

CH. n] Structural Characters : Plants 19 



greatly in height are used, dominance is complete, and the two parent forms 

 are represented as three to one in F%. No clear exception has yet been 

 observed. Peas (Pisum) exist in a vast number of distinct horticultural 

 varieties which can roughly be classified as tall (about 5 6 ft.), half-dwarfs 

 (about 4 ft), dwarfs (about 9 ins. to 3 ft.). The genetic relations of the 

 half-dwarfs to the others are not fully explored, and further investigation 

 will probably lead to the discovery of important facts. The cross half- 

 dwarf x tall giving tall as dominant has produced some extreme dwarfs in 

 F^ doubtless by recombination (g : v.\ R.E.C. 20, p. 69. The cross 

 half-dwarf x dwarf has given intermediates in F^ (ibid.). 



The cross between tall and dwarf " Cupid " Sweet Peas gives complete 

 dominance of tallness and simple segregation in F^ "Cupids" indis- 

 tinguishable from the original "Cupid" parent reappearing (Fig. i). 



Phaseolus has been investigated especially by Tschermak (278) who 

 records some apparently anomalous results, de Vries (298, n. p. 76) 

 states that he found that extracted F z dwarf Antirrhinum did not breed 

 true, but threw plants of various heights. The experiment should be 

 repeated. 



2. Branching habit and the unbranched habit. Sun- 

 flower (Helianthus, Shull, 241) and Cotton (Balls, 6). The 

 branched form of Stock (Matthiola incana) is dominant to 

 the unbranched Brompton type. In F z the unbranched 

 type reappears, but the ratio has not been determined 

 (Fig. 4). E. R. Saunders (unpublished). 



3. The straggling habit of both the tall and dwarf 

 " Cupid " Sweet Peas, and the much-branched erect habit of 

 the " Bush" Sweet Peas (R.E.C. 22). 



The relation of these two types to each other is not altogether simple. 

 As described (g.v.) F-^ from Cupid x Bush is a reversionary form exactly 

 like the normal tall variety. Neither the tall varieties nor the Cupids show 

 the profuse branching of the Bush Sweet Peas which gives them their 

 peculiar appearance. This is evidently recessive to the unbranched 

 condition, and the fact thus stands out in contrast to those observed in 

 the case of Sunflower and Cotton. But in the Sweet Pea we have the 

 additional complication that the factor which represses the excessive 

 branching by its presence gives increase of height. The tall and the Bush 

 differ from each other in respect of this factor only. It is present in the 

 tall but absent from the Bush. In the cross between Bush and Cupid two 

 pairs of factors are concerned as explained in the passage referred to. 



4. Hairiness and glabrousness. Lychnis. Matthiola 

 (Stocks). Wheat. 



The case of Lychnis has been studied by de Vries (288) and R.E.C. 

 (19). In crosses between fully hairy and glabrous strains the discon- 

 tinuity is complete. Various forms intermediate in hairiness may 

 nevertheless be found wild and are by no means rare. Silene inflata 



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