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.
Apart from the functions already described, a large number of bulbous plants are provided with roots which have the power of contracting at certain periods, and thus pull down the bulbs or corms deeper into the soil. These roots are known as "contractile ". They are generally thicker and fleshier than the more fibrous feeding roots, and are recognized by the transverse wrinkles or rings upon them. The young or new corms of Gladiolus and Crocus, and the young bulbs of many Liliums and other bulbous plants, are all provided with such roots. In the case of seedlings, Dr. Scott says, in his Structural Botany, that the young bulb "is gradually drawn down year by year owing to the shortening of the adventitious roots. As the end of the root attaches itself firmly to the soil, the effect of the contraction is to exert a downward pull on the bulb. The upper part of the root is alone capable of contraction, and is much thicker than the rest. The inner cortex is the actively contractile tissue; as it contracts, the external layers are thrown into transverse wrinkles.
Fig. 18. - Seedling Plant of Gourd (Cucurbito Pepo) with Radicle, Caulicle, and opposite Cotyledons. Liberation of the Cotyledons from the cavity of the Seed or Fruit Husk, showing in the central figures the little peg or radicle that serves to fix the seedling.
New roots of this kind are formed each year, until the bulb has reached its normal depth." [J. f.]