Winter
Plant Observations
Strategies for Survival
— Teacher Background —
It's early fall. The days continue to shorten and grow colder. This triggers chemical changes inside a plant that tells it winter is coming. Some of these changes actually help "toughen" up the plant, acclimating it to colder conditions. Here's a quick overview of some of the visible changes that take place.
Growth/Food
Production
With less sunlight available, plant leaves can't make as much food, and growth
slows down. As plant cells stop making green chlorophyll (for food production),
they begin to move sugars down to the deep roots — or to other food storage
structures, such as bulbs. These sugars act like antifreeze. (Snow also insulates
and helps protect underground plant parts.)
Leaf
Changes
As chlorophyll breaks down in deciduous trees and shrubs (those
that lose and regrow leaves each year), the green fades. In many trees,
reds, yellows, and other colors already in the leaves begin to show
through. Because the leaves are no longer needed to make food, and
their tender tissues would be damaged by winter cold, they simply drop
off the plant. (Leaves on other perennial plants also fade or die back
entirely each year.) The plant seals up the places where the leaves
attached to the branch. (Your students should be able to observe these
leaf scars.)
Water
is also in limited supply during the winter. If a tree didn't drop
its leaves, the plant would dry out as it transpired (lost water through
its leaf pores). Students may notice, however, that trees with needles
(e.g., pine) tend to hang onto them through the winter. (These are
adapted to life in very cold and/or dry climates.) Ask students to
compare these leaves with those on deciduous trees. The small size
and waxy coating on needle-like leaves prevent them from losing water.
Some can even make food slowly during winter thaws. 
Buds
on Branches
If you look closely at the ends of branches on trees and shrubs in the winter,
you'll see scaly bumps. (You should see these along the branches, too.) These
contain tender leaf buds (and in some cases, flower buds). If you cut through
one of these laterally (left to right) with a razor, you will discover the
layers of hard bud scales and cottony packing material that insulate the shoots,
leaves, and flowers that will open in the spring.
Seeds
and Seed Pods
Students may also notice evidence of seeds, old seedpods, and dispersal structures
(e.g., fluff). Plants that complete their life cycles in just one year (including
many weeds and grasses) disperse their seeds in the summer or fall. Seeds have
tough coats and are in a domant state, so they are able to survive cold, dry
winter conditions.
Winter
Survival in the Desert
If you live in an area with a desert-like climate, students can still notice
winter changes in plants. Cacti are adapted to conserving life-sustaining water,
but if this stored water were to freeze, it could damage the plant. So to prepare
for winter, many cacti lose water and spend the season looking deflated and
limp!
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