Plants grow and develop in several ways, but they all start with the meristem cells that we talked about earlier in the series. As a refresher, meristematic cells are undifferentiated, meaning that they don’t have a specific job assigned to them when they first form. That means that when meristem cells divide, they can produce daughter cells of any kind that can go on to do whichever jobs the plant needs them to do. Apical meristems are those cells at the apex or tip of different parts of the plant. For instance, there are apical meristem cells at the tips of each of the roots and at the tips of each of the branches and the stem. These apical meristems are responsible for what is known as primary growth in a plant. When a plant is exhibiting primary growth, it is building more cells to make it taller or longer.
So primary growth makes the grass in your lawn grow higher, it makes trees grow taller, it makes roots grow deeper, and it makes vines grow longer. Many plants can also grow in another way, called secondary growth. In secondary growth, the plant gets broader or thicker due to the lateral or sideways activity of meristem cells. These meristematic cells exist in the cambium, a special layer in between the xylem and the phloem in a plant’s stem. When lateral meristem cells divide, they create more girth for the plant, so the stem increases in diameter.
The best example of secondary growth is the annual growth rings of a tree trunk. If you look at the rings, you can see each growing season represented by a broad band of expansion in the tree’s trunk. This is secondary growth. So how does a plant know when to start growing, when to switch modes of growth, or even which way is up so that it grows out of the soil into the sunlight? Just like animals, plants have a series of hormones that trigger and regulate their growth. As we learned in studying human physiology, hormones are chemicals that affect processes occurring inside a living organism. It is this similarity in function, rather than their chemical structure, that causes hormones to be grouped together. The most important plant growth hormone is auxin.
Auxin is responsible for most of the primary growth in a plant, including the lengthening and differentiation of cells from the meristem. Auxin is also responsible for much of a plant’s “sense of direction.” Gravitropism, also known as geotropism, is growth in relation to gravity. The distribution of auxin within a plant’s cells changes with the relationship between the direction of growth and gravity, helping the plant to grow correctly. Roots demonstrate positive geotropism, or growth toward the pull of gravity, while stems experience negative geotropism, growing away from the direction of gravitational pull. If you tip a potted plant on its side, and a few days later the stem has become bent and is growing straight up again, that’s auxin at work. Auxin is also responsible for phototropism, or a plant’s ability to sense and grow towards or away from light.
If you notice the houseplants on your windowsill appearing to bend towards the sunlight coming through the window, that’s also the work of auxin. The next important plant growth hormone is cytokinin, named for its role in regulating cytokinesis, which is a process that occurs at the end of cell division. We covered cell division at length in the biology series, so visit those tutorials now if it sounds unfamiliar. Cytokinin is responsible for stimulating mitosis for growth, regulating how cells differentiate from the meristem, and how quickly or slowly plant tissues senesce, or age. Another group of plant hormones, the gibberellins, is primarily responsible for the reproductive parts of a plant. Production and distribution of gibberellins is what causes flowers to mature, stimulates pollinated flowers to turn into fruits, and causes seeds to mature and be ready for planting.
Scientists have discovered that preventing the normal activities of gibberellin hormones can cause fruits to be “seedless,” so that’s how we get things like seedless grapes. We can also use a spray of gibberellin hormones on crops to cause their fruits to grow bigger than they would in the wild. Next, abscisic acid can be thought of as a plant stress hormone, because it accumulates when plants are exposed to stressful conditions like a lack of water, cold air temperatures, or shorter amounts of sunlight each day, also called a photoperiod.
When a plant or seed begins to experience these conditions, abscisic acid puts the brakes on a lot of the plant’s growth functions in order to conserve its resources. Abscisic acid is important for signaling tree branches to stop growing in fall and winter, and the same hormone causes seeds to go dormant in the soil until spring, when warm temperatures return. Like gibberellins, ethylene, the last important plant growth hormone, is responsible for regulating flowers and fruits. What makes ethylene different from the other hormones we’ve covered is that it’s actually a volatile gas released by maturing and senescing parts of a plant, but the gas also stimulates other nearby flowers and fruits to mature and age. If you’ve ever used the trick of putting ripe and unripe fruit together in a paper bag to help the unripe fruit ripen faster, then you’ve used ethylene.
Commercially, farmers spray ethylene on fruit crops so that all of the fruits ripen at approximately the same time, so as to make the harvesting process more efficient. We should also note that many of these plant growth hormones can also be used as herbicides, or chemicals that target and kill plants, especially undesirable plants like weeds. Messing with the balance of a plant’s hormones can cause a weed plant to grow too fast or too slow, or cause it to not produce seeds, any of which could seriously damage or kill the plant.