Some ornamental plants, including orchids and most transgenic
crops, are reproduced by tissue culture (Figure 3.4). This involves
the infliction of a wound to the parent plant so that it forms a
callus from parenchyma cells. Parenchyma cells are most similar to meristem cells and represent up to 80% of all the cells in the
plant. Meristem cells have a large nucleus and can repeatedly
divide to produce daughter cells. Meristem cells are the plant
equivalent of animal stem cells and are found in the meristem
region at the tips of roots and shoots and in seeds. It is only when
meristem cells migrate out of the meristem region that they differentiate
into specialized cells that make up the rest of the plant.
Once a meristem cell differentiates, it generally will not divide
again, but parenchyma cells can divide in response to a wound
and produce a callus, which is a mass of undifferentiated cells.
These undifferentiated cells from the callus can be induced to
differentiate into the specialized cells that will produce a fullgrown
|Figure 3.4 Round-leaved sundews
(Drosera rotundifolia) are grown
from tissue cultures on a gelatin media
in a petri dish. In this process, known
as micropropagation, clones are grown
from single cells of a callus that formed
from a wound inflicted on the parent plant.
The callus will generate a somatic embryo
when grown on a
nutritious gelatinous media supplemented with hormones. First,
the callus is placed in a receptacle called a petri dish that contains
a mix of chemical nutrients, high concentrations of cytokinin,
and low concentrations of auxin, all of which have been solidified
with gelatin. This ratio of high cytokinin to low auxin promotes
shoot growth. After the shoots are formed, the plantlets
to a second petri dish with a high auxin to low cytokinin
ratio, which induces root growth. After root generation, they are
transferred to potting soil.