It is important to note that root nodules are not root knots, which are root swellings in response to worm invasions. Root nodules are beneficial bacterial colonies that are visible as small swellings in the root system. The bacteria aid the plant in fixing, or converting, atmospheric nitrogen into a form that the plant can use. Root nodules are found extensively throughout the legume family. A nodule develops when a substance leaked into the soil by plant roots stimulates Rhizobium bacteria to produce another substance that caused root hairs to bend sharply. The bacterium may attach in the crook of the bend and then ‘invade’ the cell with a tubular infection thread. This thread does not penetrate the cell wall and plasma membrane. The thread, does, however, grow through to the cortex which is stimulated to produce new cells that will become part of the housing for the bacterium. As the bacteria multiply and the colony grows, the nodule will swell. It is in the crook of root hairs that the nitrogen-fixing takes place.
Stems are vital to the human cause. They provide building materials, paper products, food and much, much more! Stop and think of how many things you encounter in a day that is either made of wood or plant fiber or a derivative product, chances are good they are a stem derivative.
Cacti usually have modifications in their stem or ‘trunk’ structure in order to hold extra water. Other stems may be modified into thorns or briars. It is important to remember that not all thorn-like structures are stems! Raspberry and rose prickles are extensions of their epidermis and are neither thorns nor spines. Other stems are modified for climbing, such as tendrils and ramblers.
Cladophylls are usually called the prickly part of a cactus. They are flattened and somewhat leaf-like in appearance. The center of each cladophyll usually has a node with small scalelike leaves complete with axillary buds. The scaly look to asparagus is cladophylls. These specialized stems are not only restricted to cacti but are found in some orchids and green briars.
At first glance you might think these guys are bulbs, however, the differences lie beneath the thin layer of leaves covering the outside of the corm. Adventitious roots form beneath the fullness of the base. Corms function in storing food. Crocuses and gladioli are good examples of plants with corms.
Bulbs are actually large buds with a small stem at the lower end that is surrounded with fleshy leaves. Onions, irises, and tulips are good examples of bulbs and their main function is food storage.
Tubers develop at the tips of stolons. The plant accumulates food at the stolon and the area swells at the internodes. When the tuber is mature the stolon will die and the ‘eyes’ of the potato are actually nodes arranged in a spiral around the modified stem. Each eye has an axillary bud in the axil of a tiny leaf, which is not always visible in maturing tubers.
Runners are horizontal stems that grow above ground, usually along the surface (compare with rhizomes). Strawberry plants produce runners after the first flowering of the season, they may extend out up to 3 feet or more beyond the parent plant. Along the runner, adventitious buds will develop in order to propagate new plants. Stolons are similar to runners, except that they grow roughly vertically beneath the surface of the soil. Irish potato plants have tubers at the tips of stolons.
Rhizomes are horizontal stems that grow beneath the ground, but near the surface of the soil. They resemble roots but are actually modified stems with scale-like leaves and buds at each axillary node. In addition, adventitious roots are produced along the rhizome on the lower surface in order to increase the absorption surface area.