The reproduction of plants through the formation of seeds is called sexual reproduction. It requires the fusion of two sex cells or gametes, each having one set of chromosomes (haploidal), to form a cell with two sets of chromosomes (diploidal) that is termed a zygote. The gametes are formed through a sequence of cell divisions that reduces the number of chromosomes in each cell by half. The process is termed meiosis (Figure 5-1).
The creation of a diploidal cell occurs at the time of fertilization. Through sexual reproduction, the genetic material of the cell recombines in a way that can produce new plants, differing in physical and physiological characteristics from their parent plants. Such differences as stronger stems, larger flowers, and greater winter tolerance may appear. In other instances, sexual reproduction results in new plants identical in nearly every way to their parent plants. The degree of dif-
ference is determined largely by the characteristics of the homologous chromosomes (those that associate in pairs during meiosis). When plants self-pollinate, the pairs of genes on the homologous chromosomes are basically alike (homozygous), and the plants that are produced are nearly exact duplicates. When plants cross-pollinate, the pairs of genes on the homologous chromosomes are often dissimilar. The plants that result are not exactly like either parent plant.
When an exact duplicate of a plant is desired, especially if it is not self – pollinating, asexual propagation methods must be used.
Asexual reproduction is a vegetative process that eliminates genetic variation. The result is the multiplication of plants possessing the same genetic complex. Asexual reproduction can perpetuate an individual plant essentially unchanged for generations over a period of many years.
The asexual reproduction of plants is made possible by a combination of two processes termed mitosis and cytokinesis. Mitosis is the normal division of a cell nucleus that occurs as a plant grows, enlarging from embryo to maturity (Figure 5-2). It is mitosis that occurs when new roots form on a cutting or new shoots break from a stem. The
Characteristics of Mitosis and Meiosis in
chromosomes in the cells do not reduce in number or recombine as they divide, thus allowing no opportunity for a new plant to be initiated. Cytokinesis completes the division of the nonnuclear remainder of the cell’s contents and the formation of a new cell wall. Table 5-1 summarizes the major distinguishing features of mitosis and meiosis.