Appendix E 435 



presence seems an absurdity. However, to make the facts 

 consistent with the presence and absence hypothesis, two expla- 

 nations are offered. One consists in assuming the presence of a 

 positive inhibitory factor, which prevents the production of the 

 character concerned. The other consists in assuming that one 

 "dose " of the factor concerned is insufficient to produce the 

 result, hence in its simplex or heterozygous condition, the char- 

 acter determined by the factor fails to appear, and it is only 

 when the factor is in the duplex or positively homozygous con- 

 dition that it does appear. The first of these explanations is 

 embodied under demonstration (d). The last is embodied 

 under the demonstration entitled "Explanation of So-called 

 ' Dominance of Absence.' " 



(d) Demonstration of the Presence of an Inhibitory Factor 

 (Fig. 112). 



A contains 10 cc. 2.5 % NH^OH + 2 drops phenolphthalein. 

 Ai equals A + 5 cc. 10 % HCl. 



(e) Explanation of So-called "Dominance of Absence" 

 (Fig. 113). 



A contains 10 cc. 10% NH 4 OH + 6 drops phenolphthalein. 

 a contains 10 cc. 10 % HCL 



After the zygotes of F 2 are obtained, in this last demonstration, 

 the instructor should add 10 cc. 10 % NH^OH to each Aa 

 zygote of F 2 to show that another "dose " of factor A will now 

 produce the result. 



EXERCISE 15 



A Study of Mendelian Characters in Timothy and Oats 



Purpose. To afford the student concrete illustrations of 

 Mendel's laws ; to find unit characters in plants and to see their 

 segregation and recombination. 



Materials. Mature timothy plants of various strains, com- 



