The thick and swollen modified stem which grows under the ground and lacks green color due to their underground nature with a large amount of stored food is called the underground stem. These are actually derived from tissues of the stem but grow under the ground.
Sometimes underground stems are mistaken as roots due to their great resemblance. These stems have nodes, internodes ad scale leaves. These always arise from the axils of leaves. There are various types of underground stems which are given below.
Types of Underground Stem
Underground stems are rhizome, corm, bulb, and stem tubers.
The elongated, horizontal, thick, and fleshy underground stem is called a rhizome. It has a large amount of stored food. It has distinct nodes and internodes. Nodes give off adventitious roots. It also gives off serial branches.
Types of Rhizomes
- Straggling Rhizome:
The elongated less branched rhizome which grows horizontally is called a straggling rhizome. It is present in water grasses such as Typha.
- Sobole Rhizome:
The rhizome is long and slender with long internodes. These are present in many grasses.
- Monopodial Rhizome:
These are arranged like racemose. They have a single axis which gives rise to lateral branches. This is present in wood sorrel.
- Sympodial Rhizome:
The rhizomes are arranged in a cymose pattern. In this, each branch gives rise to the new axis. These are found in ginger.
The thick, solid, rounded and condensed underground stem with few internodes is called a corm. These are covered with a number of loose sheathing scale leaves. Each axil of the leaf contains one or more buds that form a new corm.
The new corm lies on top of the old corm. The aerial portion dies in unfavorable conditions. The corm passes unfavorable conditions. It grows to form a new plant in the growing season. Sometimes, corms of three or more generations remain connected.
A specialized much reduced short underground stem composed of the broad conical disc is called a bulb. Large fleshy leaves are borne on the upper surface of the bulb. These leaves contain stored food. The fleshly leaves may be small-scale leaves like an onion.
These may contain a broad basal portion with the upper green part die away. The disc bears an apical bud and some axillary buds on its upper surface. The apical bud lies in the center. The lateral buds lie in the axil of fleshy leaves. The apical bud grows in the growing season.
Types of Bulbs
There are two types of bulbs:
The buds in which leaves surround each other is called the tunicated bulb. In these bulbs, the bases of the foliage leave store food and become fleshy. New leaves are formed on the older leaves. These are very close to each other.
The lower portion of leaves contains a large amount of stored food. But the upper portion of leaves have no stored food and are dry. These are found in onions and garlic. There are many small oval structures in bulbs of garlic known as cloves.
The bulb in which leaves overlap each other at their margins is called the imbricate bulb. These bulbs bear two types of leaves: foliage and scaly leaves. The foliage leaves die at the end of the season and food is stored in scale leaves.
These leaves become thick and fleshy. These fleshy leaves overlap each other at their margins. Outer dry tunics are absent in them. An apical bud is present in the center of the disc. One or more axillary buds are present in them.
The swollen underground tips of branches of the vertical axis which store food and use for vegetative propagation are called stem tubers. Several slender axillary branches arise from the basal underground portion of the stem.
These branches bear foliage leaves. These leaves function to prepare food and food is stored in the tips of underground branches to form tubers. These tubers consist of several nodes and internodes. The axil of scale leaves contains groups of lateral buds.
These buds are called eyes. The eyes represent nodes and are arranged spirally. When the plant dies, the tuber detaches from the plant. And the buds grow in the growing season (spring for potatoes) to give rise to the new plants.
Functions of Underground Stems
- These are used for perennation. The passing of unfavorable conditions for growth is called perennation.
- These are a great mean of vegetative propagation.
- This can increase the number of plants rapidly in the locality.
- They contain large amounts of stored food. This stored food is used by new growing plants.
- Most underground stems are used as food by humans.
Economic Importance of Underground Stems
- The underground stems have large amounts of stored food in the form of starch. This is used by humans and other animals.
- Some underground stems have medicinal importance. For example, garlic is used to control cholesterol levels in the body.
- Rhizomes can cause severe problems in agriculture. Some weeds develop underground rhizomes. These are cut into pieces during ploughing. Each piece develops into a new crop. This becomes very difficult to control such weed rhizomes.