EXTRA CHROMOSOMES — BLAKESLEE 435 



the 4ri plants have become ?ct up with capsules and ceased flowering. 

 The In and 3n types, therefore, are of floricultural vahie in species 

 in which it is possible to have a continuation of flowering by re- 

 ducing seed production. Since such unstable balanced types can not 

 breed true it would be necessary to propagate them by vegetative 

 methods such as by grafts, cuttings, bulbs, etc. 



Tetraploids (4n) arc of horticultural value from the large size of 

 (licir flowers. To the plant breeder they are of value as the means 

 of obtaining triploids (3n). The cross 4n x 2n produces 3n or tri- 

 ploid individuals. From the latter may be obtained a wide range 

 of types with extra chromosomes as will ha shown later. 



UNBALANCED CHROMOSOMAL TYPES 



In the unbalanced chromosomal types not all the chromosomal 

 sets have tlie same number of chromosomes. Some of the types are 

 shown in the diagrams in Figure 2. The simplest type is a modified 

 diploid or 2n plant with an extra chromosome in one of the 12 sets. 

 For comparison, diagrams of the chromosomes of a normal 2n plant 

 from our standard line 1 are shown in Figure 3. All except two are 

 in outline only. We will center our discussion upon these two 

 chromosomes. The largest (L) for convenience may be called the 

 white chromosome, and the small, medium one without the hump 

 (w), which is stippled in the diagram, will be called the stippled 

 chromosome. One half of each of these chromosomes has been 

 shaded to distinguish it from the other half. The outer ends of each 

 half are marked by a black dot. Models of the same chromosomes 

 are shown in Plate 5. 



The ends of all the chromosomes in the Jimson weed have also 

 been designated by numbers 1 to 24. Thus the white chromosome, 

 which is the largest of the 12, is numbered 1 • 2. Similarly the 

 stippled chromosome is represented by the numbers 17 • 18. In all 

 cases the odd number corresponds to the shaded half of the models. 



riilMARY, SECONDARY, AND TERTIARY (2N+1) TYPES 



If to the set of two white chromosomes, an extra member (1-2) 

 like the other two is added as shown by the models (pi. 5 -^1), we have 

 a (2n4-l) type of distinct appearance represented by our " Rolled " 

 mutant. From the peculiarities of this type we may learn some- 

 thing about the aggregate of factors contained in the white chro- 

 mosomes. A type, named " Sugarloaf," is known in which the 

 chromosome added to the white set consists of two unshaded halves 

 (2 ■ 2) joined together as shown in Plate 5 B. The characters of the 

 mutant type which results tell us something about the factors in the 

 unshaded (or 2-2) half of the white chromosome. A type, named 

 " Polycarpic," complementary to that just discussed, has the extra 



