Hayes and Garber - Cucurbit Breeding

266 BREEDING CROP PLANTS
probably a frequent cause for the variation in yield from different
trees of the same variety when grown in the same orchard. Selfsterility,
which is so prevalent among fruit crops, often prevents
the production of homozygous material; while the use of heterozygous
material does not allow the breeder to make systematic
crosses with a knowledge of the genetic constitution of the parents.
In spite of these difficulties which the fruit breeder must face,
there has been a consistent attempt to use fundamental breeding
principles and at present methods are becoming somewhat
standardized. The advantage which comes to the breeder from
the fact that an improved variety may be propagated asexually
and need not be reduced to a homozygous condition, tends to
offset other difficulties Some of the more general problems will
be here illustrated.
Overcoming Soil Heterogeneity. Batchelor and Reed (1918)
have made an interesting study of variability in orchard plots.
They used orange, lemon, walnut, and apple trees in the investigation.
From 224 to 1,000 trees of each of the different fruits
were studied and the coefficient of variability for yield of single
trees determined. The coefficient of variability of the clonal
varieties ranged from 29.72 to 41.23 per cent. Thirty-five per
cent, might be considered a fair average. Multiplying this by
0.6745 gives 23.6, the probable error in percentage of the mean.
The effect on the coefficient of variability of increasing the
number of trees in a plot was studied ;
a comparison of plots containing
1, 2, 4, 8, 16 and 24 trees being made. Table LXIX gives
an average of tests with oranges, lemons, apples, and walnuts.
The results are based on a study of more than 2,000 individual
trees.
TABLE LXIX.
EFFECT OF INCREASING THE NUMBER OF TREES PER PLOT
Number
of trees
per plot