There was no significant change in the number of evergreen
species offered in all of the four time periods and there was no linear
or quadratic trend with time. In the case of deciduous species, the
number of species offered in time period 1990–2011 was greater
than the number of species offered in time periods 1900–1929,
1930–1959, and 1960–1989. There was no significant difference
in the number of species offered in time periods 1900–1929,
1930–1959, and 1960–1989. A strong linear trend (p < 0.001) was
observed with time