I think your professor is trying to make you consider the
fact that water is only taken up by plants when it is in the liquid state. Winter,
accompanied by frozen ground, is sometimes referred to as a "physiologic desert",
because although there may be adequate amounts of water present in the winter
environment, if it is frozen, it is unavailable for plants to
utilize.
This is the reason that evergreens growing in
temperate climates demonstrate water-conserving adaptations. Keeping the leaves or
needles through the winter is a very successful adaptation for growing in nutrient-poor
soil, but the plant must not lose more water through evapotranspiration than it takes in
or it will reach the permanent wilting point and die. As a result, evergreens tend to
have thick waxy cuticles and sunken stomates that can close more tightly than other
plants. Broadleaf evergreens like rhododendrons will curl their leaves in response to
low temperatures and humidity, and some members of the group also have fuzzy leaf
undersides to increase the leaf boundary layer, another way to slow water
loss.
On the converse side, plants that live in alpine
zones or certain tropical regions called "cloud forests" sometimes have to access water
in the vapor form year-round; this is particularly true of epiphytes like orchids and
bromeliads, which live perched on other plants. For these plants, adaptations may
include hairs that collect moisture from water vapor in the air, leaf shapes that funnel
collected water toward the base of the plant and the roots, and specialized air roots
which can take in water vapor from the air directly.
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