402 University of California Publications in Agricultural Sciences [Vol.2 



C. hursifolia, C. taraxacifoUa Thiiill. X C. iectorum L., and C. aspera 

 X C. aculeaia (DC.) Boisii. The five species used for the crossings 

 have rj = 4 chromosomes. The chromosomes in the four species are 

 very similar in size during meiosis, except in C hiosifolia, where one 

 chromosome is conspicuously shorter than the others and difVerent 

 from them. 



The material for the present investigations was procured by the 

 senior author, who made the taxonomic studies and wrote section 5, 

 while the junior author made the cytological investigations and wrote 

 the remainder of the paper. We acknowledge with gratitude our 

 indebtedness to Mr. C. W. Ilaney, who made the crossings and pro- 

 vided observations on the fertility of the hybrids, and to Miss L. IIol- 

 lingshead, who prepared the fixations of C. taraxacifoUa X tectonim 

 and C. aspera X acnleata. Miss Hollingshead also made pollen counts 

 of the hybrids. The cytological investigations were carried out during 

 the stay of the junior author at the Division of Genetics, University 

 of California, as a Research Fellow of the International Education 

 Board. 



METHODS 



The material of Crepis taraxacifoUa X teciorum and C. aspera X 

 acideata was fixed in Carnoy's fluid. This material could not be 

 used for paraffin sections and subsequent staining with Heidenhain's 

 haematoxylin or with iodine-gentian violet because the stain disap- 

 peared from the chromosomes as soon as from the plasm, making 

 differentiation impossible. For these hybrids Belling 's iron-acetocar- 

 mine method was successfully applied by Hollingshead to Crepis 

 material fixed in Carnoy in order to have reserve material for later 

 examinations and also in order to overcome the difficulty involved in 

 finding time enough to do all the cytological investigations during the 

 period in which reduction division takes place. 



The acetic acid of the acetocarmine causes the pollen mother cells 

 and the chromosomes to expand, thus overcoming the contraction and 

 collapsing commonly caused in species hybrids by the Carnoy fixation ; 

 and in many cases the acetocarmine gives a clear differentiation 

 between chromosomes and plasm, especially when Belling 's mono- 

 chromatic gi'een light is applied. For the large chromosomes of 

 Crepis the differentiation gained through the acetocarmine method is 

 sufficiently distinct to determine pairs and univalents of chromosomes, 

 and in determining the tetrad stages it is much more distinct and 



