112 THE THEORY OF THE GENE 



former, eggs and pollen are produced on the same plant. 

 Hence if a plant lias started as a tetraploid, it will pro- 

 duce both egg-cells and pollen-cells with a diploid number 

 of chromosomes. Self-fertilization will give tetraploids 

 again. On the other hand, in animals or plants with sepa- 

 rate sexes the eggs of one individual must be fertilized by 

 sperm from another individual. Now, if a tetraploid fe- 

 male should arise, her ripe eggs, with the diploid number 

 of chromosomes, will ordinarily be fertilized by the hap- 

 loid sperm from a normal male, with the result that a 

 threefold type, or triploid is formed. From a triploid 

 the chance of recovering a tetraploid again is very small. 



The tetraploids that have arisen in pedigreed cultures 

 furnish more accurate information as to their origin 

 than do tetraploids found accidentally. There are, in fact, 

 other records where tetraploids have arisen under con- 

 trolled conditions. In Primula sinensis, Gregory has 

 found two giant types, one of which appeared in a cross 

 between two diploid plants. Since the parent plants con- 

 tained known genetic factors, Gregory was enabled to 

 study the inheritance of the characters in the fourfold 

 type. His results left him undecided as to whether they 

 indicated that a given member of each of the four like 

 chromosomes unites with a specific mate or equally with 

 any member of its group. Muller's analysis of the same 

 data indicates the latter as the more probable conclusion. 



Winkler has obtained a giant nightshade (Solanum 

 nigrum) and a giant tomato (Solanum lycopersicum) 

 through the intermediate process of grafting, which has 

 in itself, so far as known, no direct relation to the produc- 

 tion of the double forms. 



The tetraploid nightshade was obtained in the follow- 

 ing way. A piece of a young tomato plant was grafted 

 into a young nightshade plant from which the axial buds 

 were then removed. A cross cut was made, ten days later, 



