60 Inheritance in Pisum 



other parents. This provisional interpretation of increased vigour of 

 ^1 plants, has at all events the merit of being less obscure than the 

 hypotheses which are current in the literature of plant physiology 

 (Jost, 1907). 



We return now to the F^ of the crosses between Autocrat and 

 Bountiful. Since the constitution of F^ = TtLl, its gametes have con- 

 stitutions :—TL :Tl:tL: tl, and hence, when F^ plants are self-fertilized, 

 we expect the usual 9:3:3:1 ratio ; i.e. in 16 plants, 9 with both, 

 dominants {T and Z); 3 with one dominant; 3 with the other and 1 

 with the two recessives {t and I). That the expectation is realised is 

 seen from the following table in which the results already given are 

 recorded in terms of Tt and LI. 



2^8 = 



Of the 13 dwarf (^Z) plants, all but one were below 3 feet in height, 

 the three dwarfest being respectively 1^, 1|, and 2 feet. Table II 

 gives the records of height, of thinness or thickness of stem, and of 

 length of internode of the plants which we regard as true dwarfs. The 

 classification is of course open to the objection that thick and thin are 

 but qualitative terms, and that, in difficult cases, the criteria are purely 

 subjective. An answer to this objection is that the records in Table II 

 were made before we were aware that the characters "thin" or "thick" 

 were of any considerable importance. We include in Table II the 

 records of the characters oi F^ plants of a stature up to 4 feet. A com- 

 parison of the descriptions of the stems of the shorter plants (above the 

 horizontal line in Table II) with those of the stems of the less short 

 plants (below the line) confirms, as we think, the view which we have 

 expressed above, that the dwarf plants have thin stems and also short 

 internodes. With regard to the range of variation in height, both 

 among the dwarfs, and among the plants of the constitution Tl, it is 

 probable that the character of number of nodes, and also that of the 

 position of the first flowers, are also of importance. It is hoped that 

 further investigation of these characters among the F^ families will 

 determine this point. 



We conclude from the above experiments that tallness in peas 

 (P. sativum) depends on the presence of two factors, long internode, and 



