Some scientists and others blame patenting for what they say are losses of crop diversity. But the study did not find that plant patents influenced the number of newly created plants. The researchers said they did not make a major change in the number.

A patent is the legal right of an inventor to a process or a product, even a plant. To get this right, the inventor has to demonstrate the qualities that make the patented plant different from other plants. The inventor must show that the plant has been reproduced asexually. This usually means that the plant is cut or its tissues have been connected to another plant.

MARIO RITTER: Asexual reproduction proves that the inventor can reproduce the plant. Grafting is a way to produce plants from parts of existing plants instead of seeds, which cannot be used. Branches or buds are cut from one plant and placed on another plant.

American patent law says a new version of plant also can be discovered, but only on land used for growing. Wild plants in nature cannot be patented. They do not belong to any one owner.

CHRISTOPHER CRUISE: The two researchers looked at three dates when laws protecting the rights of patent holders took effect. For example, seeds were first patented in nineteen seventy. The researchers say that, as they examined the records, they kept waiting for the number of new plant varieties to drop. But that did not happen. Instead, Mr. Heald says, they had a surprise.