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]."
| Nothoscordum Bulb. Showing Contractile Roots. | 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.
| Colchicum. o. c. old corm; n. c. new growth; o. r. old roots. | 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.
