Leaves
Leaves provide trees with all their food because they turn sunlight into food energy. Chlorophyll makes this energy transformation possible. Leaves also make the oxygen in the air that we breathe.
Chlorophyll is a pigment found in the cells of leaves which is formed only in the presence of light and is the substance that colors plants green. Chorophyll is contained in chloroplasts and has the property of capturing light energy.
The process of Photosynthesis:
- Sunlight shines through the top of the leaf and reaches the next layer of cells. The light energy is trapped by the chlorophyll in the chloroplasts. In the chloroplasts, a process that uses water changes the light energy into a kind of chemical energy. This chemical energy is stored in the chloroplasts.
- The chloroplasts use the chemical energy to make food. Air enters the leaf through the stomata and moves into tiny spaces around the food-making cells in the leaf. Carbon dioxide from the air passes through the cell walls and membranes of the cells. Carbon dioxide enters the chloroplasts where the previously stored chemical energy converts the carbon dioxide into sugar.
- Tubes in the plant carry sugar from the leaf cells to other parts of the plant, such as roots, stems, and fruits. Cells in these parts of the tree store some of the sugar.
Three Main Parts to a Leaf
The base which is the point at which the leaf is joined to the stem. The stalk or petiole is the thin section joining the base to the lamina – it is generally cylindrical or semicircular in form. The lamina or leaf blade is the wide part of the leaf.
Leaves can be many different shapes. Primarily, leaves are divided into simple or compound. All blades are attached to a single leafstem. Where the leafstem attaches to the twig there is a bud.
Leaves may be arranged on the stem either in an alternate arrangement – leaves that are staggered or not placed directly across from each other on the twig; or in an opposite arrangement – 2 or 3 leaves that are directly across from each other on the same twig.
The margin may be entire, singly-toothed, doubly-toothed, or lobed.
Leaf diagrams adapted from Fifty Trees of Indiana, Purdue University, Department of Forestry and Conservation
Compound leaves may be palmate or pinnate.
The form of leaves is related with all their functions and their environment. In addition to photosynthesis, the leaf also carries out all the other exchanges with the atmosphere. It is through the leaf that the plant “breathes” (absorbs oxygen and gives off carbon dioxide plus energy) and transpires. Epidermic tissues in the leaf contain stomata. In most plants the stomata are located on the underside of the leaves. Their function is regulated so that plants living in dry climates have a substantially smaller number of them than those in humid climates, where they are numerous and prominent. Where humidity is low the stomata may actually be recessed or partly protected by soft hairs which can prevent excessive transpiration.
Flowers
All trees produce flowers of some kind. The main parts of a generalized flower are diagramed below. Flower parts occur in a standard arrangement of 4 whorls around the flower stem, or pedicel or pedicuncle. The outermost whorl is the calyx whose job is to protect the developing flower. The calyx is usually green and its separate parts ( sepals) are what we would recognize as the outside covering of a bud, as in a rose. As the flower opens, the sepals are pushed apart by the petals.
The whorl of petals is collectively called the corolla. Petals are usually bright-colored to attract pollinators to visit flowers. Inside the petals, the next whorl consists of the male parts of the flowers, the stamens. The stamen’s job is to make pollen. Tiny, microscopic pollen grains carry sperm from flower to flower. Stamens have two components: filaments and anthers. Filaments are thread-like structures like the filaments in a light bulb, that support the anthers out from the flower base. The pollen grains are produced in the sac-like anthers, which open in intricate ways to release pollen.
A generalized flower would also have female parts as the center whorl. These are known as the pistil or carpel. Like stamens, they are made of parts: the style extends from the center of the flower and supports the stigma, the sticky surface on which the pollen adheres during pollination. At the base of the carpel in the center of the flower is the ovary, where eggs are borne.
This is a generalized description; in nature, there are many variations on the general theme. Some plants have separate sexes, so that an individual bears only flowers with male (stamens) or female (carpels) parts, not both. Some flowers have colorful sepals, some have no petals, some have elaborate stamens that look like petals. Different species also may have different numbers of parts in each whorl.
Life Cycle of Flowering Plants
Seeds
Different trees produce different kinds of seeds. Some produce seeds in a fleshy covering – a fruit or berry, while other trees produce seeds tucked into the folds of cones or catkins. Other trees produce seeds with wings as well as some produce seed inside nuts or pods.
Tree seeds vary greatly in size – some of the largest trees have the smallest seeds. Tree fruits in various forms aid in the dispersal of seeds. Fleshy fruits are eaten by animals, from whose bodies the seeds may later be dropped. Winged fruits are spread by wind. “Seed trees” left after lumbering will reforest the land.
Conifers (pine and fir trees) make their seeds in cones. Pollen cones make pollen and the pollen spreads through the air like dust. Seed cones are sticky and the pollen grains stick to a seed cone. Tubes then grow from the pollen grains into egg cells in the cone. A sperm enters an egg cell and fertilizes the egg. Each fertilized egg cell grows into an embryo and is housed in a seed.
Twigs & Buds
Like a branch, a twig’s job is to support and transport. Twigs support the leaves which have the job of making food. Because leaves need to collect the Sun’s rays to make food, they must be held up as high as possible by the twigs. Twigs also transport water to the leaves and sugars from the leaves, using tiny tubes.
Both from Fun With Science: Trees and Leaves, Rosie Harlow and Gareth Morgan, Warwick Press, 1991.