Something About Bulbs and Corms.
There is so much confusion of thought as to what a "bulbous" plant really is, that it may be as well at the beginning of this volume to endeavour to clear up the haziness that exists in regard to the matter. It seems to be taken for granted that any plant with a swollen or thickened stem or rootstock is a "bulbous" one. And this impression is no doubt confirmed when one consults the bulb catalogues issued by nurserymen. In these publications—chiefly, no doubt, for the sake of convenience and to avoid unnecessary extra expense in printing—a large number of plants are enumerated as if they were really bulbous. It is, therefore, not at all unnatural that the amateur should come to the conclusion that everything mentioned between the covers of a bulb catalogue should be truly bulbous in nature. Even some publications on bulbous plants have adopted the same loose nomenclature. Thus we find such non-bulbous plants as Aconites, Anemones, Dahlias, Dicentras, Day-Lilies, Hepaticas, Solomon's Seal, Astilbe japonica, Tropæolums, Lily of the Valley, Corydalis, Torch Lilies, Pæonies, Christmas Roses, and many others described as "bulbous" plants, while some that are really so, and worthy of cultivation, are not even mentioned.
Some of the plants referred to above have thickened stems or roots, and will be found described in their proper places in the companion volume to this—"Beautiful Garden Flowers." They belong to several different families of plants. True bulbous plants, however (with which we may include those having "corms"), are confined to very few families. Indeed, they are restricted to one of the two large groups of flowering plants, viz., that in which the leaves usually have parallel veins, and the flowers have their parts in circles of three or six. This group of plants is known to botanists as "monocotyledons," and is still further distinguished by having only one seed-leaf, as may be seen when the seeds of any of them sprout, as shown in the Tulip, p. 35.
It is within the limits of this definition, therefore, that all the plants described in this book come. They all have parallel-veined leaves, and the parts of their flowers are in "threes" or "sixes," as may be seen by consulting the coloured plates.
There is an apparent contradiction to this rule in the Daffodils (Narcissi) in which the "trumpet" or "corona" in the centre makes a seventh organ. A similar growth may be seen in such bulbous plants as the Eucharis, Hymenocallis, Pancratium, &c., that are usually grown under glass. This corona is analogous to the ligules or scale-like outgrowths so noticeable on the petals of the Campions (Lychnis), the chief difference being that in the Narcissi the ligules are joined together, become much larger, and often constitute the most attractive feature of the flowers.
Definition of a "Bulb."—Perhaps the very best-known example of a true bulb is the common or garden Onion. Another example is shown in the sketch of a Hyacinth and Tigridia.
Tunicated Bulb of Hyacinth in section showing "Disc," and Scale Leaves s. l. Tigridia Bulb, Showing thick Contractile Roots.
The bulbs of Daffodils, Tulips, Snowdrops, Scillas, &c., all conform very closely to the Onion in structure. It will be noticed that at the base of the Hyacinth, for example, is a flattish or deltoid mass of tissue. This is called the "disc" and is really the stem portion of the bulb. On the upper surface it bears a number of thick scaly leaves packed very close together, and rolled round each other, with the flower-spike in the centre; while from the under surface, the roots emanate when growth takes place. It may be easily imagined by the reader that if the "disc" were drawn out lengthwise, and if a space separated one scale-leaf from another, that the bulb would be very similar in appearance to an ordinary leafy stem. Nature, however, has a certain object in view in modifying the stems and leaves in such a manner that they are tightly packed away when at rest, within a brown protecting coat, so that they resemble the large scale-protected flower-buds that may be seen in winter on Horse-chestnuts, Lilacs, Ash, &c. The thick scale-leaves are really storehouses in which food has been stored up by the larger and broader green leaves that perform the functions of assimilation, respiration, &c., above the ground during the growing period.
When the bulb begins to grow, the food in the thick scale-leaves is drawn up to supply nourishmentx to the flower-stem, until the new green leaves can manufacture or elaborate a fresh supply in the sunlight from the raw materials drafted into them from the soil by the roots. Under favourable circumstances more food is elaborated than is necessary for the wants of the plant, and then extra growths or young bulbs called "offsets" are developed at the base, or rather the side, of the older bulb.
It should be mentioned here, however, that all bulbs do not vegetate in the same way. In many cases the original bulb persists for several seasons, as in the Daffodil and Hyacinth, for example; but in others it vanishes completely during the period of growth, and is absorbed, or swallowed up, as it were, by the flower stem. The most common example of this among bulbs is the Tulip, to which more detailed reference has been made at p. 133.
Kinds of Bulbs.—Most true bulbs are constructed like the Onion, Daffodil, Snowdrop, or Hyacinth, in having the scale-leaves rolled round each other, forming different layers or coats. Such bulbs are said to be "tunicated." In the case of the Liliums, however, the scale-leaves only lap over each other at the edges, and are arranged spirally round the central axis. These bulbs are called "scaly," or "imbricated," and are shown in the annexed sketch on p. 12.
The individual scales are much thicker at the base than at the apex, and in the case of tunicated bulbs, they are also thicker on one side than the other. By this arrangement, the various "coats" can be rolled round each other more tightly, and without wasting any space.
Definition of a "Corm."—In outward appearance, many corms are so much like bulbs, that the two terms are interchangeable and loosely applied, at least, among gardeners. By cutting a "corm" through the centre lengthwise, a great difference, however, will be noticed in the structure. In the bulb, the "disc" is small and unimportant, while the scale-leaves upon it are the most conspicuous feature. In the "corm," on the other hand, the "disc" is the all-important feature, and is devoid of any thick scale-leaves upon it. The new growths appear on the top or sides, and the lines round the circumference show where the sheathing papery scale-leaves were attached. In the "corm" then, it is the disc, and not the scale-leaves, that is the great storehouse of food.
Growth of a Corm.—The vegetation of the corm is very remarkable, and somewhat resembles that of the Tulip. When a corm commences to grow, the reserve material within it is used up for the benefit of the flowers and leaves. The result of this absorbing process is that by the end of the season the old corm has almost vanished, or is reduced to a dry shrivelled, woody, and lifeless mass, incapable of further growth, and attached to the base of the new corms, as shown in the annexed sketches of Gladiolus and Crocus on page 14.
These new corms are the direct result of the work that has been done by the green leaves in the daylight, and after a period of rest, they go through precisely the same process the following season—vanishing themselves, but leaving others behind to carry on the work of producing flowers, and, if possible, seeds.
Gladiolus. o. c. old corm; c. r. contractile roots; n. c. new corms with "spawn" (s.) at base. Crocus Corm. o. c. old corm; n. c. new corm with growths.
The importance of Green Leaves to Bulbs and Corms. —If the reader wishes to be successful in growing bulbous plants in his garden he must have very great respect for the green leaves of his plants, and always endeavour to keep them in the cleanest and healthiest possible condition. From what has just been said about the production of new bulbs in the Tulip, and new corms in the Crocus and Gladiolus, it is obvious that the leaves play a most important part. Indeed, without their aid there would be neither bulbs nor corms to carry on the work of the plants from year to year. In the form of carbon-dioxide the leaves eat up the carbon and oxygen from the atmosphere. Under the influence of sunlight the gas is decomposed, so that the oxygen is given off again into the air, while the carbon is retained for the production of starch and other materials. These are elaborated in the cells of the leaves, and after undergoing certain changes pass down the stems and are stored up in the bulbs or corms beneath the surface of the soil. It is only green healthy leaves that can perform this important work satisfactorily. When the foliage therefore begins to turn yellow and wither, it may be taken for granted that its work for the season is coming to a close, and the bulbs or corms are going to enjoy a well-earned rest. It should, perhaps, be mentioned also that leaves can only become green in day light; and although some bulbous plants like a certain amount of shade, it would never do to exclude the light from them altogether, or even to plant them in places where they could not get an adequate amount of sunshine, or diffused light, during the day.