Darling: Sex us dioecious plants 183 



Wilson ( 



differences of the chromosome groups, divides the known cases 

 into five classes as follows : 



1. '' Both sexes with the same number of chromosomes, a pair 

 of equal idiochromosomes present in both. No visible difference 

 between the two classes of spermatozoa or between the male and 

 female somatic groups/' 



2. '* Both sexes and both classes of spermatozoa with the same 

 number of chromosomes ; the male with a pair of unequal idio- 

 chromosomes, half the spermatozoa receiving the large one and 

 half the small one." 



3. **The female chromosome group with one more chromo- 

 some than the male. The male with an unpaired idiochromosome 

 and an odd spermatogonial number, half the spermatozoa receiving 

 the idiochromosome and half being without it," 



4. Female group (by inference only) with two more chromo- 

 somes than the male. In the male a pair of unequal idiochromo- 

 somes, half the spermatozoa receiving both these idiochromosomes, 

 and hence two more than the other half." 



5. '^ Female group with three more chromosomes than the 

 male. Half the spermatozoa receiving three more chromosomes 



than the other half." 



Wilson ('06) has been able to show very clearly a progressive 

 series from forms in which there is a visible difference in number 

 or in size of the idiochromosomes to those in which there is no 

 visible difference in either number or size, as for example those 

 included in his first class. He regards this class " as very important 

 as a sort of connecting link to forms, perhaps the more usual, in 



which there is no visible difference. 



>> 



It is generally believed that in plants the chromosomes are 

 formed by the segmentation of the spireme thread. The careful 

 work of Allen ('05) on Lilmm, together with the works of others 

 on the same genus, seems to leave no doubt that in these forms 

 all of the chromosomes are formed in this manner. In the work 



( 



Mano 



Solaniim and Pkaseohis, and in fact in all of the literature 

 examined, I have found no case where any of the chromosomes 

 are reported to be formed in any other manner. A possible 



