In the natural world, plants reproduce both sexually and asexually. For example, new plants may arise vegetatively from the root system of a plant while the same plant is producing flowers, pollinating, and setting fruit and seed. Many plant species reproduce asexually in assorted ways, each a variation of the basic mitosis process. Although the result is the same—that is, continuation of the genetic complex in the new plants— some of the reproductive methods are faster or in other ways preferable. The method of propagation selected by a grower may be based on:
• ease of propagation
• number of plants needed
• rate of growth of the species
• characteristics desired in the new plant (when grafting of two species is involved)
• desire to avoid disease present in the parent plant
• desire to perpetuate a mutation developing in a parent plant
Sexual propagation utilizes seeds. Some seeds germinate immediately after planting. Some require light to germinate; others prefer darkness. Some seeds require pregermination treatment, such as scarification or stratification, and in some cases, both. Scarification is the breaking of a seed coat otherwise impervious to water to permit water uptake by the embryo. Stratification is the exposure of the seeds to low temperatures. The cold period is believed to initiate the formation of growth regulators or the destruction of growth inhibitors necessary before the seed can germinate. In nature, scarification may be accomplished by soil microorganisms or passage through the digestive tract of an animal. Stratification occurs naturally during the winter. The horticulturist may simulate and accelerate the activities of nature by treating the hard seed coats of certain plants with acid or by mechanically scratching their seed coats to promote water uptake. Bulk quantities of dormant seeds may be placed under refrigeration to hasten their stratification.