In a plot of turf three feet by four, twenty distinct species of plants were found to be growing, and no less than nine of these perished altogether when the other species were allowed to grow up to their full size.[5] But besides having to protect themselves against competing plants and against destructive animals, there is a yet deadlier enemy in the forces of inorganic nature.

Each species can sustain a certain amount of heat and cold, each requires a certain amount of moisture at the right season, each wants a proper amount of light or of direct sunshine, each needs certain elements in the soil; the failure of a due proportion in these inorganic conditions causes weakness, and thus leads to speedy death.

The struggle for existence in plants is, therefore, threefold in character and infinite in complexity, and the result is seen in their curiously irregular distribution over the face of the earth.

Not only has each country its distinct plants, but every valley, every hillside, almost every hedgerow, has a different set of plants from its adjacent valley, hillside, or hedgerow--if not always different in the actual species yet very different in comparative abundance, some which are rare in the one being common in the other.

Hence it happens that slight changes of conditions often produce great changes in the flora of a country.