Part 5

THE NATURAL SINKING OF BULBS AND CORMS.

In connection with the question of planting, attention may be directed to a very interesting and remarkable power possessed by the roots of many bulbs and corms. A glance at the sketches of Gladiolus, Tritonia, Nothoscordum, and Lilium, will show the reader some thick fleshy roots with conspicuous rings on them. They are readily distinguished from the finer fibrous roots, and, as may be readily supposed, their functions are quite distinct. To thoroughly understand what these thick-ringed roots are for, the reader will remember what has been said at page 13 about the way in which the old corms of Crocuses and Gladioli disappear, or are surmounted in autumn by new ones. If the plants were not disturbed for several years, one would imagine that as the new corms were always produced on top of the old ones, they would sooner or later come through the surface of the soil, and thus run the risk of being either parched by drought, or shrivelled up by the heat of the summer sun; or, again, of being frozen to death in winter. And yet, examination of the corms will show that the new ones are quite as deep down in the soil, if not deeper, than their predecessors. This remarkable state of affairs to preserve what may be called the status quo is entirely due to the action of the thick, ringed roots referred to above. These roots usually strike straight down into the soil. When they have gone as far as Nature intended them to, they begin to contract much in the same way apparently as a worm does when going into its burrow, and for this reason they have been called "contractile."

Illustration: NOTHOSCORDUM BULB. Showing Contractile Roots.

Illustration: TRITONIA CORMS.

During the process of contraction a tremendous force must be exerted to enable the roots to pull the corms or bulbs down to their proper level in the soil. The passive resistance of the latter is overcome, and as a result its particles are pressed much closer together than they were before.

Sometimes this pulling power of the roots is exerted horizontally instead of vertically, and this accounts for the spreading of many bulbous plants like Tulips, Grape Hyacinths, &c., over a large area in the course of a few years when left undisturbed.

=Bulbous Plants without Contractile Roots.=--Some bulbous plants have not the advantage of contractile roots to keep them down in the soil, so they must secure this desirable end by different means.

Illustration: COLCHICUM. o. c. old corm; n. c. new growth; o. r. old roots.

Illustration: BULBOCODIUM. o. c. old corm; n. c. new growth; o. r. old roots.

A glance at the sketches of Colchicum and Bulbocodium will show a peculiar method of growth. The new corm instead of being produced on top of the old one, is developed at the side. Note, however, that the new corm is not on the same level as the old one. That would be no advantage whatever. Therefore it takes, as it were, a step downwards, so as to be well out of reach of mowing machines, rats, and mice, and other enemies, and also probably because it knows it will be much warmer in winter when several inches below the surface. The same principle seems to be employed by the bulbs of the Dog's Tooth Violets (Erythronium), as may be seen from the sketch--the new bulb to the right being distinctly lower than the older one to the left.

Illustration: ERYTHRONIUM.

PLATE 8. FRITILLARIAS (31. MOGGRIDGEI, 32. WALUJEWI, 33. MELEAGRIS ALBA, 34. RECURVA)

PROPAGATION OF BULBOUS PLANTS.

Perhaps there is no one class of plants that have so many ways of being easily increased as bulbous plants proper. Some kinds, e.g., Liliums, Alliums, may be increased in four different ways--from offsets and "spawn," scales, bulbils, and, last of all, seeds.

=Offsets.=--The great mass of bulbous and cormous plants, however, are so readily multiplied by detaching the offsets from the parent bulb or corm, that the other methods are rarely employed except by trade growers. Nearly all hardy bulbous plants produce offsets freely. These offsets represent a superabundance of nourishment that has been elaborated in the leaves, and very often there are several smaller ones attached to the base of the larger ones that have been produced in precisely the same way.

In the case of Daffodils, Tulips, Hyacinths, Crocuses, Gladiolus, and a host of others, the new offsets are pressed against the sides or on top of the older ones. In the drawing of the Tulip (p. 30), three new bulbs are to be seen surrounding all that is left of the old bulb. This latter has practically vanished up the main axis from the disc to produce flowers and leaves--hence it follows that the Tulip bulb somewhat resembles the corm in its vegetative characters. The bulbs taken out of the soil in early summer are not those that were planted the previous autumn.

Besides "offsets," some plants produce numerous small vegetative bodies called "cloves" or "spawn." These are shown in the drawing of the Gladiolus (p. 14), where two strong flowering corms have been developed on top of the old shrivelled one. At the base of each of these are numerous small outgrowths among the contractile roots. If these growths or spawn are taken off and stored in sandy soil until spring, they may then be planted in special beds, and in the course of two or three years will reach the flowering size.

Illustration: Tulip. d. disc of old bulb; f. s. flower and leaf-stalk which have eaten up old bulb; n. b. new bulb and offsets.

The Liliums are a large and interesting group of bulbous plants. Many of them produce offsets freely round the base of the old bulb. There are several species, however (e.g., canadense, Grayi, maritimum, pardalinum, Parryi, superbum), which have creeping rootstocks or rhizomes, and the new offsets are produced along these at intervals as shown in the drawing.

=Division.=--Bulbs or corms are rarely cut up for purposes of propagation. The best example in which this method of increase is practised is the Gladiolus. The larger corms, if they show two or more crown-growths, may be carefully cut down between them with a sharp knife. The cut surfaces may be dipped in soot, not only to dry it more rapidly, but also to prevent any stray spores of fungoid diseases from germinating.

Illustration: RHIZOME (r) WITH OFFSETS.

=Leaf-Scales.=--The thick, fleshy, deltoid scales of many of the Liliums will develop buds at the base, as shown in the drawing, when detached and inserted almost vertically in sandy soil. In about three or four years flowering bulbs can be produced by this means.

A somewhat analogous process is adopted with Hyacinths. The old bulb is slashed across the base of the disc two or three times into the fleshy scales. The cut surfaces dry up, and by-and-bye small buds or bulblets, as shown on the sketch of the Lily scale, make their appearance. In due course these bulblets are detached and planted in light sandy soil. The propagation of the florists' varieties of Hyacinths by this means is not altogether satisfactory, as the old bulbs themselves undergo a deterioration in our variable climate.

Illustration: Scale leaf (s. l.) of Lily bulb showing new growth (n. b.) at base.

PLATE 9. TULIPS (35-38)

=Bulbils.=--These are vegetative growths--neither seeds, bulbs, nor offsets--that appear in the axils of the aerial leaves, as shown in the sketch. Many Liliums, like bulbiferum, tigrinum, speciosum, Leichtlini, and some of the Alliums produce them with great regularity. It is thought that bulbils are borne by some plants and not others, because the conditions for the fertilisation or ripening of the seeds are not favourable. In such cases, therefore, Nature has provided such plants with this means of reproduction by bulbils, rather than allow them to run the risk of dying out altogether. In Kerner and Oliver's "Natural History of Plants" it is stated that "There are two forms of Orange Lily indigenous to Europe. One (Lilium croceum), occurring especially in the Pyrenees and South of France, almost always ripens fruits and forms no bulbils in its leaf-axils. The other (Lilium bulbiferum), found in the valleys of the Central and Northern Alps, hardly ever fruits, but is characterised by the bulbils it produces in the axils of its leaves; bulbils which disarticulate in autumn and are scattered by the wind. But there is no difference noticeable in the structure of the flowers in these two Orange Lilies, and it is difficult to explain their difference in mode of propagation, save on the assumption that in the regions where Lilium bulbiferum grows those insects are wanting which should convey its pollen from flower to flower. As the Orange Lily possesses no arrangements for autogamy (i.e., self-fertilisation), no fruits are formed in the absence of insect visits. It appears that this plant has lost the capacity for autogamy; at any rate, if a stigma be pollinated with pollen from the same flower on plants in a garden, no result follows. On the other hand, offshoots in the form of numerous bulbils are produced by Lilium bulbiferum, by means of which it is propagated and dispersed. In several valleys of the Central Alps it does not flower at all, and thus obviously depends entirely upon its bulbils for propagation."

Illustration: BULBILS in leaf-axils.

The bulbils should not be detached from the stems until the latter are quite ripe, and the foliage shows signs of withering. They may be sown as if they were large seeds. They possess the advantage over seeds, however, inasmuch as they produce flowering bulbs two or three seasons before the bulbs from real seeds come to maturity.