268 Colchicine 



matic changes into apparently normal restitution nuclei (6 hours 

 after irradiation) . The nuclear membrane may give some protection 

 to the prophasic chromosomes. 



The results of these changes on the growth of the root tips and 

 of the leaves of bvdbs of Allium cepa have been studied.^- Exposure 

 to 0.01 per cent solutions of colchicine induces the well-known root 

 tip swelling, the so-called c-tumors, and when the plants are replaced 

 in water, growth is resumed. If the root tips are irradiated with 900 

 or 1500 r. after 48 hours of colchicine, growth is arrested and leaf 

 development is strongly impaired. These effects are greater than those 

 obtained by irradiation alone. The action of X-rays appears to be 

 independent of the nuclear division stage. After 48 hours of colchi- 

 cine, "some non-recognizable toxic effects in the cell . . . sensitize it to 

 irradiation." * The same author has published detailed results of 

 investigations on the combined action of colchicine and X-irradiation 

 on onion root tips.^'^ It appears evident that the two actions add 

 their effects, but the mechanism is not clear, and does not seem to 

 be related to an increase of mitotic cells at the time of irradiation. 

 For instance, the 48-hour colchicine bulbs are more vulnerable to X- 

 treatment, "even though the time of exposme occurred when the 

 number of dividing cells had passed the peak of metaphase arrest. "f 

 Irradiation by 900 r., which has only a temporary retarding effect on 

 growth, inhibits completely cellular multiplication and growth with- 

 out any immediate death of the tissues when the roots have been pre- 

 viously treated for 48 hours with a 0.01 per cent solution of colchi- 

 cine. A long exposiue to the alkaloid seems necessary, for, "while 

 colchicine causes analogous cytological changes at 6, 12, 18, 24 and 

 48 hours, the larger exposures induce some microscopically unrecog- 

 nizable alterations. This . . . arrests growth permanently and com- 

 pletely [with 1500 r.]"t The oiJiimum growth-inhibition effects 

 were observed after 1500 r. and a more than .^6 hours' exposure to 

 colchicine. 



On the other hand, onion bulbs treated for 45 minutes in a 0.05 

 per cent solution of colchicine, then irradiated with 300 r. and re- 

 placed in the solution, showed less chromosome rearrangements than 

 controls, while the number of breakages was not appreciably altered. 

 It is supposed that the short colchicine treatment could not have in- 

 creased the metaphases. but impairment of the sjMudle function may 

 slow the movements of chromosomes. This would leave less oppor- 

 tunity for the broken ends to reunite into abnormal structures. ^^ 



* M. Levine, "The Action of Colchicine on Cell Division in Human Cancer, 

 Animal and Plant Tissues." Ann. N. Y. Acad. Sci., 51 (1951) , p. 1400. 

 j-Ibid., p. 1397. 

 % n>i(l.. p. 1399. 



