seed formation is a survival strategy of higher plants that protects the juvenile embryo from unfavorable conditions and facilitates the dispersion of offspring into a new habitat. Plant seeds do not only benefit the ecological fitness of reproduction, but also contribute to the staple foods for human nutrition and caloric intake. A typical seed consists of highly organized structure, including a differentiated embryo, a nourish endosperm, and a protective seed coat or testa, but the physiological characters of individual seed considerably vary among the plant species. In addition, the complex web of environmental and endogenous factors appears to affect the quality and quantity of seed production. The molecular mechanisms underlying seed formation have been uncovered by the recent studies using the model plant Arabidopsis. In this article, we trace the evolutional history of land plants that have acquired the abilities to create seed, and introduce the recent achievements of plant seed biology. Future perspectives to dissect the relevance between structure and function of plant seeds are also discussed by retrieving the past information of cytological studies.