叶片是最为常见的植物侧生器官。扁平化是叶片形态的通常特征，对于增大光合作用面积具有显而易见的优势。叶片原基从接近半圆形的隆起快速形变，最终成为扁平化的叶片。经过70余年的研究，发育生物学家对叶片原基发育过程中的基因表达和形变机理有了逐步深入的认识。对叶片扁平发育机制的理解催生了在进化过程中扁平化叶片形成的新理论。

As the most typical lateral organ, leaf shape is usually flattened. Although flattened leaf shape is clearly beneficial to expand the area for photosynthesis, how it is achieved developmentally remains unanswered. Commonly, leaf primordia are close to half ball shaped right after initiation, but quickly acquire flattened shapes. Through research over the past 70 years, developmental biologists have obtained a deeper understanding of not only the gene regulatory networks but also the form acquisition process underlying leaf geometry changes. New theories have been proposed to explain leaf blade evolution, which broadly impacted on the Earth’s ecosystem.