09 



A further example of both extremes of parental characters in regard to 

 earliness being exceeded, was met with at Svalof by the writer during the 

 summer of 1910. Thus, from the crossing between the two Black oat sorts 

 Roslag (01006) and Black Bell II (0408) there were 26 separate cultures in 

 the third generation. These cultures were numbered from 235 to 260 inclu- 

 sive. Number 257 was at least three weeks later than the next latest culture, 

 parents included, remaining in the field long after all other sorts had been 

 taken to the barns. This was the most extreme case of this kind on record 

 at the above Institution up to that time. A few of the combinations on the 

 other hand, were somewhat earlier than the earlier parent, Bell II. 



In a crossing between Triticum Compactum wheat (very winter hardy) 

 and a less hardy sort, there were produced forms which were completely 

 winter killed in the 2nd generation, and others which proved quite hardy. 

 None, however, proved so hardy as T. Compactum, but were at best only 

 intermediate. Other crosses have shown similar complex results and it is, 

 therefore, believed that Winter-hardiness is not a simple character, but 

 rather is one which, like other quantitative characters previously referred to, 

 depends on different units or factors. Not only do the experimental evi- 

 dences seem to support this view, but physiological considerations also lead 

 to the same conclusion, as Johannsen (17 p. 175) has recently pointed out. 



Investigations at Svalof have shown further that certain units produce 

 only visible effects when combined with certain other units. The result of 

 such a combination is to strengthen or enhance the visible effect of one or 

 the other of the units involved. Thus has Tedin found on crossing certain red- 

 flowered and certain white-flowered varieties of pease that the resulting 

 combinations produced flowers having a Red Violet color. Numerous 

 experiments have led to the conclusion that the white sort possesses a unit 

 or factor which alone produces no visible effect, but when brought into 

 combination with the factor for rose color produces a Red Viole . 



Experiments in crossing have therefore shown that the process of 

 segregation is often of a very complex nature, and one in which many units 

 may be involved. Such characters as winter-hardiness, time of ripening, 

 stiffness of straw and resistance against disease, instead of being simple charac- 

 ters, are believed to be composed of many units or factors by reason of which 

 they are called by Johannsen " construction characters." By means of different 

 combinations of the various units, a large number of constant gradations 

 of a quantitative nature may arise, which may differ physiologically as well 

 as morphologically from the parents. These gradations or lines then, are 

 not the expression of ' a natural tendency' to vary, but rather of different 

 combinations (" combinations- variation ") which may arise from a relatively 

 small number of initial differentiating units as already explained. 



An especially complex character is yielding power. This is determined 

 by many different factors such as that for stooling, strength of straw, size 

 of head and kernel, resistance against disease, and finally upon the different 

 units which are concerned in the constitution of each of the characters them- 

 selves. It is of special interest that characters of direct practical importance 

 to the plant breeder behave in the manner indicated. 



