STERILITY AS THE RESULT OF HYBRIDIZATION AND 
THE CONDITION OF POLLEN IN RUBUS' 
CARL SHERMAN HOAR 
(WITH PLATES X—XII) 
One of the first well authenticated records of the possibility of 
crossing species dates from 1694. In that year CAMERARIUS (1) in 
a letter to VALENTINE mentions the effect of crossing the hemp 
(Cannabis) with the hop (Humulus). However, it was not until 
1760 that KOLREUTER (2) made the first plant hybrid which was 
to be of use to science. He crossed Nicotiana paniculata 4 and 
Nicotiana rustica ?. In stating his conclusions he recognizes the 
relative fertility of hybrids from crossed varieties as compared with 
the relative sterility of hybrids made between distinct species. 
As a result of KOLREUTER’s research much interest was aroused 
among plant breeders as to the réle of hybridization and as to the 
conditions which lead to sterility. Such men as KNnicHT (3) and 
HERBERT (4) will, perhaps, illustrate the diversity of opinion which 
sprang up. KNIGHT, on the one hand, held that hybrids from 
parents of distinct species were sterile, while, on the other hand, 
the varietal crosses were fertile (an opinion quite like that of 
K6LREUTER). HERBERT, on the contrary, found that hybrids 
between different species are frequently fertile, a fact which he 
interpreted as signifying that the parents had branched from the 
same main stock. Focke (5) also has pointed out the fact that no 
peculiarity of hybrids has attracted so much attention as the 
lessening of the power of reproduction, and that crosses between 
more distantly related plants are more sterile even up to entire 
sterility. Other views have been expressed since, some favoring 
one side and others being equally sure that the contrary is true. 
However, though this diversity of opinion is prevalent even to the 
present time, nevertheless it seems clear that sterility is a common 
characteristic of hybrids. 
* Contribution from the Laboratories of Plant Morphology of Harvard University. 
Botanical Gazette, vol. 62] [370 
