Glossary Plant Breeding

possible to estimate variation among row-blocks as well as among column-blocks and
to remove them from experimental error. However, this is possible only when the
number of treatments is equal to the number of blocks (replication), and thus is
completely a square. This enables the treatments to individually appear in each rowblock
and in each column-block. This design becomes impractical if the number of
treatments increases. On the other hand, if the treatment number is small, the degree
of freedom associated with the experimental error becomes too small for the error to
be estimated reliably. This design is used for field trials in which the experimental
area has two fertility gradients running perpendicular to each other or has a
unidirectional fertility gradient but also has some residual effects from the previous
trials.
Law of Homologous Series in Variation. Characters found in one plant species may
also be expected in similar species. Characters occurring in one collections made in
one place may abound in those from other places within the species. Vavilov (1951)
expressed this concept in genetic terms and used this principle as a clue to characters
remaining to be discovered. Also called law of parallel variation.
Law of independent Assortment. Mendel’s second law. During gametogenesis, the
segregation of one gene pair is independent of other gene pairs (or the inheritance of
one character is independent of the others). Notice that the phenomenon of gene
linkage is an important exception to this law.
Law of Minimum (Liebig 1840). A law stating that the growth of a plant is proportional
to the foodstuff, which is available in minimum quantity; also called the law of
limiting factor. The two key points for its practicability are: steady-state conditions
and factor interaction. Steady state implies a balance between inflow and outflow of
energy and materials. Factor interaction includes involvement of other condition with
the essential element. For instance, some plants have been shown to require less zinc
when growing in the shade than when growing in full sunlight; therefore, a low
concentration of zinc in the soil would less likely be limiting to plants in the shade
than under the same conditions in sunlight.
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