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FUNDAMENTALS OF CYTOLOGY 



the stages when the embryos are beginning their development, some of 

 these embryos may develop into tetraploid plants instead of diploids. 

 Abnormal temperature, either high or low, may induce ameiosis in flower 

 buds, so that diploid spores and gametes become available for crossing. 

 When some plants, notably tomatoes, are decapitated, the shoots arising 

 from the callus tissue are often tetraploid. At present the most popular 

 method for inducing tetraploidy and higher stages of heteroploidy is 

 the one employing colchicine. This substance, which is an alkaloid 



/ / 



Fig. 152. — Induced autopolyploidy in maize. Left: normal diploid plant, an ear from 

 such a plant, and diploid chromosome complement in root tip. Right: tetraploid plant 

 from kernel on ear subjected to heat-treatment during early stages of embryo development; 

 also mature ear and tetraploid chromosome complement in root tip. {Courtesy of L. F. 

 Randolph.) 



derived from the autumn crocus, may be applied by painting very young 

 buds with a lanolin emulsion containing it, or by standing cut shoots or 

 roots in an aqueous solution for brief periods, or by spraying young 

 plant parts with such a solution, or by treating seeds before planting. 

 Many useless malformations may result from such treatments, particu- 

 larly when the dosage is too high, but when the technique is sufficiently 

 refined tetraploid shoots may appear. Colchicine produces its effect by 

 preventing the formation of the mitotic spindle. The chromosomes pass 

 through their division cycle as usual, but since no spindle is developed 

 their halves undergo no anaphasic separation and reorganize as a single 



