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Plant Growth & Form; Cell Elongation & Division Part I: Overall plant growth by cell elongation
Part II: Overall plant shape by cell division
Under construction: How was all this discovered?
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Dr. NAD's Mini-course |
An example: Growth, not bending
Note importantly that the plant does NOT bend, per se; the new direction resulted from growth. Unlike bending/straightening an elbow, growth is not reversible; if this plant were rotated, its new growth would adjust accordingly, but the old growth would not be altered.
This is an example of "negative gravitropism"; i.e., growth away from the force of gravity. Guess what the name for the type of gravitropism shown by roots is called.
Plants exhibit other types of tropisms, e.g., towards light, called "positive phototropism."
Watch a plant move in these sped-up videos. More plant tropism images. More on tropisms. Large collection of botany course pictures |
Plant parts do not move as animal parts do. Very unlike lowering and extending our arms, a root grows downward. Have you seen time-lapse movies of plants that were laid horizontally and appeared to turn upwards eventually? See the feature at the left. That bending, very unlike bending an elbow, occurs through growth. Cells on the bottom side of the horizontally placed stem grew more than those on the top side. Imagine a ribbed hose, like on a drying machine vent or a vacuum cleaner. When bent, the inter-rib spacing is reduced on the curve's inside and widened on the outside. A plant has no musculo-skeletal system; instead, the stem creates more "hose mass" on the bottom side, forcing an upward bend.
The cells of plants are also substantially different from animal cells. Imagine an animal cell as a water balloon. Its shape depends on an internal scaffolding, termed the cytoskeleton (cyto,"cell"). The balloon is stretched over the support, much as your body's skeleton determines its shape. By re-structuring the scaffold, the balloon can assume new shapes. The balloon actually can move by disassembling the scaffold at its trailing edge while assembling it anew at its leading edge (much as a hamster running in its ball). As an animal embryo develops, its cells move to appropriate positions (explanatory diagram and actual microscope movies e.g.; e.g.). This is substantially different than the situation in plant cells. "Cytoskeleton" and "scaffolding" are misnomers for a highly dynamic, but stably organized, structure. Generally, cells within a plant do not move. They are packed amidst adjacent cells to which they are also glued, unable to move like a hamster in a ball, and without space to roam. In addition, they have cell walls. Imagine a water balloon expanded against the walls of a rigid box. The shape of the cell (the balloon) is determined by its enveloping wall (box). So, do plant cells have internal scaffolding (a cytoskeleton)? Yes. Like animal cells, it is inside the balloon. But the balloon is now inside the box. Does the scaffold, as in animal cells, affect the balloon's shape? Yes, but indirectly, by affecting the shape of the box outside the balloon. Huh? The cell wall is not entirely like a box. It actually consists of cross-linked, more or less parallel fibers (perhaps more like corrugated ridges in cardboard). Here's the prevailing paradigm: The fiber material is synthesized by machines embedded, but floating, in the balloon membrane (which is more fluid than the analogy allows). As each machine floats along, it leaves one fiber in its wake on the balloon's external face. To lay down a large wall segment requires an orchestrated flotilla of machines. The coordinated paths of the machines is believed to be determined by the scaffolding near the balloon's internal surface. Generally, the pattern of recently deposited fibers outside the cell membrane reflects the predominant order of the scaffolding inside the cell. This is quite different than how an animal cell's internal scaffold determines cellular shape. How the plant arrangement affects cell shape and growth is better illustrated next. How might a root grow, elongating more deeply into soil? Cylindrical root, cylindrical cells. How did they elongate from newborn spherical cells? Imagine holding a water balloon in your hand as you fill it. Even if the balloon is round and not cigar-shaped, it expands at the ends where your hand is not constricting it. Suppose a newborn, round cell within a vertically growing root is elongating vertically. Where is your hand? The fibers of the wall would be oriented...horizontally. The rigid wall restricts horizontal expansion, limiting elongation to the ends of the cell, up and down. Imagine the fibers as ribs in a ribbed hose. As the hose is stretched, the inter-rib space widens, so more fibers must be deposited to fill in. Fiber-making machines don't deposit fibers randomly, so their swimming must be directed. The scaffold is a likely candidate--it is internal, so perhaps more readily subject to the cell's control. Some of the internal scaffold is very close to, and probably linked to, the inside surface of the (very thin) membrane in which the machines swim (explanatory graphic). The swimming paths of the machines may be restricted in a way analogous to how swimmers in a race are restricted by lane ropes. But in the case of the plant cell, the restrictive entity must extend through the membrane from internal scaffold to external machine. Perhaps legs of the machine extend deeply enough into the membrane to interact with scaffold elements that extend outward? It remains to be better understood.
The take-home message
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At left:
Boxes represent cells. Teaser: How would you suspect the lines look in a round plant organ like an orange? |
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