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§ 37. Rhizome, tuber, bulb
Peculiar underground pantries have perennial herbaceous plants. The above-ground parts of these plants die off annually by autumn. IN soil roots and modified underground shoots remain. They don't look like regular overhead ones. It is in these modified shoots that reserves of organic matter are deposited for the winter. Modified shoots are rhizomes, tubers and bulbs.
The rhizome is found in nettle, wheatgrass, iris, lily of the valley, houseplant aspidistra.
If you dig the rhizome of any plant out of the ground, you can see that outwardly it resembles root. But the rhizome, like the above-ground shoots, has apical and axillary buds, as well as membranous scales - modified leaves. Adventitious roots grow from the rhizome 72. From the apical or axillary buds of the rhizome, young above-ground shoots develop in spring. At the same time, they use the nutrients deposited in the rhizome in autumn. If a piece of rhizome with a bud and adventitious roots is planted in the soil, a new, independently existing plant. Some perennial ornamental plants reproduce by dividing the rhizome into parts.
Tubers are characteristic of a few plants, for example, for potatoes. Underground shoots, on the tops of which tubers develop, grow from the bases of above-ground stems; these shoots are called stolons. Tubers are apical thickenings of stolons 73 . The tuber has short internodes; it does not contain chlorophyll, but when exposed to light, it can turn green.
Consider a potato tuber.
On its surface, in depressions of 2-3, there are buds, or eyes. There are more eyes on that part of the tuber, which is called the top. The opposite side - the base - the tuber is connected to the stolon. The structure of the potato tuber convinces that the tuber is a modified underground shoot 74 .
Organic matter constantly flows from potato leaves through the stems into stolons and in the form starch deposited at the top.
The tops of stolons grow, thicken and turn into large tubers by autumn. Cut a tuber and put a drop of a weak iodine solution on it. You will make sure that there is starch in the tuber.
Some wild plants also have tubers, for example, Corydalis and Jerusalem artichoke forage plant.
In the lower part of the onion bulb there is an almost flat stem - the bottom.
On the bottom are modified leaves - scales. The outer scales are dry, leathery, while the inner scales are fleshy and juicy. They contain water reserves, as well as sugar and other substances dissolved in it. On the bottom, buds are visible, located in the axils of scales 75. Thus, the bulb is a modified shoot.
If the bulb is planted in the ground, a fibrous root system grows from the underside of the bottom. Sometimes young bulbs, called children, develop from the kidneys. An independent plant can grow from each bulb of a baby.
Bulbs form onions, lilies, tulips, daffodils, wild goose onions. All of them are perennial.
1. What modified underground shoots do you know? Name plants that have a rhizome, tuber, bulb 72, 73.
2. How to distinguish a rhizome from a root?
3. How does a potato tuber develop?
4. Why should a potato tuber be considered a shoot?
5. What is the structure of the bulb?
6. How to prove that the rhizome and bulb are a modified shoot?
1. Place an onion bulb in a jar with a narrow neck so that the bulb does not fall through, but only touches the bottom of the water poured into the jar. After a few days, the bulb will develop adventitious roots and green leaves. Why does the bulb grow, although it is not in the soil, but in the water?
2. With the onset of warm spring weather, watch the flowering of wild bulbous and rhizomatous plants. 3. Watch the development of woody shoots and sprouting tubers 76, 77 . Compare them and draw your own conclusions.
Korchagina V.A., Biology: Plants, bacteria, fungi, lichens: Proc. for 6 cells. avg. school - 24th ed. - M.: Enlightenment, 2003. - 256 p.: ill.
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