Plant Structure and Function
Who showed that plant cells were totipotent?
Short-day and long-day plants exhibit a response to photoperiodism, or the changes in light and dark in a 24-hour cycle. Short-day plants form flowers when the days become shorter than a critical length, while long-day plants form flowers when the days become longer than a critical length. Short-day plants bloom in late summer or autumn in middle latitudes. Examples of short-day plants are chrysanthemums, goldenrods, poinsettias, soybeans, and ragweed. Long-day plants bloom in spring and early summer. Some examples of long-day plants are clover, irises, and hollyhocks. Florists and commercial plant growers can adjust the amount of light a plant receives to force it to bloom out of season.
In 1958 Frederick Campion Steward (1904–1993), a botanist at Cornell University, successfully regenerated an entire carrot plant from a tiny piece of phloem. Small pieces of tissue from carrots were grown in a nutrient broth. Cells that broke free from the fragments dedifferentiated, meaning that they reverted to unspecialized cells. However, as these unspecialized cells grew, they divided and redifferentiated back into specialized cell types. Eventually, cell division and redifferentiation produced entire new plants. Each unspecialized cell from the nutrient broth expressed its genetic potential to make all the other cell types in a plant.
Why was Steward successful? Like previous investigators, he supplied the cultured cells with sugars, minerals, and vitamins. In addition, he also added a new ingredient: coconut milk. Coconut milk contains, among other things, a substance that induces cell division. Subsequent research identified this material as cytokinins, a group of plant hormones (growth regulators) that stimulate cell division. Once the cultured cells began dividing, they were transplanted on agar media, where they formed roots and shoots and developed into plants.