Angiosperms, or flowering plants, have traditionally been divided into two classes: monocots and dicots. Despite growing evidence that some dicots may be more closely related to monocots than to other dicots, and that angiosperms do not all fit neatly into two clades, there are some standard differences between the two. Whereas dicots generally have two seed leaves (cotyledons) and stems with vascular bundles forming a ring around a central pith, monocots typically have one seed leaf, parallel-veined leaves, and vascular bundles distributed throughout the stem cross-section. Their flower parts often occur in multiples of three. Monocots comprise about one quarter of all flowering plant species and include lilies, orchids, palms, and grasses. Current hypotheses suggest that they diverged from their dicot relatives very early in angiosperm evolution.