both mechanical injury and drying. Bulbs of this are usually quite solid. Roots are not present during the dormant phase, but developed adventitiously during active growth.

Non-tunicate bulbs (figure 2) as represented by lilies do not have a tunic covering, but rather, have exposed scales. Those bulbs are much more subject to mechanical injury during planting or digging.

Lily bulbs must always be protected from drying out. Lilies develop roots in midsummer or later and retain them throughout the following year. Many lilies develop what is called contractile roots which will pull the bulb down to the desired level. Adventitious roots also develop along the stem just above the bulb. Lilies are never truly dormant and, therefore should be left out of the ground as short a time as possible.

Within the lilies there are sever types of bulbs, in fact identification of species and different groups is aided by observation of the bulb type. The common bulb form is called 'erect concentric' (figure 3). The majority of lilies sold in nurseries and garden centres are of this type. This group includes the Asiatic hybrids, the trumpet such as L. regale and the white and coloured trumpet hybrids.

Another kind of lily bulb is the 'rhizomatous type' (figure 4). There is an underground rootstock, which extends by branching. Scales are attached along these branches. Some of the North American species and hybrids have rhizomatous bulbs.

'Stoloniferous' bulbs (figure 5) are those bulbs which are essentially erect bulbs connected underground by a stolon or underground stem. The species L. wardii and L. duchartrei are examples of this group.

Reprinted with permission of the Prairie Garden Committee, Winnipeg Horticultural Society.

PAGE 18 THE PRAIRIE GARDEN, 1990