We sometimes see plant roots growing from stems or branches (rather than from seeds), or small, leafy shoots appearing on tree trunks far from the leafy canopy. The term used for that in biology is “adventitious growth.” The word adventitious is used in many ways and has many proposed synonyms (reflecting the varied usage), but I’ve not found one that applies very well to the biological situation. There it might be described as tissues or structures that emerge from (initially) unexpected parts of an organism—not in the main place for such tissues or structures to develop. Nevertheless, in some cases it is a regular occurrence.
Consider, first, what causes adventitious growth. Some kind of stimulus (often environmental) wakes up some cells, which start to divide, making new tissue. In some cases, undifferentiated cells (not committed to making a particular kind of tissue) are lying dormant and would remain so without that stimulus. In other cases, the stimulus causes already-differentiated cells to de-differentiate and then re-differentiate to make a new kind of tissue. Quite amazing! The nature of the stimulus is varied—maybe a stress, such as too much water or a wound or pressure, or an infection, or perhaps some unexpected input of resources.
Sometimes the extra growth can be useful to the organism, sometimes it has no evident significance (beyond the cost of making it), and sometimes it can be harmful.
Adventitious roots are common in many kinds of plants. Trunks and branches of fig trees and mangroves send down aerial roots that eventually reach the soil and serve the normal root functions of taking up water and nutrients. Corn plants commonly develop numerous adventitious roots in addition to the main root system (which comes from the seed); they grow from the lower stem and eventually provide the normal root functions and also brace the upright plant.
This photo available under a Creative Commons license shows the roots of a fig tree. Trunks and branches of fig trees and mangroves send down aerial roots that eventually reach the soil and serve the normal root functions of taking up water and nutrients. (Bernard Dupont / Flickr)
Strawberry plants send out above-ground stems called stolons (runners), and roots can develop when nodes on the stolons contact the ground; a new plant may grow there—it’s a form of vegetative propagation and part of the way strawberry plants colonize new ground. In plants such as these, adventitious roots are common, regular features that contribute to the plant’s survival and reproduction.
So-called “sucker shoots” often develop on trees that have been damaged in some way. They come from cells that were just lying dormant under the bark. But if lots of tree branches are torn or cut off, the dormant cells wake up and build new shoots that eventually produce leaves. Contrary to the name, these shoots don’t parasitically suck out nutrients; ultimately they help replace the lost photosynthetic capacity of the original branches. Kalanchoe plants, mostly native to Madagascar and tropical Africa but often cultivated, produce many tiny plantlets at the edges of the leaves; the tiny plantlets drop off and can establish themselves in the soil—a form of vegetative reproduction . Horticulturists can propagate begonias (and some other plants) from cuttings and leaf fragments, applying nutrients and certain chemicals that induce the cuttings and fragments to produce new little plantlets from previously dormant cells.
There are also cases in which adventitious growth is harmful to a plant. Crown galls are tumors or cancers (terminology depending on the researchers) that form in response to certain microorganisms or fungi. They disrupt the flow of water and nutrients, weakening and stunting growth of stems and roots. They are seldom lethal, however, because their growth circumscribed and limited.
However, in animals, it’s a different situation. Cancers are often damaging and sometimes lethal. Many kinds of factors contribute to cancer, damaging the genes that control cell division.
So the cells with damaged DNA divide uncontrollably; in some cases they are impervious to the immune system that would normally eliminate them, they may attract blood vessels that supply nutrients and remove metabolic wastes, they may even make their own nutrients in a novel way. The uncontrolled growth creates problems with normal functioning; and the cancer spread to other parts of a body, creating more problems there.
Some other examples: Skin moles are little lumps that form on human skin (other animals, too?); the causes are variable and little known. They come in various colors and shapes and can be unsightly, but they are not usually harmful (although some rarely turn cancerous). Bone spurs can develop in vertebrate joints when cushioning cartilage is worn down and adjacent bones put unusual pressure on each other, causing bone cells to grow into the joint. (It’s normal for bone cells to grow under pressures; that’s why staying active helps keep bones strong; so when spurs develop, the cells are doing the normal thing but in the wrong place). But the bone spurs can interfere with joint movements.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
