EXPERIMENT STATIONS. 187 
NEW ULM STATION. 
C. W. H. HEIDEMAN, SUPT. 
CLASSIFICATION OF THE SEXUAL AFFINITIES OF PRUNUS 
AMBRICANA VAR.* 
THE PROBLEM. 
The uncertainty of the regular annual fruiting of plums in the 
Northwest, where only the native Prunus Americana in its many 
varieties has been found sufficiently hardy to endure the climatic 
conditions, has long been a difficult problem in horticulture. 
Writers on the subject of plum culture have attributed as the cause 
of the more or less non-productivenes “the influences of domes- 
tication and consequent high culture,” “self-sterility,” etc., etc. The 
beneficial effect of cross-fertilization has been hinted at and pro- 
posed as the remedy for all cases of infecundity. Mixed and close 
planting of the varieties to better ensure cross-fertilization has 
been suggested by nearly all of them. Reports of various horti- 
cultural societies are filled with instances of the beneficial effects of 
cross-fertilization, but, reading between the lines, as many or more 
instances of the failure of good results fron cross-fertilization have 
been recorded. Cross-fertilization, therefore, unless it be effected 
in the direction of the natural affinities of the varieties, does not 
completely explain why certain varieties, even with the aid of cross- 
pollenation, may be prolific one season and the next produce no 
fruit at all; why one season the fruit will be large and fine, the next 
inferior in size and quality; why an unusually fine variety in the 
woods and thickets will be worthless when removed from its sur- 
roundings, even with subsequent best of care and culture. 
About ten years agoI began making artificial crosses for the 
purpose of breeding improved varieties. My grounds contained at 
least two hundred trees, mostly selected from the woods and thick- 
ets along the Minnesota and Cottonwood river bottoms, together 
with a few horticultural varieties of P. Americana. I soon found 
that many of my desired crosses were difficult to obtain. I ob- 
served numerous adaptations to ensure cross-pollenation, together 
with differences in morphology of the stamen and pistil. Crosses 
between certain forms were fully fertile, while with others negative 
results were invariably obtained. Reciprocal crosses between 
varieties and between species were not equally fertile. I determined 
to go into the matter systematically,keeping a careful record of each 
cross made and noting the result, raising hundreds of seedlings and 
again experimenting with them. 
*A paperread before the Minnesota Academy of Natural Sciences, Jan., 1895, 
