Chapter 25: Plant growth—patterns, limitations and models
Chapter 25: Plant growth—patterns, limitations and models
One of the marvels of life is growth, the ability of organisms to get bigger. This is especially the case for plants since, as described earlier, plants grow ‘ out of thin air.’ And while plant growth is a very ordinary phenomenon, its explanation is very much ‘extra-ordinary,’ as they are able to accumulate scattered pieces of their environment and assemble them into an organic entity. In contrast, animals need only to find pre-assembled pieces of life, perhaps still living, perhaps not, and reconfigure them to their own use. And while the controls on animal growth are straightforward, being tied to ingestion, what controls plant growth is less obvious.
When plants grow, they generally get bigger both in size and in weight and the process of growth can be linked to increases in either. However, for a variety of reasons, the growth of plants is defined as an irreversible increase in size, not an increase in weight. Part of the reason for this is because water absorption and loss can change plant (wet) weight substantially because of processes that most would not consider growth. For example, trees gain a considerable amount of water overnight to replenish that lost during the day; most would not consider the overnight weight gain to be growth nor the loss during the day to be ‘negative growth.’ To avoid the dynamics of water, one might monitor growth with increases in ‘dry weight’, a consequence of the accumulation of carbon, nitrogen, phosphorus, etc., and the synthesis of organic molecules such as carbohydrates and proteins. But defining growth by an increase in dry weight would lead to some counterintuitive results. Most would consider that trees grow in the spring when the shoots elongate and leaves appear. At this time the tree is actually decreasing in dry weight. During the summer in temperate areas, as trees photosynthesize and absorb nutrients, their dry weight increases, yet many are not getting bigger in terms of longer shoots. Similarly, a sprouting seed, which most would consider to be growing, is actually decreasing in dry weight until its photosynthetic rate exceeds its respiration rate; this generally doesn’t happen until the seedling is a couple of weeks old and already of substantial size. Consequently, plant growth is typically defined as an irreversible increase in size.
TOPICS
- Growth processes
- Limitations on plant growth
- Growth models
Growth Processes
Increases in the size of plants come about as individual cells, produced by cell divisions in the meristems, expand. While this may seem at first a simple phenomenon, consider the following aspects of the process:
- All plant cells, even small ones, are surrounded by a confining cell wall, whose most basic function is to prevent expansion. This allows the cell to pressurize and this is important to plant cell water balance and functioning.
- As the cell expands the thickness of the cell membrane and the cell wall ou