EFFECTS OF EXPOSING GERM-CELLS TO RAYS OF RADIUM 255 



In conclusion then, the double plant may be interpreted as, in 

 reality a hybrid between elementary forms, the characters of one of 

 which had had expression only potentially in the conjugating gamete. 

 In normal cases of hybridity every cell in a given plant of the first 

 generation resulting from the cross is of hybrid nature. The charac- 

 ters of the two parents are diffused throughout the entire organism, 

 though, in Mendelian cases, they may segregate in the I^^ and sub- 

 sequent generations, according to the well-known laws of Mendel. 

 But, in the case of the double plant, segregation of characters 

 occurred before the embryo was complete. Not every cell in the 

 mature plant, then, is of hybrid nature (considering merely the 

 characters that distinguish the two shoot systems), though hybridity 

 may be postulated of the organism as a whole. 



Discussion of the possible causes that might produce a plant with 

 two different shoot systems on one root should not be closed without 

 reference to the case of anomalous mitosis illustrated in plate 6, 

 FIGURE I, and described on page 233. There it is seen that two 

 mitotic spindles have developed within one cell. Without going into 

 details, it is merely noted here that such a result in nuclear division 

 suggests that some such departure induced in a fertilized egg-cell by 

 radium rays, might have been the first step in the production of the 

 double plant of Onag7'a biennis. 



Heritability OF Induced Changes : Carefully guarded, and 

 also unguarded* seed, of the aberrant forms 10a, 11b, i^a broad, 

 and i^a broad x i$a narrow were collected, and the second gen- 

 eration of plants is now (June, 1908) under observation. Attempts 

 to secure seed from 15a narrow were unsuccessful, as were also 

 attempts to cross this form with pollen from 15a broad. Seed was 

 obtained, however, by crossing i^a broad with pollen from i^a 

 narrow, but only one seedling from this cross appeared in the seed 

 pan. 



Thus far inheritance of the variations is manifested in only one 

 instance, viz., 11b. Two plants in the second generation, from 

 unguarded seed resemble the parent in every fundamental character, 

 while a third resembles it in every way except in the flower-bud, 

 where the character of the stout, spreading tips of the sepals is lack- 

 ing, being replaced by the biennis character. It is proposed to con- 



* Mj absence from these plants at the time the seed was maturing resulted in the 

 failure to secure carefully guarded seed in some instances. 



