Biology of Propagation
Plant Propagation is defined as a method of increasing plant material by
natural or artificial means. Natural reproduction involves the management of
two developmental stages of plants – sexual and asexual reproduction.
Sexual reproduction entails the union of male (sperm) and female (egg) sex
cells, followed by the formation of embryos and seeds, and finally ending
with a population of seedlings with new, similar, or differing genotypes.
The genotypic composition of a plant or group of plants depends on the
transmission from one generation to the next of a particular combination of
genes present on the chromosomes in the cells. The genotype produces a
plant of a given outward appearance referred to as the phenotype. One of
the goals of plant propagation is to preserve a particular genotype that will
reproduce the kind of plant being propagated.
Biology of sexual reproduction also includes cell division, called meiosis, in
which the chromosomal number is reduced by half. A diploid cell completes
two successive divisions to form four new haploid cells. Because
fertilization restores the original number of chromosomes, the new plants
contain chromosomes from both the male and female parents. Meiosis
occurs in the gametophyte stage of Bryophytes (mosses, liverworts, and
hornworts), Pterophytes (ferns), Gymnosperms (pines, junipers, ginkgos),
and Angiosperms (flowering plants). In all plant life cycles, called
Alternation of Generations, meiosis occurs in the sporophyte (diploid)
generation. During the meiotic process, specialized diploid cells undergo
cell division and create four haploid cells. The Bryophyte and Pterophyte
haploid cells divide, called mitosis, and grow and develop into plants called
gametophytes. Reproductive male and female sex cells are produced on the
gametophyte stage, thus allowing fertilization to occur and the creation of
the diploid sporophyte generation.
In higher forms of plant life, the reduced gametophyte plant occurs only in
the reproductive organs. In Gymnosperm species, a fleshy or woody cone
bears the sex cells, called pollen and egg. Wind or insect carried pollen
initiates fertilization and the development of a sporophyte generation as an
embryo inside a seed. The embryo and seed developing process also occurs
in the Angiosperm plants.
The outward appearance (phenotype) of a plant and the way characteristics
are inherited from generation to generation are controlled by the action of
the genes present on the chromosomes. Some traits are controlled by a pair
of genes (allele pair), while other traits are controlled by two or more sets of
genes (alleles). The terms homozygous and heterozygous are frequently
used to describe the genotype of parents or progeny of monohybrid or
dihybrid crosses. If a gene type is present on both homologous
chromosomes, then the plant is homozygous and will express a predictable
trait if self-pollinated or if the other parent is genetically similar. One the
other hand, if the genes on one chromosome differ from those on the other
homologous chromosome, then the plant is considered heterozygous. In this
case, future generations of plants may not possess phenotypic traits of the
parents, and the seedlings may also differ in appearance not only from the
parents but also from each other.
Vegetative cell division, called mitosis, is possible because each cell of a
plant contains all the genetic information necessary for growth and
development. Regeneration of a new plant by asexual methods occurs
naturally in higher plants. Mitosis is a chromosomal duplication as well as a
chromosomal and cell division process, which creates two new identical
cells. As a result of this process, the characteristics of a new plant that
grows will be the same as that from which it originated.
Mitosis occurs in specific growing points of a plant to produce growth.
These are the shoot apex, the root cap, the vascular cambium, and the
intercalary areas (nodes). Mitosis also occurs when callus forms on a plant
wound and when new growing points are initiated on roots and stem pieces.
Callus parenchyma consists of new cells produced from cut tissues in
response to wounding. When new growing points are initiated on roots,
stems and leaves, they are referred to as adventitious parts or adventitious
shoots.
Mitosis is the process of vegetative growth, regeneration, and wound healing
which makes possible such asexual propagation methods as cuttage,
graftage, layerage, separation, division, and tissue culturing.