148 PLANT GROWTH 



duced doubling of chromosomes in a number of plants in- 

 cluding: Datura, Portulaca, Cosmos, Phlox, Petunia, Digi- 

 talis, and Cucurbita. 



A very successful student experiment with radish seeds 

 was carried out by putting dry seeds in a i per cent solution 

 of colchicine for six hours. The seeds were then washed in 

 water and planted. Other concentrations and length of 

 soaking period can be tried. Not all seeds require the same 

 treatment. In this experiment nearly 50 per cent of the 

 treated seeds showed signs of having been properly treated. 

 About 25 per cent of the treated seeds were killed and about 

 the same number appeared as normal. The treated seedlings 

 had the characteristics of the cosmos seedlings shown in 

 Plate IX. The plumule and the root are very slow in start- 

 ing to grow, and may be so severely injured that the plant 

 will die before it begins to grow properly. After the seedling 

 stage growth is better and the plants are likely to survive. 

 The leaves are often wrinkled, indicating regions with larger 

 cells due to the increased chromosome number. 



Simple methods have been found to detect the plants in 

 which a mutation is caused by colchicine. In addition to 

 abnormal appearance of the leaves, microscopic examination 

 shows the leaves to have larger cells, and as a direct result 

 fewer, larger stomata in a given area. Pollen studies made 

 of most of our common plants show that the colchicine treat- 

 ment causes the pollen grains to be uniform and larger in 

 size and to contain increased numbers of chromosomes. 



The importance of these simple methods of inducing poly- 

 ploidy may prove that plant improvement by this method 

 may be very nearly as important as were Mendel's laws. 

 Larger flowers, fruits, seeds, stems, leaves, and roots have 

 been grown in this way. It has also been used in many cases 

 to build closely related plants with a difference in their chro- 

 mosome number into plants with a like number, which is 

 necessary before they can be crossed successfully. 



