Topics in Depth
The Theme of Germination and Development in Plant Biology
Now we know how seeds develop, and how they get from place to place. But how does a seed, which can lie dormant in the soil for weeks, months, or years, develop into a plant?
Seeds have to take in water to germinate. Drinking in water causes the seed to expand and burst its seed coat (imagine all the buttons popping off!). Inside the seed, hormonal changes cause the endosperm to break down and let the cotyledons absorb nutrients. The first thing to emerge from the seed is the embryonic root, called a radicle. Next the shoot starts to emerge, and has to push its way up through the soil to the surface where it can start getting light. This is done differently in eudicots and monocots.
In eudicots, the shoot pushes up through the soil in a bent-over, U-shaped position. The area just below the cotyledons is called the hypocotyl, and the stem area above the cotyledons is called the epicotyl. The hypocotyl acts as a battering ram for the shoot. By emerging this way, the fragile apical meristem is protected from abrasive particles in the soil.
Aboveground, the stem straightens out, the two cotyledons unfold, and the epicotyl regains its place at the top of the stem. These two first leaves are "seed leaves" and not "true leaves," since they existed in the seed. The cotyledons will nourish the seedling for a short time, but will shrivel and die quickly once true leaves have formed.
Monocots have a different way of doing things. Rather than make a battering ram out of a hypocotyl, monocots use a tunnel to get to the surface. Monocot seeds don’t come with shovels; they use the coleoptile, a protective sheath covering the embryo, for their transportation needs. The coleoptile pushes up to the surface and the shoot grows straight up through the tip of the coleoptile.
Another difference between eudicots and monocots is the way their roots grow. Eudicots usually grow a main root, called a taproot, that grows deep and other roots branch sideways from. Monocots often just have a shallow fibrous root system but no main root.
Acorus calamus is a monocot, whose roots look like this:
In this picture of, you can clearly see the taproot:
Once a seedling gets its roots going and grows some new leaves, it is pretty much established. However, it’s not ready to start a family yet; plants have juvenile and adult stages just like humans, though plants get to skip those awkward middle school years. Plant development is dictated by the all-important shoot apical meristem.
At some point determined by environmental conditions and hormones internal to the plant, the juvenile plant will become an adult. When the plant passes from its juvenile phase to its adult phase, the shoot apical meristem starts producing adult leaves that do boring adult things like balance their checkbooks and listen to talk radio. The shift to adulthood only affects new growth on the plant—leaves that already exist and were made during the juvenile phase retain their youthful look.
Two important factors for flowering, photoperiod and hormones, are discussed in their own sections. However, there are also genetic controls on flowers. When the plant becomes an adult, the apical meristem converts into a floral meristem. Somehow the apical meristem needs to develop all the flower parts. Good thing there’s an instruction booklet.
The instruction booklet for floral development is DNA, which also gives directions for how to make everything else in the plant. First, a stimulus called florigen moves up
the vascular tissue to the shoot apical meristem. This causes certain genes to be expressed.
The general idea for how genes control flowers is called the ABC model. In the ABC model, there are three switches that can be turned on in a cell to dictate what floral part develops there. The switches are really groups of genes, and gene expression is equivalent to flipping the switch on.
The A group by itself causes sepals to develop. The C group by itself causes carpels to form. When A and B are both turned on, petals form, and when B and C work together, stamens develop. Scientists have figured out what these genes do by studying mutant plants that are can’t express one of the genes. Unfortunately, the mutant plants don’t have superpowers and won’t be joining the X-Men anytime soon, but they have at least helped us figure out how flowers form.
A = sepals
A + B = petals
B + C = stamens
C = carpels
Bean sprouts, which are just seeds that are germinating, are a health nut’s dream. Learn how to make your own here.
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