ASEXUAL PROPAGATION AS AN AID TO THE BREEDING 
OF ROOTSTOCKS 1 
By Walter Scott Malloch 
Assistant in Genetics , University of California 
It has been stated 2 that— 
nearly all plants may be propagated by cuttings 
from one or another of their parts. The ease with 
which plants may be multiplied in this way varies 
greatly in different species, and even in different 
varieties of the same species. 
Climate exerts a marked influence upon the tend¬ 
ency of plants to develop from cuttings. In certain 
localities in southern Europe and in parts of South 
America branches of the common apple tree, 
sharpened and driven into the ground as stakes, 
often take root and sometimes even bear fruit 
during the same season. 
A comparatively warm soil and a 
cool atmosphere with abundant soil 
moisture are favorable to the rooting 
of cuttings. A coarse, sharp, clean 
sand has been recommended as the 
best material for use indoors, since it 
provides ample drainage and is com¬ 
paratively free from the damping-off 
fungus. For cuttings which form roots 
readily, a mixture of one part of light 
garden loam to two parts of sand may 
be used. The cuttings should be kept 
in an environment sufficiently moist 
to prevent loss of water by evapora¬ 
tion and sufficiently warm to favor 
moderate root growth. The tops 
should be kept cool enough to prevent 
the early growth of leaves. 
Cuttings of larger diameter root 
more readily than those of smaller 
diameter, which are more apt to 
shrivel and die. Wounds on the lower 
end of the cuttings start to heal by the 
production of a loose cellular mass of 
tissue known as a callus. The roots 
do not arise from the callus itself but 
from internal tissue. In many plants 
the roots bear no relation to the callus 
in position, as, for instance, in the 
figure shown in Plate 1, Group 3, C, 
where the roots are seen arising from 
the bark in the middle of the cutting. 
Bailey 3 states, however, that “as a 
matter of practice, best results are 
obtained from callused cuttings, parti¬ 
cularly if the cuttings are made from 
mature wood, but this is probably due 
to the fact that considerable time is re¬ 
quired for the formation of adventitious 
buds which give rise to the roots, not to 
any connection between the callusing 
and rooting processes themselves.” 
It has been the purpose of the pre¬ 
ceding paragraphs to review briefly a 
few of the principles of the propagation 
of plants from cuttings in order that 
the results reported in this paper may 
be interpreted to better advantage. 
Let us now turn to a brief consideration 
of the desirability of asexual propaga¬ 
tion of rootstocks from the standpoint 
of the breeding of improved types of 
pomological plants. 
The breeding of improved pomo¬ 
logical varieties has occupied the 
attention of horticulturists for a number 
of years. Most of this breeding work 
is of value for the number of varieties 
which have been produced rather than 
for an analysis of hereditary characters, 
as was pointed out by the writer in 
1923. 4 This condition has resulted from 
the fact that long-life-cycled plants do 
not meet the requirements of favorable 
genetic material discussed by Babcock, 5 
Malloch, 6 and others. The breeding 
of horticultural forms has been largely 
directed toward the improvement of 
the scion wood rather than of the root- 
stock, As soon as a desirable new type 
has been discovered, it has been prop¬ 
agated by asexual means, thus 
utilizing immediately the favorable 
morphological variations as well as any 
increased physiological vigor which the 
new type may possess. The breeding 
of scion wood has proceeded along these 
lines, however desirable a genetic analy¬ 
sis might be from the standpoint of 
future improvement. 
The breeding of improved types of 
rootstocks has not received the atten¬ 
tion it deserves, owing partly to the 
increased difficulty and expense of 
conducting such investigations and 
partly to the less obvious value of such 
endeavors. The more thoroughgoing 
1 Received for publication June 20,1924— issued February, 1925. 
2 Goff, E. S.— principles of plant culture. Ed. 3, p. 200. Madison, Wis. 1906. 
3 Bailey, L. H.— the nursery book. Ed. 16, p. 56. New York. 1911. 
4 Malloch, W. S.— experimental accuracy in fruit breeding. Amer. Nat. 57: 435-442. 1923. 
6 Babcock, E. B.— crepis—a promising genus eor genetic investigation. Amer. Nat. 54: 270-276. 
1920. 
» Malloch, W. S.— value of the hemp plant for investigating sex inheritance. Jour. Heredity 
13: 277-283. 1922. 
Journal of Agricultural Research, 
Washington, D. C. 
(515) 
Vol. XXIX, No. 10 
Nov. 15,1924 
Key No. Calif.-38 
