[cannebosanac]

poglavlje 2 nastavak

In Cannabis, mitosis takes place in the shoot apex
(meristem), root tip meristems, and the meristematic cam-
bium layer of the stalk. A propagator makes use of these
meristematic areas to produce clones that will grow and be
multiplied. Asexual propagation techniques such as cuttage,
layerage, and division of roots can ensure identical popula-
tions as large as the growth and development of the paren-
tal material will permit. Clones can be produced from even
a single cell, because every cell of the plant possesses the
genetic information necessary to regenerate a complete
plant.
Asexual propagation produces clones which perpetu-
ate the unique characteristics of the parent plant. Because
of the heterozygous nature of Cannabis, valuable traits
may be lost by sexual propagation that can be preserved
and multiplied by cloning. Propagation of nearly identical
populations of all-pistillate, fast growing, evenly maturing
Cannabis is made possible through cloning. Any agricul-
tural or environmental influences will affect all the mem-
bers of that clone equally.
The concept of clone does not mean that all members
of the clone will necessarily appear identical in all charac-
teristics. The phenotype that we observe in an individual is
influenced by its surroundings. Therefore, members of the
clone will develop differently under varying environmental
conditions. These influences do not affect genotype and
therefore are not permanent. Cloning theoretically can pre-
serve a genotype forever. Vigor may slowly decline due to
poor selection of clone material or the constant pressure
of disease or environmental stress, but this trend will re-
verse if the pressures are removed. Shifts in genetic compo-
sition occasionally occur during selection for vigorous
growth. However, if parental strains are maintained by in-
frequent cloning this is less likely. Only mutation of a gene
in a vegetative cell that then divides and passes on the mu-
tated gene will permanently affect the genotype of the
clone. If this mutated portion is cloned or reproduced
sexually, the mutant genotype will be further replicated.
Mutations in clones usually affect dominance relations and
are therefore noticed immediately. Mutations may be in-
duced artificially (but without much predictability) by
treating meristematic regions with X-rays, colchicine, or
other mutagens.
The genetic uniformity provided by clones offers a
control for experiments designed to quantify the subtle
effects of environment and cultural techniques. These
subtleties are usually obscured by the extreme diversity
resulting from sexual propagation. However, clonal uni-
formity can also invite serious problems. If a population of
clones is subjected to sudden environmental stress, pests, or
disease for which it has no defense, every member of the
clone is sure to be affected and the entire population may
be lost. Since no genetic diversity is found within the
clone, no adaptation to new stresses can occur through
recombination of genes as in a sexually propagated
population.
In propagation by cuttage or layerage it is only neces-
sary for a new root system to form, since the meristematic
shoot apex comes directly from the parental plant. Many
stem cells, even in mature plants, have the capability of
producing adventitious roots. In fact, every vegetative cell
in the plant contains the genetic information needed for an
entire plant. Adventitious roots appear spontaneously from
stems and old roots as opposed to systemic roots which
appear along the developing root system originating in the
embryo. In humid conditions (as in the tropics or a green-
house) adventitious roots occur naturally along the main
stalk near the ground and along limbs where they droop
and touch the ground.
Rooting
A knowledge of the internal structure of the stem is
helpful in understanding the origin of adventitious roots.
The development of adventitious roots can be broken
down into three stages: (1) the initiation of meristematic
cells located just outside and between the vascular bundles
(the root initials), (2) the differentiation of these meristem-
atic cells into root primordia, and (3) the emergence and
growth of new roots by rupturing old stem tissue and
establishing vascular connections with the shoot.
As the root initials divide, the groups of cells take on
the appearance of a small root tip. A vascular system forms
with the adjacent vascular bundles and the root continues
to grow outward through the cortex until the tip emerges
from the epidermis of the stem. Initiation of root growth
usually begins within a week and young roots appear within
four weeks. Often an irregular mass of white cells, termed
callus tissue, will form on the surface of the stem adjacent
to the areas of root initiation. This tissue has no influence
on root formation. However, it is a form of regenerative
tissue and is a sign that conditions are favorable for root
initiation.
The physiological basis for root initiation is well un-
derstood and allows many advantageous modifications of
rooting systems. Natural plant growth substances such as
auxins, cytokinins, and gibberellins are certainly responsible
for the control of root initiation and the rate of root for-
mation. Auxins are considered the most influential. Auxins
and other growth substances are involved in the control of
virtually all plant processes: stem growth, root formation,
lateral bud inhibition, floral maturation, fruit development,
and determination of sex. Great care is exercised in appli-
cation of artificial growth substances so that detrimental
conflicting reactions in addition to rooting do not occur.
Auxins seem to affect most related plant species in the
same way, but the mechanism of this action is not yet
fully understood.