AN INTERPRETATION OF STERILITY IN CERTAIN 



PLANTS.i 



By E. M. east. 

 {Read April 23, 191 5.) 



It is obvious that it is impossible to investigate the cause of 

 steriHty in hybrids by the pedigree culture method when such 

 sterility is complete. Occasionally, however, one finds hybrids 

 which are not wholly sterile. Such is the case in the historic cross, 

 Nicotiana rustica L. X Nicotiana paniculata L. This hybrid holds 

 an enviable position in experimental botany, since it was the first 

 artificial hybrid to be studied. It was made by Kolreuter in 1760 

 and was studied by him for several years by means of back crosses 

 with each parent. 



This cross I repeated in 1909, using as the A^. rustica parent a 

 small variety N. rustica humilis Comes obtained from Dr. Comes 

 through the kindness of Dr. D. G. Fairchild. It has now been 

 studied through five generations both in the field (general morphol- 

 ogy) and in the laboratory (histology and cytology). The essen- 

 tial points noted, as I see them, are as follows : 



Two species giving extremely uniform progeny when selfed 

 have, when crossed, given an intermediate F^ population as uniform 

 as themselves, and an inordinately variable F^ population. 



The germination of F^ seeds varies in different samples from 

 20 to 60 per cent. 



Practically no two F^ plants are alike, and the parental forms 

 are recovered once in every 100 to 200 F^ plants. 



In F^, from i to 6 per cent, of the 5 gametes are functional. It 

 is impossible to determine the percentage of viable J^ gametes formed 

 from the pollen mother cells, but from 2 to 6 per cent, of the 



1 It is impossible to reproduce the photographs shown by means of lan- 

 tern slides, but an illustrated paper giving the details of the investigation is 

 to be published shortly. 



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