This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
A bulb is really a very much enlarged bud, consisting for the most part of leaves, with a very short and thin flat stem. Lilies have scaly bulbs (fig. 23), while Hyacinths, Tulips, Daffodils, and Onions (fig. 23) have tunicated bulbs, so called because the sheaths are continuous all round, like a tunic.
If the sheaths or scales are pulled off one by one there will remain a thin, solid part, which is the stem. The Tiger Lily, and several others, bear bulbils, or small bulbs in the axils of their leaves, and their structure is similar to the parent bulb. The bulbils form a ready means of propagating the plant (fig. 24). Corms are produced by the Crocus, Gladiolus, Colchicum (fig. 25) and others. They consist of a short, flattened stem, covered with dry, scalelike sheaths, or modified leaves, and surmounted by a tuft of perfectly developed green leaves. They root from the base. They produce a new conn, or several, on the top of the old one every year, the old one dying.
Fig. 23. - Lily - Scaly Bulb. Onion - Tunicated Bulb.
Fig. 24. - Bulb-bearing Lily - Portion of Stem.
Fig. 25. - Colchicum - Corm.
Small ones are produced at the base of the Gladiolus corm and serve for propagation.
A tuber is a short, not flattened, but fleshy stem, growing a little at the apex and dying a little at the base every year. Roots are given off from the sides, as in the Arum Lily and Caladium. The tuber of the Potato is the thickened and fleshy end of an underground branch, which develops into a new plant, the stem of which bears the roots, while the old tuber dies. The Potato stores starch, while the tuber of the Jerusalem Artichoke stores inulin, and both are cultivated for these reasons, being useful for food. The tuber of the Jerusalem Artichoke is of similar origin or structure to that of the Potato.
Rhizomes are creeping, underground stems, giving off roots below and leaves and flower stems at the end of the main axis or side branches.
Fig. 26. - Rhizome of Iris.
Fig. 27. - Rhizome of Sedge (Carex).
Examples of stout rhizomes are those of Solomon's Seal and many of the Irises (fig. 26). Slender rhizomes are produced by Couch Grass, Sedges (fig. 27), Lily of the Valley, and many others. The stem nature of these rhizomes may often be testified by the presence of very much reduced scale-like leaves, as in the cultivated Spearmint and Peppermint.
The object of bulbs, corms, and tubers is to store food with which to commence growth, flower, and fruit in the following year. Rhizomes serve a similar purpose as well as to increase the plant and extend it into fresh ground. Mints are noteworthy in this respect. All of these types are of easy propagation by offsets and divisions in gardens. By virtue of the stored food in their bulbs Hyacinths, Tulips, Daffodils, Snowdrops, and others may be grown in the dark till their flowers are visible, when they must be placed in the light for the benefit of the young leaves. The Hyacinths and Daffodils may be, and are, grown entirely in clean water till they have finished flowering, solely as a result of the starch stored in the leaves and fleshy stems constituting the bulbs. They require to be grown in good soil for some years afterwards in order to recuperate before they can flower as well again.
Besides such modified stem structures as corms, tubers, and rhizomes there are many plants in which the stems have assumed the form of leaves. One of the best-known examples is the Common Butcher's Broom(Ruscus aculeatus); others are Ruscus Hypoglossum (fig. 28) and the Alexandrian Laurel (Dancea Laurus or Ruscus racemosus). These plants, being apparently unable to develop true leaves, have modified some of their shoots for the purpose of assimilation. These shoots are flat and leaf-like, but it will be noticed that they bear flowers near the centre, a thing no true leaf does. It will also be noticed that instead of spreading out horizontally these peculiar shoots (known botanically as cladodes or phyllo-clades) are set more or less vertically. Other plants with modified structures are to be found in such genera as Asparagus, Acacia, Eucalyptus, Grevillea, Phyllanthus, Phyllocactus, Phyllocladus, Semele, etc. [J. F.].
A DUTCH TULIP FARM.
DOUBLE TULIPS GROWING AT WISBECH, CAMBRIDGESHIRE (Mr. J. W. Cross).
DUTCH AND ENGLISH TULIP FARMS.
Fig. 28. - Plants with Leaf-like Branches.
1, Young shoot of Ruscus Hypoglossum. 2, The same branch fully grown with flowers on the cladodes. 3, Young shoot of Ruscus aculeatus. 4, The same branch with flowers on the cladodes.