Self-sterility in Dewberries and Blackberries 
13 
of the total number of flowers inclosed, and whatever fruit was in evi¬ 
dence was composed of only a few (one to eight) druplets. 
Even from the commercial standpoint, that variety which produced 
large numbers of imperfect fruits, as recorded in Table No. 3, cannot 
be classified with those which produced no fruit whatever, and therefore 
must be classified as being only imperfectly self-fertile. 
By an examination of the records of the hybrids in Table No. 1, and 
in Table No. 2, we notice the interesting fact that among them fertility 
ranges all the way from absolute self-sterility, as in the case of McDon¬ 
ald, Sorsby, Spalding, and Haupt, through partial sterility, as in the 
case of Batlibun, up to complete self-fertility, as is found in the varieties 
of Cox, Ruth, and Wilson. 
Sterility, from the evidence at hand, apparently occurs only in dew¬ 
berries and their hybrids. Its cause, therefore, must be studied with 
the aid of dewberry and hybrid material. Blackberry varieties will 
henceforth in this bulletin be considered only in their relation as polli¬ 
nators for the berry plantation. 
TIIE PROBABLE CAUSE OF STERILITY. 
Having determined the fact that some of our varieties of blackberry- 
dewberry and dewberry-blackberry hybrids, and some of our true dew¬ 
berry varieties, are self-sterile, we shall now proceed to investigate the 
cause or the causes that might be responsible for this phenomenon of 
sterility. 
Theories Advanced. 
The investigations as to the causes of sterility will proceed along eight 
lines, or theories, which may be advanced as an explanation of this 
phenomenon. These eight theories may be stated as follows: 
1. The factor of the origin of the varieties. 
2. The structure of the flower, and disease (Double Blossom). 
3. The daily blooming period of varieties. 
4. The amount of pollen produced by the flower. 
5. The factors of environment. 
6. Sterility due to hybridism. 
7. The percentage of defective pollen grains produced. 
8. Natural antipathy between the pistils and the pollen of the same 
variety. 
Experimental Evidence and Discussion. 
Origin of Varieties .—From our results secured on the self-sterility and 
the self-fertility of varieties of dewberries, the fact becomes quite clear 
that the ancestral species of our cultivated varieties of dewberries fall 
into two well defined groups, namely, those that are self-sterile and those 
that generally are self fertile. The tabulated data in Table No. 1 
reveals the fact that all of the varieties that are pure lineal descendants 
