This section is from the book "The Gardener V2", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
The Primrose is distinguished from the Cowslip by the flower-stalks having the appearance of springing directly from the root, and bearing each only one flower: there is, however, a common footstalk so short as to be concealed by the leaves. The corolla in the Primrose is larger than that of the Cowslip, and flat. There are many very interesting and beautiful varieties of Primrose, both single and double; the single varieties need not be enumerated, but are very useful for planting out in clumps or patches, as recommended for the commoner forms of Cowslip. The best of the double varieties are the flesh-coloured, the white, the sulphur, the lilac, the red, the coppery, and the dark purple: the three last are rare, and difficult to procure. All are invaluable for the spring flower-garden, for rock-work, and for many other purposes in the kitchen and fruit gardens.
In Primula elatior, the Oxlip, the common flower-stalk is generally shorter and stouter than that of the Cowslip, but is variable in length and strength; always, however, showing the umbel above the foliage, and the corolla is broader and flatter than that of the Cowslip. This is the least constant form of the three, and its varieties are less numerous and interesting than those of the other two; they are mainly useful for planting along with the commoner sorts of Cowslip and Primrose in woods and suchlike places.