88 THE CHROMOSOMES 



biennis two rings (of eight and six chromosomes 

 respectively) and in Oe. franciscana a ring of four 

 and five bivalents ; Oe. Hookeri is completely homo- 

 zygous and forms seven bivalents. We can represent 

 the chromosomes of the first two species as follows 

 (the ' complexes ' are called velans and gandens in Oe. 

 lamarckiana, rubens and albicans in Oe. biennis) : 



Rings of four, six or more chromosomes are formed 

 at the first meiotic division in a number of other 

 plant genera {Campanula, Pisum, &c.) and from a 

 study of these cases (some of which, notably Pisum, 

 have arisen under experimental conditions) it has 

 been possible to explain the origin of the whole 

 mechanism (Chap. VI). 



