Growth Sites and the Timing of Growth in Plants

Plant Growth is Localized and Precisely Timed

Jul 8, 2009

Unlike animal bodies in which all parts grow proportionately, growth in plants occurs only in specialized regions known as meristems.

Meristems are classified according to their position in the plant. Based on location, there are two main categories of meristems: apical meristems and lateral meristems. Apical meristems are found at the tips of the shoot and roots while lateral meristems lie along the sides of stems and roots.

Cells in the apical meristem of the shoot grow and develop into more stem cells, leaf buds, and flower buds while the apical meristem in roots produces more root cells, lateral roots, and root hairs. Lateral meristems are found primarily in woody plants. The cells produced in lateral meristems cause woody plants, such as trees and shrubs, to increase in diameter year after year.

Growth Rate in Plants Varies by Species

There is great variability among different plant species in the rate at which stems grow. This is not due to different rates of growth in individual cells, most of which tend to double their size in a day, but to the length of the section of stem tip that is actually involved in the growth process.

Among slow growing plants, only about a quarter of an inch at the end of the stem does the growing. But among the fastest-growing stems, such as bamboo shoots, as much as two feet of the end of the stem tip may be involved in the growth process. This enables bamboo to reach their full size of 100 feet in only a few months, whereas 100 feet of growth in a tree might take several decades. The difference is not due to a faster growth rate of individual cells in the bamboo, but to the vastly greater number of cells that are all stretching at the same time. A combination of cell division and cells stretching all along this two-root growth zone can produce as much as a foot of new stem a day in bamboo.

Also, while the shoots of most trees grow for only about a month each year, bamboo shoots grow without interruption, a prodigious feat requiring enormous amounts of food. This the bamboo stores in a huge root system, replenishing it with sugar produced through photosynthesis in all the leafy shoots.

Trees continue to grow taller and broader year after year because of their apical and lateral meristems. Animals lack such localized meristematic areas. Therefore, animal growth (including humans) stops after a finite period of time. However, some animals, such as elephants, continue to grow as long as they live. It is easy to pick out the oldest elephants in a herd because the oldest are also the largest elephants in that herd.

Plant Growth is Precisely Timed

Growth in any category of creature must be carefully regulated and controlled by internal factors; it never happens chaotically. These internal factors regulate not only the development of specific structures but also the timing with which these structures develop.

Every once in a long while – 33 to 66 years (every one or two human generations) – bamboo plants commit “suicide.” For reasons we do not fully understand, the giant bamboos burst into bloom, sending up enormous flowering shoot which take the place of leafy shoots. These flowering shoots use up all the plant’s reserve of food, and the inevitable result of this spendthrift flowering is the death of the plant.

Another curious aspect of this phenomenon is that all bamboos of the same species flower at exactly the same time in the same year, regardless of where they are growing. A Japanese bamboo transplanted to Hollywood will bloom at the same time as its brothers and sisters back home. The Japanese cherry trees along the tidal basin in Washington all flower simultaneously. In the same way, all the beech trees in a town leaf out on the same day.

Obviously, the trees in one part of town or on different continents do not know what the others are doing. Nor do they live in exactly the same environments but all are synchronized when the moment comes. The inescapable conclusion is that plants must contain some sort of remarkable internal timing mechanism. The degree of coordination of these timing signals has been compared to that displayed by an organist using all 10 fingers and both feet to play a complicated musical piece.

The copyright of the article Growth Sites and the Timing of Growth in Plants in Botany is owned by Dennis Holley. Permission to republish Growth Sites and the Timing of Growth in Plants in print or online must be granted by the author in writing.