Characteristics of Growth

Arithmetic growth is the slower of the two basic growth patterns a plant organ can follow. In this pattern, after a cell divides only one of the two resulting daughter cells goes on dividing again, while the other stops dividing and instead matures into a differentiated body cell - so each division round adds just one new mature cell to the tissue, not a doubling population (Figure 15.3). Plotting an organ's length against time under this pattern gives a straight (linear) line (Figure 15.4), expressed by the equation Lt = L0 + rt, where Lt is the length at time t, L0 is the starting length, and r is the constant rate of elongation. Because only one new cell is added per round, arithmetic growth can only build very small structures in a realistic timeframe - the textbook's own worked example notes that reaching a typical hair's cell count this way would take on the order of thousands of years if grow …

Geometric growth is the faster, exponential alternative to arithmetic growth, and it is the pattern most higher plant organs actually use during active growth. Here every cell in the dividing tissue keeps dividing mitotically, rather than just one cell per pair as in arithmetic growth, so the total cell number doubles with each successive round - 2 cells become 4, then 8, and so on (Figure 15.5). This kind of growth is expressed by the exponential equation W1 = W0.ert, where W1 is the final size, W0 the starting size, r the relative growth rate (also read as an 'efficiency index' of how well the plant produces new material), t the growth period, and e the base of natural logarithms. Geometric growth is common in young, actively expanding tissue in both plants and animals, but in plants it is largely restricted to embryonic and very …

A whole developing plant is never purely arithmetic or purely geometric in how it grows - it typically starts out as a young embryo growing geometrically throughout its body, then, as development proceeds, active cell division becomes restricted to the meristems at the root and shoot tips while the rest of the plant shifts into the slower arithmetic mode and its cells begin maturing and specialising (Figure 15.6). The result is that any real plant is a mixture of older, differentiated, arithmetically-produced tissue and younger, still-dividing, geometrically-produced tissue concentrated at its growing points. Growth between organs or plants can also be compared in two complementary ways, summarised in Table 1. Absolute growth rate is simply the total increase in size of an organ measured over a given unit of time - it treats a large and a small organ the same if they add the same amount of new tissue. Relative growth rate instead expresses that same increase as a fraction of the organ's own starting size, so a small organ that adds the same absolute amount as …

Plant growth responds to a combination of factors from outside the plant and factors generated within it. Among the external factors, water is essential for cell enlargement (turgor pressure drives expansion) and provides the medium in which growth-related enzymes work; nutrition supplies the macro- and micro-elements, such as carbon and oxygen, that are assimilated during photosynthesis and built into new protoplasm; temperature has an optimum window (roughly 28-30 degrees C) beyond which, above about 45 degrees C, the protoplasm itself is damaged; oxygen is required for the respiration that releases the metabolic energy growth depends on; and light drives photosynthesis and stimulates healthy growth, with its absence causing the yellowing condition called etiolation. Among the internal factors, genes set the underlying genetic programme for growth; phytohormones such as auxin, gibberellin and cytokinin actively regulate growth processes; and the ratio of carbohydrates to nitrogenous compounds in the plant's tissue (the C/N ratio) shapes the overall growth pattern - tissue rich in nitrogenous compounds relat …

Because plant growth typically proceeds too slowly to observe with the naked eye, it needs purpose-built techniques to measure it. The arc auxanometer is a classic mechanical instrument for this (Figure 15.8): a thread is tied from the tip of a growing stem, run over a small pulley, and attached to a hanging weight at its other end; a long pointer fixed to the pulley's axis sweeps across a graduated arc as the pulley turns. As the stem tip elongates and pulls the thread, the pulley rotates and the pointer sweeps a proportionally larger distance across the arc than the actual growth, which is then calculated back using the relationship: actual growth in length = (distance travelled by the pointer x radius of the pulley) / length of the pointer. A simpler, more direct method uses an ordinary scale together with waterproof …

Development is the broad umbrella term for every change an organism undergoes across its entire life cycle, from a germinating seed all the way to senescence, and at the level of an individual cell it proceeds through a defined sequence of states, shown in Figure 15.9. Differentiation is the process by which a generic, undifferentiated meristematic cell matures into a specific cell type built to perform a particular function. Dedifferentiation is essentially a reversal of this: a living, already-differentiated cell that had lost its ability to divide regains that capacity under the right conditions - this is exactly how the interfascicular and vascular cambia arise from mature tissue. Redifferentiation then closes the loop: cells produced by dedifferentiation, after a further round of dividing, once again lose the capacity to divide and mature into a new specialised cell type performing a specific function, such as secondary xylem or secondary phloem. Finally, plasticity is the plant's broader capacity to follow different developmental pathways depending on the environment or its own life stage, producing structurally different organs from the same genetic programme - the classic example is he …