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Buds and Twigs

Buds

Structure

A bud is a “condensed” shoot. Its stem is very short and its leaves are so close that they overlap, each one wrapping round the next above it. The inner leaves are crinkled and folded, since a large surface area is packed into a small space. The outermost leaves are often thicker and tougher, and sometimes black or brown. These are the bud scales and they protect the more delicate, inner, foliage leaves from drying up, from mechanical damage by birds, insects, etc., and, to some extent, from extremes of temperature. At the end of the bud’s short stem is either a flower, or a growing point where rapid cell division will take place later on when the next bud is forming.

Function

The bud forming habit gives the plant advantages of being able to present a photosynthesizing surface to the atmosphere very rapidly after seasonal conditions become favorable. The leaves, already formed in the bud are available almost at once, whereas it would take many weeks for them to grow from the single cells at the growing point. In most plants the buds are formed when conditions are favorable for plant growth, e.g. in the summer months, and the close packing of the leaves and the thick bud scales protect the leaves from desiccation and low temperatures during the winter.

Types of bud

Terminal buds are formed at the ends of main shoots or branches during the season’s growth. There are also buds in the axils of the leaves. These are called axillary or lateral buds. If they do not grow in the following year, they are also described as dormant buds. Terminal buds wen they grow, continue to grow in length, whereas lateral buds make new branches. Either type may produce a flower instead of, or in addition to, a leafy shoot. If this happens in a terminal bud, growth is continued in the following season by one or more lateral buds, since the flower or inflorescent drops away leaving no growing point.

Growth

In the spring, the stem of the bud begins to elongate and the bud scales are pushed apart. As the stem grows in length it spaces out the leaves, which unfold and spread out their surface. The bud scales often curl back and in a few weeks fall off. On exposure to light the chlorophyll in the leaves develops fully, and photosynthesis beings soon after.

Twigs

Leaf fall or Abscission

Most trees shed their leaves. Deciduous trees do so in the autumn while evergreens shed theirs in small numbers all the year round. In certain trees, abscission takes place as follows: cells at the base of the leaf stalk divide and form layers of cells across the region where the petiole joins the stem. The new tissue nearest to the stem becomes corky and the vessels become blocked to that the leaf is deprived of water. Before this occurs, the contents of the leaf cells begin to break down chemically, producing the characteristic red and yellow autumnal tints. The cells’ contents are digested and the soluble products absorbed back into the tree. The cells beyond the corky layer degenerate, and the dried up leaf falls, leaving a scar on the stem protected from the entry of bacteria and fungi by impermeable cork. The details of abscission vary in different species. Earlier in the year, when the terminal bus was sprouting, the bud scales, unlike the foliage leaves, were not spaced out on the stem. There are commonly called girdle scars and, since they mark the position of each year’s terminal bud, the length of stem between each set of girdle scars represents one year’s growth.

Winter Twigs

The leaf scars on twigs are a characteristic shape for each species, the sealed vascular bundles making a pattern of dots in them. Since each leaf usually has a bud in its axil, above each leaf scar there should be a lateral bud.

Biology


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