Hayes and Garber - Cucurbit Breeding

142 BREEDING CROP PLANTS
index is obtained by dividing the average leaf breadth by the
average length and multiplying by 100. The table shows that
by crossing, a form, No. 38, was obtained which combined somewhat
the desirable features of the parents (Nos. 26 and 27)
and is superior in both number and size of leaves to the better
pure-line obtained by selection (No. 33). As an indication of
the commercial reception of
Fio. 32.-Tobacco No. 38. This strain
this new form, it was estimated
that at least 10,000
acres of No. 38 were grown
in Wisconsin in 1919 out of
a total of about 40,000
acres. Here we have an
example of crossing two
closely-related forms and
obtaining from the resultant
progeny a strain of
more commercial value
than either parent.
The illustrations bring
out more clearly some of
the features of the parents
and progeny.
Summary. In this
chapter
concrete evidence
f the Value f Crossing as
was produced by crossing No. 26, which ex- a means of producing imcels
in leaf breadth, with No. 27, which is
nrftvpr V o i r ; Pti*p^ nf
homozygous for high leaf number. This new Pr Ved Varieties
strain is more desirable than any other pure fertilized Crops has been
line form obtained It is widely grown in presented< OSSeS should
Wisconsin. (After Johnson, 1919.)
be made with a definite
purpose in view and the parents should be selected on the basis of
performance records. Just as a chemist requires a certain knowledge
of the elements which he synthesizes into compounds, so
also the plant breeder may make crosses much more intelligently
if he is thoroughly acquainted with the prospective parental
material. Promiscuous crossing as a means of producing improved
forms is discouraged.