What argument is Pollan making by using the apple and tulip?

The apple is the primary subject of Chapter 1 of the text. The apple is the fruit of the apple tree, or Malus domestica, which is one of the most widely-cultivated fruit trees in the world. The tree is small and deciduous, and comes from an original range in Kazakhstan. Apple trees do not breed true from seed, and thus nearly all modern domestic or garden apple trees are established by grafting material taken from well-known cultivars.

Tulips are plants of the genus Tulipa, which is comprised of about 100 species. From its original range in Turkey, the tulip was sent to Europe, where it enjoyed particular popularity in Holland throughout the 1600s. By the early 1700s, breeders in Holland had created so many unique and beautiful cultivars that considerable sums were spent to bring cultivar bulbs back into Turkey. Tulips are the primary subject of Chapter 2 of the text.