Background

A monocotyledon flower has one cotyledon, which is the embryonic leaf of a seed-bearing plant. A dicotyledon flower has two cotyledons. These two types of flowers have many differences. A monocot has leaf veins branching out and flower petals in multiples of four or five. A dicot's leaf veins are parallel and have petals in multiples of three. (http://www.diffen.com/difference/Dicot_vs_Monocot). Since monocots and dicots are both flower producing plants, it can be deduced that they both evolved from the same flower producing plant millions of years ago. Although the flowers are not always conventional looking, all monocot and dicot plants produce some sort of flower. (http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html) The evolutionary divergence date for monocots and dicots was approximately 200 million years ago, which results in the distinct differences between the two. They have had millions of years to evolve on completely separate paths and develop their unique traits. This date was estimated by reconstructing phylogenetic trees from chloroplast DNA sequences. Two methods were used to approach this. One method involved calibrating the rate of synonymous nucleotide substitution from the divergence of maize, wheat, and rice; while the second involved calibrating the rate of non synonymous substitution from the divergence of angiosperms and bryophytes.  (http://www.pnas.org/content/86/16/6201.full.pdf)