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2.
In other years there may be other basic n's, in Chrysanthemum
x = 9, and x this genus, species and hybrids we know with all
multiples of 9 from 18 to 90. In the potato and tobacco genus
(Stamen and Nicotiana) x = 12, and the scutic n's are, for instance,
24, 48 and 72. In the apple genus, Malus x = 13; in poplars and willows
(Populus and Salix) x = 19. Most basic n's, however, are lower than 12,
and higher n's are usually secondary.
The occurrence of a multiple chromosome n'ts series is known
as polyploidy (Winkler 1916) and it represents a highly interesting regularity.
This phenomenon occurs in a vast majority of higher plants known; it is more
frequent under extreme conditions, and is typical of many ornamental plants.
In the wheat, the diploid wheats are most primitive and of little use at present.
In the cereals, the diploids (Avena) and the tetraploids (Triticum) are the Mediterranean and Egyptian
cereals, whereas the hexaploids are the modern breadwheats (Triticum).
In several other cases, for instance in oats (Avena), we meet with similar
conditions. As a rule, those species in a polyploid series that have the highest
chromosome numbers are also the most valuable as cultivated plants. Besides
wheat, and oats, this is true for potatoes, tobacco, white clover, alfalfa,
timothy, Kentucky bluegrass, oats, orchard grass, and the banana, apple, orange, sugarcane,
and figs. On the other hand, rye, barley, and beets have low, monoploid
or diploid chromosome n'ts.
It is known that most genera, which are the subject of presently
living higher plants, are highly polyploid, and so are also the mosses, whereas
polyploidy is quite rare in the conifers. In pines, the phenomenon
is also frequent, though it is often masked by chromosome units
concerned with sex.
The origin of a polyploid series has been elucidated & explained
in investigation. It is known, for instance, that species representing high levels in
a series have arisen from older species with lower chromosome n'ts. It has also
been found that polyploids may be of three different kinds, which we designate
as autopolyploidy, allopolyploidy and triploidy. With autopolyploidy we are
concerned with a purely quantitative increase in the number of chromosomes
sets, that originate from the same species or from a hybrid between two races.
Allopolyploidy involves a summation of chromosome sets from different species.
Whereas autopolyploidy signifies an increase in the number of chromosomes
within a cell. In connection with autopolyploidy, it may also be appropriate
to mention triploidy, which is the occurrence of individuals with half the
number of chromosomes of the species in question. It may be best to
discuss these phenomena separately each in its turn.
