io BOTANICAL GAZETTE [jaxuarv 



Under normal conditions the splitting of this latter species occurs 

 in nearly equal groups of both the twins, but as a matter of fact 

 the external circumstances are, with us, often such as to diminish 

 the percentage of laeta quite sensibly. These deviations, however, 

 have been amply studied in my work on Gruppenweise Artbildung; 

 they show what size the cultures must be in order to prove the 

 absence of laeta. I cultivated 60-80 offspring for each culture, and 

 repeated the same cultures during 2 years, making on the average 

 140 specimens for each cross. The first year I compared them with 

 the hybrids laeta and velutina derived from the corresponding 

 crosses of my new dimorphic mutant O. cana* which happened to 

 be at hand in a complete set; but the second year I have sown the 

 twin hybrids issued from crosses of 0. Lamarckiana itself with the 

 same species as were used for the crosses of O. blandina. The 

 material is externally the same in both cases, and quite as good for 

 the comparison, but in the latter instance the proof is a more direct 

 one. I grew the hybrids of O. blandina and the control cultures 

 side by side, and compared them from the beginning of germi- 

 nation in February until the ripening of the first fruits in Septem- 

 ber. The differences between the two twins of a cross are large 

 and striking, 5 from their very first leaves, and it was therefore 

 impossible that a laeta among the hybrids of 0. blandina could have 

 escaped observation. As a matter of fact, no single laeta appeared, 

 although altogether over 500 hybrids were cultivated. 



For these crosses I chose 2 heterogamic and 2 isogamic species, 

 and made the combinations in both groups in the opposite direc- 

 tions, so as to use 0. blandina twice as a pistil and twice as a pollen 

 parent. I made these pollinations in 1913, in the third generation 

 of my race, choosing the most vigorous individuals which had had 

 a normal green color from their very youth. The crosses with the 

 heterogamic species were O. syrticola Bartlett 6 XO. blandina and 

 0. blandinaXO. biennis Chicago. Those with the isogamic forms 



*De Vries, Hugo, New dimorphic mutants of the Oenotheras. Bot. Gaz. 

 62 : 249-280. j£g,y. 5. 1916. 



s For descriptions and photographs as well as for colored plates of the twins, see 

 Gruppenweise Artbildung. 



6 This is the O. muricata L. of my Gruppenweise Artbildung. 



