[THOMPSON] RIPENING PERIODS IN WHEAT 73 



decided to choose one characteristic and judge each plant by it alone. 

 After many observations and experiments the characteristic chosen 

 was the condition of the endosperm. A plant was called ripe as soon 

 as the central kernels reached the dough stage. In regard to this 

 point there is very little variation in the judgment of different in- 

 dividuals. 



In addition to the period of ripening the flowering (heading) 

 time was also studied. This not only furnishes a different character- 

 istic but serves as a check on the results obtained in regard to the 

 ripening period. The length of the period from planting to heading 

 is given in Table II. It will be observed that there is considerably 

 less difference between the means of the different varieties than is 

 the case in regard to the ripening period. In the latter case the hered- 

 itary differences have a longer time in which to express themselves 

 and the end results become more different. 



A special point appears in connection with Preston wheat. 

 While its heading period is almost identical with that of Red Fife, it 

 ripens 2 or 3 days earlier. The condition of the plants at ripening 

 time explains this difference. The grain ripens while the rest of the 

 plant, including the glumes, is still comparatively green. The heading 

 period associates itself with the ripening of the plant as a whole 

 rather than with that of the grain. The same characteristic appears 

 in the hybrids of this plant but the hereditary basis has not yet been 

 determined. 



It is of course well known that the length of the heading and 

 ripening periods vary from year to year depending on climatic and 

 soil conditions. In 1916 for example all varieties were later than 

 in 1917, as is hsown in Table III. This table also shows that the 

 effect of climatic conditions in prolonging the ripening period was 

 almost the same in the three earliest varieties (11 or 12 days), and that 

 the two middle varieties were affected to the same extent (23 days) 

 though very differently from the three earliest. The hereditary basis 

 for this difference between the early and middle varieties has not been 

 determined. F2 plants of a cross between an early and a middle variety 

 grown in a season of slow growth should be compared with sister 

 plants grown in a season of rapid growth. The differences between 

 the 1916 and 1917 seasons in regard to the latest varieties could not be 

 determined with accuracy because the late varieties were not quite 

 ripe in 1916 when frost came, though they were sufficiently matured to 

 germinate readily the next season. They would all probably have 

 ripened in about 140 days making the effect of climatic conditions 

 about the same as in the case of the medium varieties. Observations 



