ld find
their way into the Black Hills, save that, previous to the ice age,
this species was generally scattered over the territory, and that,
during the ice age, the species was perpetuated in the hills, but was
killed out between there and the Rocky Mountains where it is found in
abundance. These are some of the natural reasons for the existence of
varied plants in different localities. They are sufficient to explain
the reason for the existence of local floras.

But nature has provided untold ways for the perpetuation as well as
the dispersal of plants for the purpose of, so far as possible,
enabling the plants of the world to take possession of all parts of
the earth's surface. If this adjustment were complete, the plants
would be practically alike all over the surface of the earth, but we
have already explained why this cannot be and why we have a different
flora in each zone, whether it be marked by lines of latitude or
height of {120} the mountains. Plants are perpetuated by seeds, by
bulbs, and by woody parts. Some seeds are highly perishable and must
be sown as soon as ripe; others remain years without losing their
power to produce plants. Some grow as soon as they come in contact
with the soil; others must fall, be buried and frozen before they will
germinate. Some plants are perpetuated by bulbs, tubers, or roots in
which a supply of food material is stored away to carry the plant over
a period when its above-ground parts cannot thrive owing to frost or
drought. Upon the return of favorable conditions, these resting parts
throw out shoots and again make the round of growth, usually producing
both seeds and underground parts for the preservation of the species.
There are both wild and cultivated plants in nearly all sections which
illustrate these methods of preservation. Besides plants which have
bulbs, tubers, or perennial roots, we have the large, woody plants
which live many years and so perpetuate themselves, not only as
individuals the same as plants with perennial roots; but they, too, as
a rule, produce seed for the multiplication of their kind.

{119}

[Illustration: _Pinkster Flower_--It shows its pink flowers in rocky
woods and thickets during spring.]

[Illustration: _White Pine_--Common evergreen tree of the Northeastern
states. Needle-like leaves in bundles of five.]

[Illustration: _Butterfly Weed_--The bright, orange colored flowers are
conspicuous in dry meadows from June to Septem