EXTRA cniiOMOSOMES — BLAKESLEE 441 



DOUBLE (211 + 1 + 1) TYl'ES 



III luklitioii to tyi)os with a sin«;le chroiiiosuiue oxlra, tomhiiui- 

 tions of these types in double (2n+l+l) forms are known. The 

 simplest double type is a combination of two primaries. In Plate 



11 is shown a capsule of the double type " Ilex-Globe " to«;ether with 

 capsules of the (*in + l) types " Ilex " and " Globe ", and of a normal 

 (2n) for comparison. The adjoininj^ diagrams show the chromo- 

 somes. In Ilex the smallest chromosome is extra. Among other 

 eifects, it reduces the size of the capsule and causes the si)inos to be 

 slender. In Globe, the next to the smallest chromosome is extra. 

 The extia Globe chromosome is res[)oiisible for a depressed capsule. 

 In Ilex-Globe, in which both the Ilex and the Globe chromosomes 

 are extra, the capsule has the globose shape due to the Globe chromo- 

 some and the fine spines and small size due to the Ilex chromosome. 

 The characters of the cap^ide ami other parts of the plant in d(Mible 

 mutant types are the resultant of the interacting factors in the two 

 extra chromosomes. 



Double (2n+l + l) types are occasionally found occurring spon- 

 taneously. They are found in large numbers in the offspring of 

 triploids (3n) along with the 12 primaries and normals (2n). Plate 



12 shows capsules from primary and double chromosomal types 

 which appeared among 1,152 olispring of triploids (3u) grown in 

 1925. The double types which were recognized at that time by other 

 characters but which had no capsules suitable for the photograph 

 are indicated by a +. Other double types which have been found 

 in later cultures are indicated b}' a ( + ) in parentheses. The lower 

 horizontal row contains capsules of 11 of the 12 primary (2n + l) 

 types. The twelfth (Sp) is less common than the rest and was 

 omitted from the photograph. Each double type nniy be consid- 

 ered a combination between two primary (2n+l) types, since it has 

 two extra chromosomes in different sets corresponding to the extra 

 chromosomes in the two respective primaries listed at the ends of 

 the intersecting columns. 



COMPENSATING TYPES 



In compensating types there may In; only a single extra chromo- 

 some, so far as the counts show, but two modified chromosomes are 

 actually present. The condition may be illustrated by the compen- 

 sating type "Nubbin "' (Nb). A capsule of Nb, together with cap- 

 sules of the types which it throws in its offspring, is shown in Plate 

 13, Figure 1. Bk, Kl, and I2c are primary (2n + l) tyi)es and need 

 not concern us here. Ph and Ilg are tertiary (2n + l) types which 

 have as their extra chromosomes the modified chromosomes present in 

 Nb. The nuitter will be made clearer by the diagrams of the chromo- 

 somes of Nb shown in Figure 5. The Kollcd (Kl) chromosome (1 • 2) 



