Unlike people, plants have a special ability to grow continuously throughout their lives. Most will continue making new roots, stems, and leaves until they die.
The continuous growth of plants takes place thanks to meristems, perpetually embryonic plant tissues made up of specialized cells called meristematic cells. Often compared to the stem cells found in animals, meristematic cells can continuously replicate themselves and are undifferentiated, meaning that they have the potential to turn into all different types of plant tissues.
There are several different types of meristems: Some make stems and leaves, some make new roots, some help the plant’s stems grow outward, and some do all three. Meristems aren’t distributed equally around a plant; instead, they’re located in small, distinct compartments, often at the tips of shoots and roots.
Small, densely packed cells (stained red in this microscopic image) are new shoots emerging from the meristems of a coleus, a popular houseplant. Src: BlueRidgeKitties on Flickr
The location of any given meristem determines the type of tissue it can become. For example, you probably noticed new roots sprouting from the meristems found in the bottoms of your root vegetables, as well as the bottoms of lettuce, bok choy, and celery heads, and the stems of mint or basil. Meristems capable of making leaves are the source of any leafy growth you noticed in the tops of your root vegetables, or at the tops of your severed celery and lettuce heads.
Some vegetable parts probably didn’t show any new growth. Lettuce and herb leaves separated from their heads or stems don’t contain any meristems capable of sprouting new stems or roots.
At some point, despite the presence of meristematic cells, your vegetables will probably stop growing. While you may have supplied them with sunlight, water, and air, the main things that plants need for growth, they will eventually run out of other essential nutrients, such as nitrogen, phosphorous, and potassium. As anyone who grows vegetables hydroponically (that is, in water without soil) can tell you, these essential nutrients must be added to the water for robust growth.
To keep the growing going, take the vegetables that have sprouted roots (such as herbs, celery, or potatoes) and plant them in soil outside for eventual harvest.