374 
The Garden Magazine, August, 1921 
closely inbred for three or four generations all kinds of mal- 
formed, dwarf, weakly, sterile, and semi-sterile plants may 
appear along with others of the normal type. But let it be re- 
membered that these same forms may be found, though rarely, 
when no forced inbreeding has been practiced. This shows us 
that some of the plants, at least, have the potentialities for pro- 
ducing these abnormal plants, and it only needs certain condi- 
tions to make them appear. The necessary condition is the 
chance for the meeting of the same kind of germ cells during 
fertilization; and inbreeding furnishes these conditions. 
Most of such abnormal forms are “Mendelian recessives,” 
which require that the character determiner be received from 
both the male and female germ cells of the parents in order for 
the character to appear. If the germinal material of a plant 
does not contain the factors for these recessive characters then 
no amount of inbreeding can produce them. The only injury 
proceeding from inbreeding comes from the inheritance received 
from the parents. If these abnormal characters show up after 
inbreeding, it is only because they already existed in the stock 
and were able to persist in the germinal material for generation 
after generation under the protection of more favorable domi- 
nant characters which kept them from appearing. 
Inbreeding is a method of purifying the hereditary material of 
a species or variety. It causes the undesirable forms to appear 
and be discarded, leaving only desirable plants. The accompany- 
ing illustrations show some of the abnormal forms which may 
appear as a result of inbreeding a naturally cross-fertilized plant. 
Inbreeding in naturally cross-fertilized plants often causes a 
general reduction in size, productiveness, and vigor of all the 
plants in addition to exposing to view the “family skeletons.” 
Vigor as great or even greater than that of the original plants 
may be secured by crossing the inbred strains. This increase in 
growth and productiveness is known as “hybrid vigor or 
heterosis.” Plant breeders of the eighteenth century noted the 
occurrence of hybrid vigor, but only recently have attempts 
been made to take advantage of it in a practical way in increas- 
ing crop yields. 
The greatest vigor and productiveness is obtained in the first 
hybrid generation; second and later generations from the same 
hybrid show less and less of this increase and, because of this, 
whoever plans to profit by this heterosis should use only first 
generation seed. 
The definite production of hybrid seeds depends on the hand 
pollinations of the two parent varieties. It must be borne in 
mind that this principle can be utilized advantageously only with 
such plants as produce a large number of seeds from a single 
flower or which have the pistillate and staminate flowers on 
separate plants. Some of the garden and crop plants falling in 
the first category are Tomatoes, Tobacco, Squashes, Pumpkins, 
Canteloupes, Watermelons, Corn, and Cucumbers. In the 
latter class are Asparagus, Hemp, and Hops. 
Inbreeding in naturally self-fertilized plants such as Lettuce, 
Beans, Peas, Wheat, Barley, and Oats does not cause the appear- 
ance of abnormal plants nor a reduction in vigor, for the reason 
that they are highly inbred due to their method of pollina- 
tion. All abnormal and weakly plants have been eliminated 
long ago and only the purified strains remain. They then refuse 
to show family skeletons because they have none hidden away 
behind dominant characters. 
We may conclude that inbreeding in itself is not a harmful 
practice; that it only serves to produce pure types from a mixed 
hereditary material, and the number of types produced will 
depend upon the number of hereditary recessive factors in the 
germplasm of the individuals with which the inbreeding is be- 
gun. Any particular individual may be vigorous or weak, 
fertile or sterile, normal or abnormal, good, bad, or indifferent, 
depending upon the combination of hereditary factors received 
from the parents. Many of the types produced this way will 
be recessives or combinations of recessives which seldom are 
seen under ordinary circumstances, because hidden by the 
dominant factors of the germplasm. These recessives are the 
“corrupt fruit” which give the bad name to inbreeding, for 
they are nearly always undesirable from the grower’s point of 
view. 
Professor Collins’s second article, to appear in September, further discusses the relation between Hybrid Seed and Increased Crops. 
THE OLD GARDENS OF PENNSYLVANIA 
X. COMPTON, AN ARBORETUM 
NEAR PHILADELPHIA 
JOHN W. HARSHBERGER 
Professor of Botany, University of Pennsylvania 
;0 ESTATE near Philadelphia promises to develop into 
a finer arboretum than Compton, which is situated on 
one of the principal streams feeding the Delaware River 
HU system, and commands some magnificent views over 
Piedmont country from the hilltops within its boundaries. 
Its founder was fortunate in having ample means to develop 
the arboretum from artistic and scientific points of view. His 
tastes also enabled him to plan for the future development of 
the place on broad lines. Although the trees and shrubs of 
rare botanic interest have been planted but a short time, rela- 
tively speaking, they have been placed so that they can reach 
their best development as the years go by. Besides native 
trees and shrubs, which are well represented at Compton, many 
of the new introductions from China and Japan raised at the 
Arnold Arboretum in Massachusetts have been planted. 
The figures in the 
map indicate the 
locations of the 
gardens and their 
sequence in the 
series 
The former owner of Compton, now deceased, was one of the 
contributors of the fund raised to send Mr. E. H. Wilson to the 
Orient in search of plants suitable for American and European 
gardens. He, therefore, received as his quota a number of con- 
signments of the Chinese and Japanese shrubs and trees which 
promise to stand the climatic conditions of eastern North America. 
