Some Learning Goals

Contrast growth, differentiation, and development and distinguish among nutrients, vitamins, and plant hormones. Identify the types of plant hormones and describe the major functions of each; discuss commercial applications for each. Distinguish among the various types of plant movements and identify the forces behind them.

4. Define photoperiodism and make distinctions among short-day, long-day, intermediate-day, and day-neutral plants.

5. Explain what phytochrome is and how it functions.

6. Summarize or outline the role of temperature in plant growth.

7. Describe dormancy and stratification and give examples.

rom time to time, I grow a few vegetables and

F berries in my back yard. When Italian squash plants are producing fruit, I have occasionally seen a young squash just beginning to develop, and I have made a mental note to harvest it within a day or two. Then, other matters have distracted me, and I have forgotten to follow through, only to discover a few days later that my squash has grown into an enormous "monster" the size of a watermelon. I then have wondered, How did it grow that big that fast? The words grow, growing, or growth are used in several ways. If, for example, you see a rubber balloon being inflated with gas, you may refer to its gradual increase in diameter as its growth in size. Or if there is a leak in the roof, you might say that the puddle beneath is growing bigger. In the biological realm, however, growth is always associated with cells. It may be defined simply as an irreversible increase in mass due to the division and enlargement of cells, and may be applied to an organism as a whole or to any of its parts.

Many plants, such as radishes and pumpkins, go through a sequence of growth stages. They grow rapidly at first, then for a while they show little, if any, increase in volume, and eventually, they stop growing completely. Finally, tissues break down, and the plant dies. Such growth is said to be determinate. Parts of other plants may exhibit indeterminate growth and continue to be active for several to many years.

All living organisms begin as a single cell and increase in mass through cell division and enlargement until a body consisting of possibly billions of cells is formed. Once cells have enlarged, differentiation occurs; that is, the cells develop different forms adapted to specific functions, such as conduction, support, or secretion of special substances. If differentiation did not occur, the result would be a shapeless blob with no distinct tissues and little, if any, coordination. The coordination of growth and differentiation of a single cell into tissues, organs, and the whole organism is called development.

What regulates development? What determines where and when growth and differentiation occur? The answers to these questions lie in the organism's genes and its environment. In the 1950s, F. C. Steward and his colleagues at Cornell University illustrated this concept with a tissue culture experiment. They isolated parenchyma cells from a carrot root and placed them on nutrient media. The cells divided, grew, and developed into embryo-like structures. The embryos eventually grew into new carrot plants, demonstrating that a cell has all the genes necessary to form a new individual. Genes, the basic units of heredity, control the synthesis and development of enzymes, which catalyze every metabolic step within cells. Certain genes are programmed to synthesize specific enzymes continually in every living cell, while other genes catalyze the production of enzymes only in specific tissues or at specific times during a plant's life cycle. The environment determines which of those genes will be expressed. In an organ or an organism, the environment includes both external and internal factors. The internal environment includes nutrients, vitamins, and hormones. The external environment includes water, minerals, gases, light, and temperature, among many factors.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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  • Marko Sinervo
    What is define as an irreversible increase in mass due to the division and enlargement of cells?
    8 years ago

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