Regulation of Plant Growth

HORMONES:  Plants have five major hormones that are relatively well understood. Each has numerous effects on the plant. Your text summarizes these effects. More extensive treatments of all five hormones is found at McDaniel College (Links to an external site.) and at the Colorado State (Links to an external site.) site. One of the effects of two hormones interacting is the phenomenon of apical dominance. In this situation, an apical meristem appears to inhibit growth of meristems imediately below it. The farther away a meristem is from the apical meristem, the less it is inhibited. This action leads to the typical plant shape--narrower at the top and increasingly broad toward the bottom (think of the "perfect" christmas tree.) We also manipulate hormones to deliberately control aspects of plant growth. Examples are solutions put on roots as new plants are planted and the use of ethylene to control ripening. A commercial site about ethylene (Links to an external site.) shows some of the applications.

TROPISMS:  Tropisms are growth responses of plants to environmental stimuli. There are three basic tropisms:  gravitropism (sometimes called geotropism, it is a response to gravity), phototropism (response to light), and thigmotropism(response to physical contact). The McDaniel College (Links to an external site.) site covers these topics very well. These sites have good information on all three tropisms:  Gravitropism page (Links to an external site.), phototropism page (Links to an external site.) (and gravitropism), and thigmotropism page (Links to an external site.). There are also some excellent movies of plant motions.

FLOWERING:  Flowering in plants is also a response to external cues, primarily day/night length. this response is called photoperiodism. Plants contain light sensitive pigments that change form depending on the wavelength of light to which they are exposed. Some plant flower only when days are getting shorter (nights longer); they flower in the Fall. Others flower only as days are getting longer (nights shorter); they flower in the Spring and summer. There are plants that do not respond to these cues at all; they are day-neutral and flower anytime. Dandelions are a good example of day-neutral plants.

The McDaniel College (Links to an external site.) site covers this topic at the end of the same page that covered tropisms. This site also has information on flowering along with links to other plant physiology topics.  Lastly, there is an excerpt from the book The Rose's Kiss (Links to an external site.) that has a well-written discussion of flowering written for the layperson.