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.
Fig. 19. - 1, 2, Seedling of Nasturtium (Trvpceolum majus). 3, 4, Seedling of Water Chestnut (Trapa natans) with section of seed. 5, 6, Seedling of Austrian Oak (Quercus austriaca). 7, 8, 9,10, Stages in the germination and growth of Date Palm (Phoenix dactylifera) with sections. 11, 12, 13, Seed and germination of same of Reed Mace (Typha Shuttleworthi). 14, 15, Seedling of Sedge (Carex vulgaris). 1-8 nat. size; 9, 10, x 8; 11-13, x 4; 14, 15, x 6.
It forms part of the embryo, while still in the seed. A Stock, China Aster, Cabbage, or Gourd seedling (fig. 18) will serve the purpose. Between the ground and the seed leaves the short, upright portion is the first visible part of the stem, to which various names have been given, including caulicle, which means little stem. Structurally it is made up of cells, fibres, and vessels, built up in the form of tissue characteristic of a stem. Its functions are to hold up the seed leaves to the light and supply them with water and food materials. Between the seed leaves the first bud of the plant, known as the plumule, will be noticed. It is really the apex of the young stem, covered with the rudiments of the first true or rough leaves. Many variations are met with amongst seedlings. For instance, in the Scarlet Runner, Broad Bean, and Oak the seed leaves remain in the seed, below-ground, during and after germination. In these cases the caulicle remains very short, the seed leaves do not make their appearance, and the plumule is the first part to rise above ground. The caulicle undergoes modification in other ways in certain plants. The upper portion of the tuberous swelling of the Turnip and Radish consists of the caulicle, enlarged and fleshy, to serve as a store for reserve food.