Unit 8. Plant structure.

WARM-UP: Think of words and word-combinations you associated with the structure of the plant and complete the picture.

VOCABULARY: Learn the following words and word combinations.

to anchor tap root lateral roots root cap root hair node internode scar dormant period annually runner tuber twinning stems stoma (pl. stomata) terminal bud needle leaf to penetrate to be covered with soil vein midrib

Guess the meaning of the following words.

To absorb; horizontally; absorption of water; region of growth; non-green leaves; epidermis; derived from; function; delicate; inorganic materials.

Practice the following for pronunciation.

Reservoir [´rezəυwa:(r)]

Penetrate [´penıtreıt]

Node [nəυd]

Annually [ænjuəlı]

Aerial [´aəriəl]

Vein [vein]

Midrib [mid´rib]

Tuber [tju:bə]

Vine [vain]

Twine [twain]

succulent [´s۸kjulənt]

anchor [´ænkə]

stomata [´stomətə]

spongy [´sp۸ndʒı]

palisade [ֽpæli´seid]

READING: Read the text and do the tasks that follow.

The part of the plant most intimately related to the soil is the root. The root has two major functions – to anchor the plant and to absorb water and certain important nutrient materials from the soil. The root often serves as a storage reservoir for food. The most common type of root is a slender and branched structure, penetrating the soil in all directions. Less common are types which possess a single main root or tap root, penetrating deeply into the soil and much stouter than the lateral roots which arise from it. Tap roots usually store food and become large and fleshy. It normally grows directly downward, but most of the lateral roots at first grow horizontally, and only later they turn downward.

Absorption of water and nutrient materials is carried on by the younger portions of a root, near its tip. The tip is usually covered with a root cap, which protects the delicate tissues. Back of this is a short region of growth. Behind this is the absorbing region, the surface of which is covered with thousands of delicate root hairs. The root hairs are hair-like projections from the root, and by means of them water and salts are taken in from the soil. They may reach a length of several millimeters. The root hairs are very numerous, but they live for a few days only and then die.

The typical root system develops from the tap root, but under certain conditions roots may also appear on other parts of the plant, mainly on stems and leaves.

Stems are usually distinguishable from leaves and roots. A stem develops from the bud of the embryo and usually bear leaves and flowers. Leaves and branches occur only at certain places on the stem called nodes. The part of the stem between two nodes is the internode. The length of a branch and the height of the plant depend upon the elongation of the internodes. When trees lose their leaves annually the scar is left on the stem. The distance between two scars is a measure of the growth in length of the stem during one year.

During dormant periods one can easily see buds on the stem. The number of buds at a node is different depending upon the species of plants. The bud at the apical end of the stem is the terminal bud, all others are lateral buds.

There are not only erect aerial stems but also underground ones. They have scale-like, non-green leaves and lateral buds from which aerial branches develop. These stems may be thick and fleshy, or slender and woody. Slender stems are termed runners. They are distinguished from roots by the presence of nodes and internodes. In some species of plants such slender stems end in thick structures known as tubers. There are also stems that use some kind of support for their growth. These stems are called twining stems. Plants possessing this type of a stem form the group termed “vines”.

A leaf is a flat organ. The blade of a leaf consists of an epidermis. Between the upper and lower epidermis there are several layers of the mesophyll. The upper layers have elongated vertical cells, forming the palisade layers; and the lower have irregular cells more or less parallel to the leaf surface, forming the spongy layers. The blade is covered with numerous veins and the midrib is several times as thick as the rest of the blade. In the lower epidermis there are openings called stomata. When open, the stomata connect the air within the leaf with the atmosphere. The opening and closing of the stomata depends on light, water and temperature.

Leaves develop from buds. With the warm weather in spring growth of leaves starts. After the opening of the buds, leaves double their size within twelve hours. In plants with small leaves the whole development occurs during ten days. In large leaved plants, the leaves grow for a month. Leaves size and structure depend on light. Leaves that develop in full sunlight are usually smaller and thicker that the similar leaves that develop in shade. The larger size of shade leaves is due to the greater enlargement of the cells rather than to the increase in number of cells.

The variety of leaves is great. The smallest flowering plant is a duckweed. The longest leaves are in palms. Pines have needle leaves. The cactus has small succulent leaves. The primary activity of green leaves is the manufacture of food from certain simple inorganic materials – carbon dioxide and water – by energy derived from sun. This process of photosynthesis is fundamental in organic nature.