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.
The first structure that emerges from the interior of a germinating seed is the primary root or radicle, which goes perpendicularly down into the earth. If the minute structure of the tip of this is examined it will be seen to consist of very small square cells at and behind the growing point (fig. 12). Around and in front of this is a layer of brick-shaped, corky cells, most of which are empty and dead. This is the root cap, which is intended to protect the tender growing point as it pushes its way amongst the particles of soil. The nature of a young root may readily be seen by filling a punnet with light sandy soil and scattering some Mustard seed thinly over the top. Stand it in a warm, moist, shady place for a few days till the seed germinates, and then in a well-lighted position. From the sides of the radicle numerous hairs arise, enter the soil in a horizontal direction, and place themselves in close contact with the particles of soil (figs. 13, 14, 15). These are the root hairs, and their function is to absorb water containing food in solution. The interstices or spaces between the particles of soil are filled with air, or should be, for land plants, but the particles themselves are covered with a thin film of water, and this is all that the root hairs can absorb. If the interspaces are filled with water, the soil is water-logged and the radicle and root hairs cannot breathe, but soon get asphyxiated and perish. The radicle does not absorb water at the tip but some way behind it, and only while the outer walls remain quite thin. The root hairs continue their work for a few days or weeks, then die away and leave no trace behind; but as the radicle lengthens and secondary roots are formed, new root hairs are continually being produced, thus tapping fresh areas for food. In Dicotyledons generally the primary root is permanent, and, if undisturbed, may attain a great size and age in forest trees. From an early stage of growth it commences to give off secondary roots which branch repeatedly, permeating the soil in every direction with their finer ramifications or root fibres. In Monocotyledons, like Lilies, Daffodils, Onions, Palms, and Grasses, the primary root soon ceases to lengthen or dies, but its place is taken by numerous, secondary, and even adventitious, fibrous roots. Some of these attain a considerable thickness in large Palms and Screw Pines (Pan-danus), but in grasses they remain slender and fibrous (fig. 16).
Fig. 12. - Section through the Root Tip of Pentstemon. The bowl-shaped mass at the tip is the root cap; x 60.
Fig. 13. - 1, Seedling with the long absorptive cells of its root ("Root Hairs") with sand attached. 2, The same seedling: the sand removed by washing.
Fig. 14. - Root Tip of Pentstemon with Root Hairs penetrating between the particles of soil; x l0.
Fig. 15. - Root Hairs or Absorptive Cells of Pentstemon with adherent particles of earth.