174 INTRODUCTION TO BOTANY 



165. Unit characters. 1 Much more definiteness has recently 

 been given to the discussion of questions relating to plant 

 breeding by the introduction of the idea of unit characters. 

 Every species or variety of animal or plant is considered by 

 many authorities to be composed of a set of unit characters, 

 or simple features by which it is distinguished from other 

 species or varieties. Just what this statement means will ap- 

 pear more clearly if some of the unit characters which belong 

 to different varieties of a familiar species of plant are set 

 down. In the case of timothy, our most important grass for 

 haymaking, twenty-eight opposing character pairs have been 

 noted. Some of these are 



Heads 



Long or short 



Continuous or interrupted 



Large-seeded or small-seeded 



(and 8 others) 

 Leaves 



Long or short 



(and 4 others) 

 Stems 



Tall or short 



Many or few branches from the base of the stem 



(and 4 others) 

 Nodes 



Near together or widely separated 



(and 2 others) 

 Habit characters 



Inclined to blow down or remaining erect 



Rusting readily or rust-resistant 



Early or late maturing 



Careful breeding would probably soon give a variety with 

 any one of these twenty-eight characters strongly marked, 

 and several of them could be combined in a single variety if 

 desired. 



1 See the valuable summary of this subject, given by Professor Herbert J. 

 Webber in his address before the American Breeders' Association, Science, 

 April 19, 1912. 



