48 



biotype in fact represents a definite combination of characters. Those familiar 

 with the law of Mendel may find in this fact, further support for, the concep- 

 tion that these forms have arisen through natural crossing. 



Interesting observations regarding the variability and multiformity of 

 distinct hereditary types found within this same variety of oats (Probstier) 

 as well as in Squarehead wheat and Two-rowed barley have been recorded by 

 Vestergaard of Denmark (80 p. 77-119). In Squarehead Wheat he dis- 

 tinguishes ten distinct biotypes on the basis of form of head. Although this 

 variety is normally smooth chaffed he has -found forms with velvet chaff which 

 he believes to haye originated from the genuine squarehead type. Bearded 

 heads have also $een found although Squarehead is a bald wheat. Certain 

 cultures were fuHher found to represent distinct biotypes on account of 

 differences in size, fbrnTor color of leaves, although no marked differences 

 in the head could be noted. Many cultures were readily distinguished on 

 the basis of form, size and color of kernel and the degree of susceptibility to 

 disease by which each was characterized. 



In Denmark the most commonly cultivated two-rowed barleys are the 

 so-called Danish Native barley, Chevalier barley and Prentice barley. Of the 

 latter Vestergaard has cultivated about 400 separate cultures, a number of 

 which have shown themselves to be distinct biotypes, although as a whole 

 this variety has proven much less composite in character than has the 

 common Danish barley. Reference has already been made in another con- 

 nection (See p. 39) to the groups of distinct forms which have been taken 

 out of this variety and specially investigated. 



In view of the independent nature of the different characters which go 

 Numerous to make up the. individual it is possible for these to group themselves into 

 Combinations a } mos ^ every conceivable combination by cross-fertilization, artificial or 



e natural. That such grouping actually takes place seems to have been shown 

 crossing. . 



conclusively by the enormous amount of work in artificial hybridization 

 which has been prosecuted with all kinds of Agricultural plants during the 

 past ten years. It seems natural to suppose therefore that at least the great 

 majority of the different strains or biotypes found within the old Probstier 

 and other races and which represent different combinations have arisen by 

 Natural natural crossing. That natural crossing between sorts, even in such self- 

 crossing in fertilizing genera as those to which wheat, oats and barley belong may oc- 

 cereal grains, casionally take place has been clearly pointed out by such recognized authori- 

 ties as Rimpau (60), Kornicke (20), Kiessling (19 p. 73), Nilsson-Ehle 

 (49 p. 15) and Tedin (71 p. 119). 



The multitude of distinct hereditary combinations which may arise 

 through a relatively small number of independent differentiating units was 

 pointed out by Mendel who showed that only 10 such units are necessary 

 to make possible as many as 1,024 different constant (homozygous) com- 

 . binations. Since it is easily within the range of two hereditary types to 

 differ in as many as 10 different characters it is only necessary that these 

 become crossed in order to produce a multiformity of combinations cor- 

 responding exactly with that which is represented in an old mixed variety. 



The progeny of a crossing are of course hybrids and according to the law 

 of Mendel, a certain proportion of these become practically constant in sue- 



