PLANT CHROMOSOMES 679 



A mixture of 100 c.c. 7 per cent, urea in 

 absolute alcohol and 33 c.c. glacial 



acetic acid 3 parts 



Transfer to the following fixative, freshly prepared, for twenty 

 to twenty-four hours at 37° C. : 



Mix 75 c.c. saturated aqueous picric acid, 5 c.c. glacial acetic 

 acid and 2 grm. urea, heat to 37° C. and add 1-5 grm. of chromic 

 acid. To 3 parts of this, add 1 part each of 2 per cent, osmic acid 

 and 40 per cent, formalin. Wash in 70 per cent, alcohol. 



1359. Time at which to Fix. Mitosis is a relatively rapid 

 process and in root-tips shows a diurnal variation in its frequency. 

 Kellicot {Bull. Ton: Bot. Club, xxxi, 1904, p. 529) found Allium 

 root-tips had a maximum of divisions at 1 p.m. and 11 p.m., 

 and a minimum at 7 a.m. and 3 p.m. Karsten {Zeits. Bot., x, 

 1918, p. 1) found no rhythm in Vicia and Zea roots. 



See also Friesxer, Amer. J. Bot., vii, 1920, p. 380 ; Stalfelt, 

 K. Svensk. Vet. Akad. Handl., Ixii, 1921, p. 1. 



In general it is unimportant at what time of day root-tips are 

 fixed. 



INIeiosis is a much more prolonged process. Pachytene lasts 

 several days, but the later stages from diakinesis onwards are gone 

 through fairly rapidly. In the pollen mother-cells of many plants 

 the late prophase and metaphase stages are most frequently found 

 between 9 a.m. and 2 p.m. They cease earlier in warmer weather. 



1360. Fixation of Root-tips. Fresh roots are taken from an 

 actively growing plant. Collection is easiest from potted plants 

 when the roots have reached the outside of the ball ; such tips 

 are usually free from soil particles. To obtain root-tips from 

 plants growing in the experimental field it is a good plan to dig 

 a hole close beside the plant, fill it with a rich light compost (free 

 of weed seeds) and keep the plant well watered ; active root-tips 

 may be obtained in two or three weeks. The tips should be 

 removed about 5 to 6 mm. from the apex with a pair of sharp 

 forceps and immediately plunged into a small bottle holding 

 about 10 c.c. of the fixative. When enough tips have been 

 collected, place the bottle under a vacuum pump for a few 

 minutes to exhaust the air and aid penetration. Large roots should 

 be split longitudinally. Air roots with a mucous layer and roots 

 with a suberised root-cap should have slices shaved off their sides 

 with a safety razor blade. Kagawa {Proc. Imp. Acad. Japan, iii, 

 1927, p. 304) finds that if root-tips are narcotised in dilute aqueous 

 solution of chloral .hydrate or ether and washed in running water 

 before fixation, the chromosomes appear thicker and their con- 

 strictions more clearly defined. Fix in a Flemming or Navashin 

 fluid. 



1361. Fixation of pollen mother-cells may be carried out by an 

 aceto-carminc method (q.v.), a sjnear method (q.v.), or fixation of 



