In botany, available space theory (also known as first available space theory) is a theory used to explain why most plants have an alternating leaf pattern on their stems. The theory states that the location of a new leaf on a stem is determined by the physical space between existing leaves. In other words, the location of a new leaf on a growing stem is directly related to the amount of space between the previous two leaves.[1] Building on ideas first put forth by Wilhelm Hofmeister in 1868, Snow and Snow hypothesized in 1947 that leaves sprouted in the first available space on the stem.[2]