Many leaves have white or nongreen veins, causing the leaf to appear variegated (Examples: an aroid, Sanchezia, striped inch plant, and milk thistle). The white vein is caused by the absence of chlorophyll-bearing cells above or below the conducting tissues of the vein. Most commonly, no stomates are found directly on a vein, so the cells, incapable of receiving adequate carbon dioxide, are not equipped for photosynthesis. Leaves with white veins should not be classified as variegated, although they are sometimes also grown as horticultural oddities.