30 Genetic Studies in Potatoes 



protected in bags, whilst four members of the original " prostrate " 

 family and sixteen different " upright " varieties were used as parents 

 in the crosses. 



The upright varieties include two ordinary cultivated varieties, the 

 remainder being of varied origin, derived partly from cultivated varieties, 

 partly from the Congo potato and partly from S. edinense. Although 

 diffefing somewhat in their habit of growth in the same sense that 

 ordinary cultivated varieties differ, they were all unmistakably " uprights" 

 and their stems developed an abundance of secondary wood. The four 

 *' prostrates " used in these crosses were all true to type and identical in 

 habit of growth and anatomical structure. 



Evidence as to the genetic constitution of the prostrates has already 

 been given. Selfed families have also been raised from twelve of the 

 sixteen " upright " parents — from some of these two or three such families. 

 In no case has a single true " prostrate " plant occurred. From the 

 remaining four no selfed families have been obtained, three of them 

 indeed produce no good pollen and the fourth could not be induced to 

 set selfed seed, so that we must look to the evidence from crosses with 

 other " uprights." This evidence, as far as it goes, confirms that obtained 

 from selfing, for in no case have these " upright " plants when crossed 

 with other " uprights " given rise to any " prostrate " plants. Two 

 " upright " parents remain, however, which have neither given selfed 

 families nor entered into any other crosses. 



The results from the crossing of " upright " and " prostrate " plants 

 (Fi) are shown in Table IV. 



On the whole the plants were remarkably healthy and luxuriant, 

 the mortality at transplanting unusually low. Twenty out of the 

 twenty-three of the families, including all those from protected flowers, 

 consisted entirely of uprights. Some of the plants were quite erect, 

 the majority more or less spreading as the season advanced, but except 

 in three families all were decidedly " upright." 



The anatomical structure of the stems of a large number, in some 

 cases of complete Fi families, was examined by section and several by 

 the method of maceration. While in some plants the interfascicular 

 wood was thin and in others absent in patches, especially just below the 

 leaf insertion, in the great majority there was strong interfascicular 

 secondary growth. None showed the extensive deficiency and isolated 

 strands of wood characteristic of " prostrates." 



In three families, however, derived from two distinct " prostrate " and 

 three distinct " upright " parents, a small proportion of true "prostrate" 



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