A Simple Chemical device 



149 



the same as the blue ones with a single added unit of acid. Have 

 one test-tube rack for the pollen-parent which produces the 

 sperms or male germ-cells (all red for instance), and the other 

 for the pistil-parent which produces the eggs or female germ- 

 cells (all blue). Then take the test tubes with the red solution 

 one by one and empty them into those having the blue solution, 

 explaining that similarly in fertilization a sperm meets and 

 fuses with an egg in the formation of a new plant. The result 

 of this process is the production of four red solutions representing 

 the first generation hybrid between the red and the blue solutions; 

 i. e. the dominance of the red or acid over its absence. * 



Next explain the production of the second generation hy- 

 brids, thus: Each vegetative cell of the first generation hybrid 

 contains side by side the elements received from the two parents, 

 one of these having a unit of acid the other lacking it. When 

 the germ-cells of these hybrids are being formed these elements 

 received from the parents are separated again so that half the 

 eggs have the acid unit and half lack it, and in the same way 

 half the sperms have the acid and half lack it. Illustrate these 

 germ-cells of the hybrids by placing in the "female" test-tube 

 rack two test-tubes one-quarter filled with the original blue 

 solution, and two one-quarter filled with the original red solu- 

 tion. In like manner illustrate the fact that the male germ 

 cells are of two kinds, half having the acid unit and half lacking 

 it, by a similar set of test-tubes in the "male" test-tube rack, 

 two with the blue solution and two with the red. As the test- 

 tubes are taken one by one from the "male" rack and emptied 

 into the test-tubes on the "female" rack, see that the combina- 

 tions are fully understood. One sperm having the acid unit 

 fertilizes an egg which also has the acid unit, illustrated by pour- 

 ing the red solution from a test-tube of the "male" rack into one 

 with a red solution on the "female" rack, resulting in a pure- 

 bred red, (positive homozygote). Another red or acid-bearing 

 sperm meets an egg which lacks the acid, as the test-tube con- 



*It may seem better, pedagogically to have blue" 1 "dominate" over red, as it actu- 

 ally does when blue and red flowers are crossed. This can be easily managed by making 

 the original solution which represents the negative homozygote acid for this particular 

 experiment, adding two equal portions of alkali to produce a blue positive homozygote. 

 As the present experiment was dev'.sed to show the dominance of absence over presence 

 it was thought best to make (B) coincide with actual experince in blue X red crossds, 

 giving 3 blue to 1 red. The experiment is equally apt ether way and no misapprehension 

 need result. 



