Seed are sometimes sold commercially while still enclosed within the hard wall of the fruit, which must be split open to reach the seed. Different groups of plants have other modifications, the so-called stone fruits (such as the peach) have a hardened fruit layer (the endocarp) fused to and surrounding the actual seed. Nuts are the one-seeded, hard-shelled fruit of some plants with an indehiscent seed, such as an acorn or hazelnut. Seeds are produced in several related groups of plants, and their manner of production distinguishes the angiosperms ("enclosed seeds") from the gymnosperms ("naked seeds"). Angiosperm seeds are produced in
Evolutionary history of plants they were too scarce to impact the atmospheric composition until around . These organisms, although phylogenetically diverse, were probably small and simple, forming little more than an algal scum. However, evidence of the earliest land plants occurs much later at about 470Ma, in lower middle Ordovician rocks from Saudi Arabia and Gondwana in the form of spores with decay-resistant walls. These spores, known as cryptospores, were produced either singly (monads), in pairs (dyads) or groups of four (tetrads), and their microstructure resembles that of modern liverwort spores, suggesting they share an equivalent grade of organisation. Their walls contain sporopollenin –