68 Heterostylism [ch. 



introduced. The next case also illustrates the appearance 

 of a novelty in F^y but, as will be seen, it is one of the 

 groups of 3 members which manifests it. 



Heterostylism in Primula, 



The dimorphic, heterostyled condition of Primula plants 

 is well known and need not be described in detail. The 

 two types are distinguished thus : 



A. Thrum, or Short-styled type. 



1. Style short, the stigma standing at the level of 



constriction of the tube. 



2. Anthers at the mouth of the tube. 



3. Pollen grains large. 



B. Pin, or Long-styled type. 



Is Style long, the stigma usually standing in the mouth 

 of the tube. 



2. Anthers at the level of constriction. 



3. Pollen grains small. 



Experiments made by Mr R. P. Gregory in conjunction 

 with me showed that the inheritance of these two types 

 is Mendelian^, the short-styled or thrum behaving as 

 dominant. The long-styled, being recessive, always breed 

 true to that type on self-fertilisation or when bred inter se. 



In connection with this fact it is interesting to observe 



* I know no authentic case of the presence of both long- and short- 

 styled flowers on the same plant. Such occurrences are frequently 

 announced, but so far as I can discover the records are based on 

 mistakes. In occasional flowers on long-styled plants, especially in the 

 beginning of the flowering period, the style does not attain its proper length. 

 Such flowers are through carelessness sometimes misdescribed as short- 

 styled. The anthers however are at the lower level, and the pollen-grains 

 are small, so the essentially long-styled nature of these plants is quite clear. 



The statement is also sometimes made that pin plants have produced 

 thrum-eyed offspring without the intervention of a cross. This mistake is 

 due to the appearance of a type with "exsert" anthers. Such anthers 

 project from the mouth of the tube and give a thrum-like look to the 

 flower. But careful examination shows that the anther-filaments are 

 inserted at the lower level, and the pollen-grains are small. Such plants 

 are therefore long-styled. 



