624 ON PHYSIOLOGICAL CHROMOMERES. 



Among the others some have homogeneous pollen and ovules as, 

 e. g., blandina and decipiens and the mutants with brittle fibres, 

 viz. deserens, tarda and fragilis. Others must be considered to be 

 the half mutants of these, and among them problandina, erythrina 

 and rubrinervis may be quoted here. Still others have homogeneous 

 pollen (at least so far as the functional grains are concerned), but 

 two types of ovules, one of which reproduces the race after selffer- 

 tilization, while the other results in barren seeds, as in 0. simplex 

 and its derivatives. Finally the new mutant 0. pulla, mentioned 

 above, is to be included in this class, although the number of its 

 chromosomes is 15. 



The characters of almost all of these mutants behave like those 

 of Drosophila and other organisms; they appear singly or in very 

 small complexes. Shull has shown that the methods of Morgan may 

 be applied successfully to their study and in doing so he could 

 prove the existence of linkages between about a dozen of factors 

 of this category. By this means he found connections between the 

 dwarfish stature, the red hypanthium of 0. rubricalyx, the red cone 

 and green hypanthium, the buds without pigment, the intensely 

 reddened stems, the sulfur colored flowers, the revolute leaves of 

 0. funifolia, the basal ramification of 0. Lamarckiana which is 

 absent in 0. simplex, the narrow petals and four lethal factors (one 

 in the pollen and one in the eggs of 0. biennis and two in the germs 

 of 0. Lamarckiana). As far as investigated all of these marks are 

 borne by plants with 14 chiomosomes. All of them belong to one 

 linkage group, which is evidently the same as our central group of 

 mutants. 



The most interesting members of this central group of mutations 

 are perhaps the lethal factors of the two components of (Enothera 

 Lamarckiana, viz. laeta and velutina. They are called balanced zygote 

 lethals. Whenever a germ gets the same of them from both the 

 paternal and maternal sides, it will die after a few celldivisions, 

 although the walls of the seed may develop normally. But when 

 the germ receives one lethal from the paternal and the other from 

 the maternal side, the effects are not combined and the normal 

 development is not endangered. Thence the name of balanced lethals, 

 proposed by Muller. These lethals are linked to groups of visible 

 characters, one of them to broad leaves and green stems a. s. o., 

 the other to narrower leaves, a reddish foliage, slender spikes, much 

 branched stems and others. Gametes of the first kind give hybrids 

 of the type of laeta, whereas the others result in hybrids of the aspect 



