BREAKAGE AND REUNION 69 



Thus we see frequently that trisomic plants give rise to diploid 

 tissues or whole shoots (Crepis, Navashin, 1931 a ; Sphcerocarpus, 

 Knapp, 1935 h). In high polyploid mosses artificially produced, 

 Wettstein (1924) found that chromosomes were frequently lost. 

 This " vegetative regulation " is probably the result of a similar 

 competition of cells with different nuclear complements. 



4. STRUCTURAL VARIATION 



(i) Classification. The permanence in structure of the chromo- 

 somes is due to the permanence in linear order of a series of different 

 particles, one of which must always be a centromere, the rest 

 chromomeres. We can then represent two chromosomes by taking 

 a letter to stand for each chromomere and an unspaced colon for 

 the centromere, as ah : cdef and ghj : klm. 



We may postulate several types of change in the structure of the 

 chromosomes that are compatible with this linear arrangement, and 

 with having one centromere in each chromosome. The following 

 are the simplest : — 



(i) Inversion of de segment : 



ah : cdef —> ah : cedf. 



(ii) Translocation of the intercalary de segment, either from one 

 arm of a chromosome to the other, or to an arm of another 

 chromosome, as : 



ah : cdef + ghj : klm -^ ah '. cf -\- ghj : kdelm. 



(iii) Deficiency or loss of part of a chromosome following its 



breakage, 

 (iv) Interchange between arms of different chromosomes : 



ah : cdef -\- ghj : klm ah : cdlm + ghj : kef 



Interchange between two arms of one chromosome is the same 

 thing as inversion of the centric or centromere-containing segment 

 between them. 



The detailed evidence of meiosis shows that these a priori types 

 of structural change actually occur. The evidence of their occur- 

 rence that can be derived from mitosis, however, is incomplete, 

 indirect and sometimes even misleading. The changes that are 



