Learning About Trees

Science: 1. The student understands and uses scientific concepts and principles. To meet this standard, the student will:

1.1 ...use characteristics to categorize living things

1.2 recognize the components, structure, and organization of systems and the interconnections within and among them

1.3 understand how interactions within and among systems cause changes in matter and energy

3. The student understands the nature and contexts of science and technology

3.2 know that science and technology are human endeavors, interrelated to each other, to society, and to the workplace

VOCABULARY:

angiosperm:flowering plant bearing seeds enclosed in a fruit
annual:    a plant that lives only one year
bark:    a general term for all tissue outside the cambium; parts are inner and outer bark
cambium:    the thin layer of dividing cells located between the phloem and xylem of a tree; it produces new phloem cells toward the outside of the tree ( inner bark) and new xylem cells toward the inside (sapwood) and is responsible for growth in diameter.
crown:    the leafy canopy of a tree including the branches
perennial:    lives more than one year
diameter:    distance across a circle at the center
deciduous:    drops all its leaves at one time (fall)
evergreen:    drops its leaves gradually; never leafless
gymnosperm:    a plant bearing its seeds exposed, as in a cone
hardwood:    a broad-leaved tree
perennial:    a plant that lives more than one year
phloem:    the food-conducting tissues of a plant; located outside the cambium and beneath the outer bark of a tree
root:    the below-ground plant which anchors and absorbs water and minerals
softwood:    a needle-leaf tree; a conifer
trunk:    the above-ground, woody stem of a tree
xylem:    the water- and mineral-conducting tissue of a plant; located inside the cambium

What is a tree?

A perennial, woody, green plant, usually leafy and with a single main stem (trunk)

A perennial lives more than one year.

A green plant converts sunlight energy to chemical energy; produces carbohydrates from carbon dioxide and water in the presence of chlorophyll using sun energy.

A woody plant does not die back to the ground each year, but accumulates growth year after year in the trunk.

What kinds of trees are there?

Basically, there are two types of trees based on reproduction – gymnosperms and angiosperms.

Gymnosperms: (JIM no-sperms) [Gk. gymnos, naked + sperma, seed] Seeds are not enclosed in flowers/fruit, but are borne naked, e.g. in scales of cones. Conifers and ginkgos are gymnosperms

Angiosperms: (AN-gee-oh-sperms) [Gk. angion, vessel + sperma, seed] Seeds are borne in an enclosed “vessel” – the mature ovary or fruit. Broad-leaved trees are angiosperms(All flowering plants are angiosperms).

Other common tree categories:

Hardwood and softwood trees

Hardwoods: another term for broad-leaved trees. Most but not all “hardwoods” have hard wood, e.g. maples, oaks

Softwoods: another term for needle-leaved conifers. Some “softwoods”, like yellow pine or yew, actually have very hard wood.

Deciduous and evergreen

Deciduous trees drop all their leaves every year and are bare in winter, e.g. oaks, maples.

Evergreen trees produce new leaves each year, but do not drop all their leaves at once. Leaves are lost gradually over two to four years, so the tree remains continuously green, e.g. pine, spruce, holly.

How is a tree structured?

A tree can be considered as having three major parts – the crown, the trunk and the root system.

Crown - The crown is the uppermost part of the tree and includes large branches, small branches or twigs, leaves, and buds. It allows the tree to capture sunlight and also is the site of major water loss (transpiration is why a tree cools the air). Leaves are the green, photosynthetic- and water-losing organs. They will be covered in detail in another session.

Twigs consist of nodes (where leaves are attached), internodes (between nodes), and buds. Terminal and lateral buds are where the tree grows in above-ground height and canopy spread. A terminal bud is formed each fall and resumes the tree’s growth the next spring. Branches are older and radially enlarged twigs; they merge basally with the trunk. Twigs, buds, leaves, and leaf scars left when leaves fall off are all useful in identifying different tree species. Terminal bud scars offer a way to see how much growth in length a twig has made over the last few years.

Trunk - The trunk is the woody stem of the tree which supports the crown. A trunk merges into the crown/canopy at the top and the root at the bottom. Once formed, a trunk grows sideways along its length, but does not grow longer except at the top of the tree. The trunk’s chief functions are structural support and conduction. Food (largely sugar) moves downward in one set of cells (phloem), while water and minerals move upward in a separate set (xylem). Old xylem accumulates in the center of the tree and becomes the wood.

Roots - The root system grows something like the crown, but in reverse, extending outward and downward through the soil from the base of the trunk. Large, older roots separate into ever smaller and finer branch roots. Roots, however, do not have nodes, internodes, or buds. Like the trunk and older branches, older roots only grow thicker while growth in length and spread of the root system occurs at the root tips. Roots function to anchor and support the tree and to absorb water and minerals from the soil. Water and minerals are absorbed principally at the young, small root ends. Older roots grow bigger radially like branches, but growth in length and spread of the root system occurs at the tips of the roots. Some trees may have a tap root, but major support and virtually all uptake of water and minerals occurs through the lateral root system which extends out beyond the branch spread and is concentrated in the top one to two feet of soil.

How does a tree work? –a further look at the trunk.

In cross section, a trunk is seen to have five major layers – outer bark, phloem (living or inner bark), cambium, young xylem (sapwood) and old xylem (heartwood).

The outer bark is the outer layer that consists of all the tissues outside the active phloem including older phloem and specially produced “corky” cells. Bark serves to protect the tree from injury and pest attack. Bark can also be useful in tree identification.

The phloem is a thin layer of cells just beneath the outer bark and outside the cambial layer. Phloem cells carry food (sugars) produced in the leaves to all the other parts of the tree. Movement of phloem sap is principally downward during the growing season.

The cambium is a very thin layer of dividing cells located between the phloem to the outside and xylem to the inside. It is the source of all new phloem and xylem cells and thus is ultimately responsible for the tree’s wood.

The xylem can be considered in two parts. The youngest xylem, just inside the cambial layer, is the water-conducting tissue of the tree. Like small pipes, xylem vessels conduct water from the root upward to the leaf where most of it evaporates into the air. Young, functional xylem is also called sapwood. As xylem ages, it stops conducting water and is converted into wood, also called heartwood. This heartwood is the chief supporting tissue of the trunk. Each year’s xylem production can be clearly seen in a trunk cross section and is useful in determining a tree’s age. Tree ring dating will be covered in more detail in another session.

Trees preserve a long-term record of their past growth in yearly xylem production as well as a shorter term history in twig and bark growth. Examining this record can reveal periods when stress was severe.