148 



The Plaxm World. 



To a moderately dilute solution of litmus add a small quan- 

 tity of sodium hydroxide or aqua ammonia. In the presence 

 of the class, divide this solution into two of the flasks, calling 

 attention to the fact that the same solution is put into both. 

 Then add to the one flask in the presence of the class a certain 

 quantity * of hydrochloric acid, which changes the solution from 

 blue to red. See that it is perfectly clear to every member of 

 the class that there is now just one difference between the solu- 

 tions in the two flasks, namely, the one contains a certain portion 

 of acid, the other has none. As it is desired that the solutions 

 in these two flasks should represent respectively the negative 

 and positive homozygotes which are to be crossed together, 

 and as they are homozygotes because they inherited the same char- 

 acter from both parents, the acid should be added in two equal 

 portions with the explanation that one acid-producing unit or 

 gene came from the mother and one from the father. The 

 experiment will be rendered more impressive if a red flower and 

 a blue flower of any variety, such as the sweet-pea, is standing 

 in a vase on the teacher's desk, for then it may be explained that 

 while it would take two years to demonstrate Mendelian in- 

 heritance by an actual cross between these two flowers, the whole 

 process may be shown in a few minutes by letting the blue 

 solution represent the blue flower and the red solution the red 

 flower. See that the class appreciates the fact now that just 

 as in the case of the solutions, the red and blue flowers dilTer 

 from each other in only one quality, one-half of which came from 

 one of its parents, the other from the other parent. 



Several different experiments may be performed as follows : 

 (A) To demonstrate the more common type of Mendelian 

 inheritance, that in which presence is dominant over absence: Add 

 sufficient acid to the solution in the one flask to change the color 

 from blue to red, and then add a second equal quantity. These 

 two solutions now represent the two plants to be crossed. Fill 

 four test-tubes one-quarter full from each flask and let these 

 represent the germ cells of the two plants to be crossed. See 

 that is it still kept in mind that the red germ-cells are exactly 



*The amount of acid to be added will differ according to the requirements of the 

 particular experiment to be performed, and this must be determined by the teacher in a 

 preliminary experiment. 



