558 DOUBLED CHROMOSOMES OF OENOTHERA 



are distributed among the two daughter nuclei according to the 

 laws of chance. 



A main condition in this discussion has been that all or almost 

 all the seeds are viable. If this is not so, the result may be different. 

 In the case of semigigasxvelutina, only about one-quarter of the 

 seeds were well developed, and the question arises, what influence 

 this may have on the figures. In order to answer this, we assume 

 that in our case the simultaneous occurrence of two or more doubled 

 rods in the nuclei of a seedling is detrimental for its development. 

 The doubling is assumed to be connected with the same mutations 

 of the normal characters with which it is connected in the old hetero- 

 gamy races, and these mutations are probably more or less in- 

 compatible with one another. We may conclude this from the fact 

 that mutations with 15 chromosomes, that is, with one doubled rod, 

 are frequent, whereas those with 16—20 chromosomes as yet have 

 not or almost not been discovered outside of the progeny of semigigas. 

 Their chance of survival, therefore, must be very small; most of 

 them, when produced in the germ, must be lost before the ripening 

 of the seed. 



As yet there is no reason to assume a lessened viability for plants 

 with only one doubled rod. They must be as good, in this respect, 

 as the normal races with 15 chromosomes. This leads us to expect 

 a high proportion of seedlings with this number, and gradually 

 smaller figures for those with more chromosomes. 



Number of double chromosomes among progeny of 

 Oenothera (lata x Lamarckiana) semigigas 



Our culture embraced 81 seedlings, which were planted singly 

 in small pots as early after germination as possible. As soon as a 

 sufficient number of root leaves had developed, root tips were 

 prepared, and thereupon the young plants were placed in the beds 

 of the glass covered part of the garden, without their pots. Their 

 development differed very much in rapidity; accordingly the fixa- 

 tion was performed at different times in May and June, 1923. 



The fixations were made early in the morning, using Bouin's 

 fluid. The preparations measured 5—10 ft, mostly 10 p, in thickness, 

 and were stained with the haematoxylin solution of Haidenhain. 

 The general results of the counting were as follows: 



Chromosomes 14 15 16 17 18 19 20 



Number of plants 3 35 19 13 3 4 4 



