However flowering plants originated, we now know there are two main groups: monocots and dicots. A cotyledon is defined as a seed leaf, the first leaf that emerges as the seed germinates. Dicotyledonous plants (Class Magnoliopsida) are thought to be the most primitive flowering plants. They are defined by several characteristics. Similar to gnetophytes, all dicots germinate producing two seed leaves, hence dicotyledons. All dicots have other similar characteristics. For example, nearly all dicots have branching venation, and flower parts in multiples of fours or fives. Common examples of dicots include: roses, oak trees, and dandelions.