A leaf can be considered a plant organ,
since it is made up of different tissue layers. The main function of a leaf is
to produce food for the plant. The leaves are the sites where photosynthesis
mainly takes place.
All during spring and summer the leaves
manufacture the food needed for plant growth, especially in trees. This
food-making process takes place in the leaf in numerous cells containing the
pigment chlorophyll, which gives the leaf its green color. Along with the
green pigment leaves also contain yellow or orange carotenoids which, for
example, give the carrot its familiar color. Most of the year these yellowish
colors are masked by the greater amount of green coloring. But in the fall,
partly because of changes in the period of daylight and changes in
temperature, the leaves stop their food-making process. The chlorophyll breaks
down. The green color disappears, and the yellowish colors become visible and
give the leaves their changing color.
- In this lab, students are to bring in 4
different types of leaves to class. They must be relatively fresh in order for
students to make their observations. Leaves vary in their arrangements on the
stem, their form, their distribution of veins (venation), their structure, and
many other characteristics. This lab will focus on 2 leaf categories.
- First, students will have to determine
if the leaves come from angiosperm (broad leaf) or gymnosperm (needle leaf)
trees. Most students will bring in broad leaves, as they will probably not
recognize pine tree needles as leaves. Under the "broad leaf"
category, your students may bring in either dicot or monocot leaves. However,
most students will not realize that grasses (monocots) are leaves and will
probably not bring them, so most of the students will bring in dicot leaves.
- The guide sheet points out the
characteristics of the leaves that the students should observe. A typical
dicot leaf consists of two principal parts, the blade and the petiole or
stalk. The blade is thin and expanded, the petiole is slender. The thin blade
is supported by a distinct network of veins.
- The leaves of a plant can be one of two
types: simple or compound. A simple leaf is one in which the blade is all one
piece. A compound leaf is one in which the blade is composed of a number of
separate leaf-like parts called the leaflets.
- The shape of the blade can be long and
slender, or oval, or heart-shaped or triangular. The top of a leaf may be
pointed, rounded or flattened. The margin may have no indentations, or may be
toothed, scalloped, wavy or cut into a number of lobes.
- The veins in plant and tree leaves have
two major purposes. First, they strengthen and help maintain the shape of the
leaf and just as our bones in our bodies keep us straight and erect, so do the
veins of a leaf. Secondly, veins transport or carry water which contains all
the essential minerals and food the plant needs to live. The arrangement of
the veins of a leaf can be parallel (found in monocots) or netted (found in