CHAPTER LI 



PLANT CHROMOSOMES f 



1355. Many methods adapted to rapid chromosome counting, 

 though useful for genetical purposes, are not suitable for exact 

 studies of the nuclear cycle. But the line of demarcation is not 

 sharp and numerous workers have used inferior methods for 

 supposedly critical work. It is a common mistake to suppose 

 that because cells appear unshrunken and the clu'omosomes 

 distinct at metaphase, all other cell division stages (and cyto- 

 plasmic structures) must be correctly preserved. 



Belar (Zeits. Ind. Abst. Vererb., Suppl. 1, 1928, p. 402), from a com- 

 parison of living and fixed material, finds that even the best technical 

 methods produce more or less artificial changes in leptotene and inter- 

 kinesis stages. It IS imsafe to draw conclusions concerning the nuclear 

 cycle from material fixed by the older methods, involving unopened or 

 even opened flower buds, whole anthers and cells in depths of root -tips 

 in higher plants, or comparable conditions in lower plants. Possibly 

 there has been no really satisfactory fixation of many stages of mitosis 

 in plants owing to the difficulty of securing immediate access of the 

 fixative to the cells. Probably considerable success would attend fine 

 choppmg of root -tips under the surface of the fixative. 



The nuclear structures are often well preserved although the 

 cytoplasm is badly fixed. But the space relation between the 

 formed elements is then often seriously disturbed and, unless the 

 karyolymph is well and finely precipitated, the chromosomes 

 tend to clump and contract. Exclusion of such artifacts is of great 

 importance in studies of somatic pairing and of secondary pairing 

 at meiosis in allopolyploids. Organic adjuvants, such as urea, 

 various sugars and malic acid, assist in fixing the karyolymph, in 

 preventing clumping, and in maintaining the spaces between 

 longitudinally split chromosomes and between closely paired 

 chromosomes in meiotic prophases. 



The best standard of quality of a preparation is experience. 

 In general, with a given material, greater ease of observations 

 denotes a better preparation. Direct comparison with living cells 

 can be made, but the technique of such observations and of those 

 on material under weak intravital staining is very difficult ; 

 further, the basis of interpretation is different from that involved 

 in the examination of material in resinous media. 



The karyolymph should be coagulated as a fine granular soft mass, 

 thereafter requiring gentle hardenmg in close grades of alcohol. The 



t Read § 621 et seq. 



674 



