32 



been given the credit of refutiug it. Wolf, in liis doctor's thesis on the 

 •'Theory of Generation," maintained that the embryo and organs of a plant 

 dfvelop not by the unfolding of parts already present in miniature, but 

 tl^at thej- grew out of undifferentiated rudiments, the theory of epigenesis. 

 However, Wolf's arguments were far from convincing, as he held that the 

 act of fertilization was merely another form of nutrition. 



About the same time experiments in hybridization were being carried 

 on by several investigators, and the results obtained supplied much more 

 convincing proof against the old theory of evolution. Among the fore- 

 most men in this held were Gottlieb Koelreuter and Christian Konrad 

 fSprengel. Wliile K.olreuter brought together many important observations 

 on the sexuality of plants, yet his greatest service consisted in the produc- 

 tion of hybrids. In this connection it may be of interest to note that his 

 first hybrids were produced between two species of tobacco plants. Nico- 

 tiana paniciiin and N. rnstica. What he accomplished did not require be- 

 ing changed, but when combined with later observations has been used in 

 the discovery of general principles of hybridization. His work seems to 

 belong to our time. Koelreuter sliowed that only closely allied plants, and 

 not always these, were capable of producing hybrids, and that the mingling 

 of parental characters in the hybrid was the best refutation of the theory 

 of evolution. It was no easy matter to place the proper estimate upon the 

 value of the contributions of this gifted observer. The collectors of the 

 Linnaean school, as well as the true systematists at the close of the IStli 

 century, who wielded a powerful influence upon botanical thought, had lit- 

 tle understanding for such labors as Koelreuter's, and incorrect ideas of 

 hybrids prevailed in spite of botanical literature. Hybrids were also incon- 

 venient for the believers in the constancy of species. 



Koelreuter's studies were not confined to hybridization alone, for he di- 

 rects attention to the natural way of the transfer of pollen from stamen 

 to stigma, being the tirst to recognize the agency of insects. He studied 

 pollen grains, showing that fertilization followed pollenation in the ab- 

 sence of light, and rejected the idea that the pollen grain passed bodily 

 into the ovary. With the microscope, however, he was less skillful than 

 as an experimenter, for he supitosed the pollen grain to be solid tissue, and 

 the fertilizing substance to be oil which adheres to the outside of the grain 

 and finds its way to the ovule. The pollen tube had not been discovered, 

 although the time was one hundred years after the discovery of the cell 

 by IJobert Hooke. 



