The tallest trees in the world are the California coast redwood trees (Sequoia sempervirens), which can grow to over 100 meters (328 feet) tall.
That’s higher than a 30-story building.
These huge trees can grow so big mainly because their growing cells can enlarge 100 to 1,000 times their initial size.
Indeed, “…the size and shape of a plant are to a large extent determined by the amount and direction of cell enlargement. For example, we calculate that if the cells in a redwood tree behaved like typical liver cells and only enlarged to 20 μm [micrometre], the redwood would have a maximum height of less than 2 feet [0.6 meter].” (From Ref. 1 below)
To think that the giant tree in the picture above likely consists of a fewer number of individual cells than the person hugging it is amazing.
This is a fairly dramatic example of the fundamental difference between plants and animals in how they grow.
Basically, animals develop and grow primarily by making new cells, that is, through cell division (mitosis). In other words, animals grow mainly by increasing the NUMBER of cells. It’s like adding more and more bricks to construct a building.
Unlike animal development, however, the growth and development of plants is essentially determined by the increase in VOLUME of plant cells. It’s like increasing the size of a building through enlarging the size of the bricks already in place, rather than by adding lots more bricks.
Reasons for this difference between plant and animal growth have to do with the two basic structural features that plant cells have but that animals cells, in general, do not: (1) a cell wall and (2) a large central vacuole.
Plant Cell = “Water Balloon Inside a Cardboard Box”
In a previous life, when I was teaching (boring?) undergraduates about plant cells, I’d sometimes use the analogy that a plant cell was sort of like “a pressurized water balloon inside a cardboard shoebox”.
I used the “water balloon” analogy because, unlike animal cells, most of the volume (up to 90%) of a plant cell doesn’t consist of protein-rich cytoplasm, but, instead, a water-filled vacuole. (Please note: the vacuole isn’t filled with pure water. It contains some organic and inorganic substances, and even some enzymes.) If the cytoplasm is analogous to liquidy, raw egg whites, then the vacuole solution is somewhat analogous to sea water.
The fact that plant cells are largely filled with a dilute salt solution allows them to, metabolically-speaking, get away with growing very, very large, compared to animal cells.
Also, please keep in mind that our plant cell “water balloon” is “pressurized”. This is because of the turgor pressure of plant cells, generated by the osmotic uptake of water coupled with the fact that plant cells are surrounded by a “cardboard box”, namely the plant cell wall.
Think about pumping up a bike tire. As you pump air into the bike inner tube (water osmotically drawn into the plant cell) the tire pressure (turgor pressure) increases due to the rigid bike tire (rigid cell wall) that encases the inner tube.
But if the plant cell wall is a rigid structure, like a cardboard box, then how are plant cells able to enlarge so much?
Solving the Enigma of Plant Cell Enlargement
The exact mechanisms involved in plant cell growth have perplexed plant scientists for well over fifty years.
Indeed, it was nearly fifty years ago that the so-called “acid growth” theory of auxin-induced plant cell elongation was first proposed and developed.
Though the acid growth theory is largely accepted (see, e.g., Ref. 2 below), it is still being refined, and the exact mechanisms have yet to be solved.
What prompted this post were two recently-published updates on the growth of plant cell walls (Ref. 3 below) and on acid growth (Ref. 4 below).
Next Time: More about how plant cells grow.
1. Rayle, D. L. and R. Cleland (1977) “Control of plant cell enlargement by hydrogen ions.” Current Topics in Developmental Biology, Vol. 11, pp. 187-214. https://doi.org/10.1016/S0070-2153(08)60746-2 (Abstract)
2. Rayle, D. L and R. E. Cleland (1992) “The acid growth theory of auxin-induced plant cell elongation is alive and well.” Plant Physiology, Vol. 99, pp. 1271-1274. (Full Text)
3. Cosgrove, D. G (2018) “Diffuse growth of plant cell walls.” Plant Physiology, Vol. 176, pp. 16-27. https://doi.org/10.1104/pp.17.01541 (Full Text)
4. Arsuffi, G. And S. A. Braybrook (2018) “Acid growth: an ongoing trip.” Journal of Experimental Botany, Vol. 69, pp. 137–146. https://doi.org/10.1093/jxb/erx390 (Abstract)
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