1917] Goodspeed-Clausen : F^ Species Hybrids in Nicotiana 315 



parent one-third in determining the characters of the hybrid. These 

 figures are for the most part arrived at from a consideration of meas- 

 urements of nineteen arbitrarily selected characters. For any par- 

 ticular character a certain percentage influence is assigned to each 

 parent and the percentage totals of the set of characters selected are 

 supposed to represent rather accurately the relative degree of influence 

 of each parent. This matter, however, is so largely one of arbitrary 

 interpretation that it necessarily can possess but little value. Particu- 

 larlj^ is this true in cases in which the hybrid in general surpasses both 

 parents. It is a well-known horticultural fact that plants in general 

 may show marked increases in the size of vegetative and even floral 

 organs due to stimuli furnished by favorable conditions of soil and 

 climate. Elsewhere (Goodspeed and Clausen, 1915) we have shown 

 that even such relatively constant characters as those which go to 

 make up the flower complex may be influenced in definite directions 

 by external environmental conditions. Conceivably the case may be 

 similar here, for if the sijlvestris elements be regarded as providing 

 merely a stimulus for the Tahacum reaction system which is concerned 

 in directing the course of somatogenesis, then the sylvestris elements, 

 while in a sense the cause of the increased growth, are not a part of 

 it. Such a conception is borne out rather strikingly by a consideration 

 of certain of the measurements given by Brown which appear to fur- 

 nish a clue to the explanation of this phenomenon. If a comparable 

 series of measurements dealing with length of leaf, length and spread 

 of corolla, etc., be examined we find a rather striking proportional 

 increase over the Tahacum parent throughout, which is not the case 

 when these measurements are compared with those given for sylvestris. 

 When particular closely related characters are considered the evidence 

 is even more strikingly confirmatory. The length of calyx in the 

 hybrid is about 1.2 times that in the Tahacum parent, and it is 1.6 

 times that of the sylvestris parent. The corolla tube length of the 

 hybrid is about 1.3 times that of the Tahacum parent, but only .75 of 

 that of the sylvestris parent. Corresponding measurements which we 

 have obtained on parents and hybrids showed practically proportional 

 increases in corolla spread and length over the Tahacum parent when 

 the spread of the Tahacum and sylvestris parents was practically the 

 same, but the length of the corolla tube of sylvestris was about twice 

 that of tlie Tahacum parent concerned. The same considerations 

 would appear to apply to the histological features which were studied. 

 It seems, therefore, reasonable to regard the presence of the sylvestris 



