W. Everett Carr, Mt. Vernon writes: I am interested in propagating plants and in plant breeding. Can you suggest some book titles for a beginner?

A. The Brooklyn Botanic Garden, 1000 Washington Ave., Brooklyn, N.Y. 11225, publishes a number of handbooks on various aspects of horticulture. Titles that would be informative in the areas of your special interest are No. 24, Propagation, and No. 75, Home Plant Breeding. Handbooks can be ordered direct from the Garden at $1.75 each, postpaid.

Wyman's Gardening Encyclopedia, available in the reference section of many public libraries, is one source of information on these topics and in addition includes recommended propagating procedures for many of the individual popular plants described.

Some popular books related to these topics are "Create New Flowers and Plants - Indoors and Out," by John James, Doubleday & Co., 1964; "Plant Propagation in Pictures," by Montague Free, Doubleday & Co., 1957; "Simple, Practical Hybridizing for Beginners," by D. Gourlay Thomas, St. Martin's Press, 1962.

Mrs. Jerry Porter, of Columbia, asks: How does fluoridation of water affect flowers and plants?

A. Fluoride in water moves through the plant in the water transport system. It collects at the ends of the veins of leaves. As the accumulation builds up, plant cells are killed. The effect differs with different species and some species are less tolerant than others.

Browning of leaf tips of Chlorophytum (spider plant) is one example of the effect of fluoride. In some Dracaenas, fluoride causes chlorosis, loss of chlorophyll, and leaves become yellowish. Cankerous lesions of leaft margins are sometimes evidence of fluoride injury.

Plants especially sensitive to fluoride are:

Chlorophytum comosum - spider plant.

Cordyline terminalis - Ti plant.

Dracaena deremensis "Janet Craig" and "Warneckii."

Pteris ferns.

Plants slightly sensitive to fluoride are:

Dracaena fragrans massangeana.

Maranta erythroneura and Maranta kerchoveana - prayer plant.

Spathiphyllum - Peace lily.

Yucca elephantipes - spineless yucca.

To reduce chances of flouride injury, the Extension Service of the University of the District of Columbia recommends the following practices:

Avoid the use of fluoridated water.

Do not use superphosphate as fertilizer in mixes for growing susceptible plants.

Try to use a mix that does not contain perlite when potting sensitive plants. Perlite contains a toxic amount of fluroide.

Add dolomitic limestone or limestone to the soil at time of mixing at the rate of 1 teaspoon for 2 quarts of soil mix.

Provide an environment that will reduce water uptake; for instance, increase the humidity by use of a pebble tray or by grouping plants together.

Factors other than fluoride can cause tip burn and lesions. Brown leaf tips or leaf scorch may be caused by over-watering, under-watering, high soluble salts from too much fertilizer, insects, disease, too much or too little light and low humidity. Review your cultural practices to be sure that one of these factors, or a combination, is not causing the problem.

B. Jean Patton, of Alexandria, writes: I started an avocado tree from a large California avocado over a year ago. I did not prune it when it was young; it isnow a tall stem, fully leafed, 4 1/2 to 5 feet tall, leaning, and top heavy, and still growing toward the ceiling. Can you advise me what to do with it to get it to bush out?

A. The recommended time for pruning, or cutting back, an avocado is when the plant is 5, 6 or 7 inches tall. The only step you can take now is to experiment. Cut back about 18 inches, or as much as your conscience will allow. This operation will put the plant in shock and it may be some time before it resumes its normal urge to become a large tree by putting out branches. Its inclination is to grow straight up and the branches will follow this urge. You can experiment also with weighting or tying the branches as they develop to give a drooping or seeping effect.