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2.

Classification of plants into a system that indicates the true
relationships of all the taxa has long been the aim of plant taxonomists,
Although much in this field has been achieved by aid of observations of
morphological and geographical characteristics in the past, great difficulties
have frequently been met with in deciding upon the level of some taxa, and
then especially in distinguishing between the category of species as
contrasted to the interspecific units of subspecies and variety. The
biological definition of these categories that considers the occurrence
of an internal reproductive barrier to be an essential attribute of the
species and the lack of such a barrier between its subunits has greatly
clarified this matter, although the disclosure of such a barrier between
two taxa sometimes requires the applications of methods other than those
of classical taxonomy.

It has been known for nine decades that the chromosomes and their
number are of an immense significance for studies of reproductive barriers,
because every well-defined species is characterized by only a single
chromosome number. When related species are characterized by the same
chromosome number, differences in chromosome morphology may indicate
the occurrence of reproductive difficulties, but hybridization experiments
are usually required to clarify their nature and importance. In the case
of polyploidy reflected in differences in chromosome number, however, the
case is much simpler, because thousands of experiments have clearly shown
that taxa with different chromosome numbers either are completely incompatible
or their more or less rare crosses are unable to reproduce.

Because of these observations, it has long been evident that a knowledge
of the chromosome numbers of the taxa of a flora is an important contribution
to the correct evolutionary classification of its species and their subunits.