Morphologie etc. — Varietäten etc. 279 



which may possibly be attributed to contact with the tips of the 

 recurved involpcral bracts. This recurving of the bracts may be 

 the result of limited Space due to the subtending structures. 



The spines of the bur are probably modified floral bracts. 



The beaks seem to be modified portions of the receptacle. 



The terminal heads became staminate, because the vascular 

 supply was inadequate to compensate for the excessive transpira- 

 tion, and hence the pistils have aborted. 



The axillary heads became pistillate by the abortion of stamens, 

 owing to the pressure and crowding incident to the formation of the 

 flowers in depressions. 



Many of the characters in which the pistillate and staminate flo- 

 wers of Xanthium differ have been causative factors in the origin 

 and development of dicliny in this form. Jongmans. 



Bartlett, H. H., Additional evidence in Oenothera. (The Bot. 

 Gazette. LIX. p. 81—123. 17 Fig. 1915.) 



The principal conclusions of these researches are suramarized 

 by the author as follows. 



Oenothera pratincola, a recently described small flowered self- 

 pollinating species from. Kentucky, is in a mutating condition 

 comparable with that of 0. Lamarckiana. 



The most striking of the mutations, O. pratincola mut. nummu- 

 laria, occurred in strains derived from 7 wild mother plants out of 

 8 selected at random. 



In two of these strains the mutation was found in both the F^ 

 and F2 generations from the parent plant. In a third strain the 

 mutation was found only in the F2 generation, but a sufficient num- 

 ber of Fl plants had not been grown to insure its detection in that 

 generation. 



Mut. nummularia appears to occur with a frequency of about 

 one individual to each 300—400 seeds planted. The several proge- 

 nies showed no significant Variation in the mutation ratio. 



The mutation ratio cannot be explained on Mendelian grounds. 



Mut. nummularia is better adapted than the parent type to 

 withstand influences unfa vorab! e to germination. In every case 

 where a progeny contained an unexpectedly large number of mu- 

 tations, the germination was correspondingly poor. 



Selective germination and differential mortality among dormant 

 seeds may be important factors in natural selection. 



Mutation is a distinct progress from Mendelian segregation, and 

 the phenomena exhibited by Oenothera Lamarckiana, O. biennis and 

 O. pratincola cannot be attributed to heterozygosis. Jongmans. 



Bartlett, H. H„ Mass mutation in Oenothera pratincola. (The 

 Bot. Gazette. LX. p. 425-456. 15 Fig. 1915.) 



Mass mutation consists in the production of unexpectedly large 

 numbers of mutations, in some cases amounting to 100 per cent of 

 the progeny. 



The phenomenon is known in two species of Oenothera: O. 

 Reynoldsii, in which it was first described, and O. pratincola, the 

 subject of this paper. Four characteristic mutations have been found: 

 mut. formosa, mut. albicans, mut revoluta and mut. setacea. Beside 

 these some other less characteristic mutations were observed. 



