LOCK ON THE WORK OF KOLREUTER 85 



supposed examples of such hybrids required for their substantiation the 

 experimental proof, which could only be afforded by making actual artifi- 

 cial crosses between the putative parent species. 



The first hybrid made artificially by Kolreuter was obtained ia 1760 

 by applying the pollen of Nicotiana paniculata to the stigma of Nicotiana 

 rustica. The hybrid offspring of this cross showed a character intermediate 

 between those of the two parent species in almost every measurable or 

 recognizable feature, with a single notable exception. This exception was 

 afi'orded by the condition of the stamens and of the pollen grains pro- 

 duced by the hybrids. These organs were so badly developed that in all 

 the earlier experiments, self-fertilization of the hybrid plants yielded no 

 good seed at all, nor were the pollen grains of the Inbrid any more eflfec- 

 tive when applied to the stigmas of either of the parent species. On the 

 other hand, when pollen from either parent was applied to the stigmas of 

 the hybrid plants, a certain number of seeds capable of germination was 

 obtained, although this number was much smaller than in the case of 

 normal fertilization of either parent species. This partial sterility, affect- 

 ing in particular the stamens and the pollen which they produce, is a 

 feature common to the majority of hybrids between different natural 

 species. Many such hybrids, indeed, are altogether sterile, so that a 

 further generation cannot in any way be obtained from them. On the 

 other hand, the members of different strains or varieties which have arisen 

 imder cultivation N-ield, as a rule, when crossed together offspring which 

 are perfectly fertile. 



In subsequent years Kolreuter was able to obtain a very few self-ferti- 

 lized offspring from hybrids of the same origin as the above. The resultuig 

 plants were described as resembling their hybrid parent so closely as to be 

 practically indistinguishable from it. 



The offspring obtained by crossing the hybrid plants with pollen from 

 either parent showed in each case a form more or less intermediate between 

 that of the original hybrid and that of the parent species from which the 

 pollen was derived. But the plants were not all alike in this respect, some 

 of them being much more like the parent species than others, and some, 

 again, varying in other directions. There were also considerable differ- 

 ences between the different indi\'iduals in respect of fertility, so that some 

 of the plants were more and some less sterile than the original hybrids. 

 Also, there was some tendency to the production of malformations of the 

 flowers and other parts. 



One of the most noted of Kolreuter's experiments was that which con- 

 sisted in repeatedly crossing a hybrid plant with one of the parent species 

 from which the hybrid was derived. By continuing to pollinate the mem- 

 bers of one generation after another with the pollen of tlu> same parent 

 species, plants were at last arrived at which were indistinguishable from 

 the parent in question. We shall return to this fact later on, when the 

 reader will be in a position to appreciate its importance more fully. 



Kolreuter found that the result of reciprocal crosses is usually identical 

 — that is to say, the offspring (obtained by fertilizing a plant A with pollen 

 from a plant B are not to be distinguished from those obtained when B is 



