96 CARNEGIE INSTITUTION OF WASHINGTON. 



ADJUSTMENTS NECESSARY TO SURVIVAL. 



The cultures of introduced species now at Tucson were subjected 

 to rather severe tests in the year 1917, with the result that they show, 

 after six generations of Uving in the desert complex, quite perfect 

 adjustment to the survival end of the problems that they have to 

 encounter. In the earlier generations after introduction, in the drj'^ 

 fore-summer, as soon as the temperature reaches the point at which 

 they would normally emerge for reproduction, they will do so regard- 

 less of whether the moisture conditions are favorable or not. After 

 the sixth, or at the latest the eighth generation, none emerge until the 

 water-relation has also attained the requisite concentration. Thus 

 in the drj^ fore-summer of 1917 in no cultures older than Fg did any 

 emerge, although they were easily brought from the soil by the addition 

 of water. Under Fe they were found to emerge in proportion to the 

 number of generations they had hved in the complex. 



The cultures that showed adequate adjustment and did not emerge 

 until the total complex was normal for reproduction also showed the 

 highest capacity for retaining water against any desiccating activities. 

 It is certain, therefore, that complete adaptation, gradually attained, 

 and in directions induced by the environment, has been attained in 

 these cultures. 



ADJUSTMENTS NECESSARY TO REPRODUCTION. 



It is not certain that any changes that are the product of the intro- 

 duction have taken place in matters that affect reproduction. None 

 of them will reproduce without a fairly high water-content in the 

 medium and in the food — that is, water in very considerable amounts 

 must be available. In the dry fore-summer of 1917 a growth of 

 Solanum eleagnifoUum was attained, then deprived of water of irriga- 

 tion, so that the plant had a low water-content, and upon this T.99, 

 the most fully adjusted stock, was placed, but refused to breed and 

 finally returned into hibernation, while a second culture of T.99, in 

 which water was given to the food by irrigation, did breed, showing 

 clearly that available water in the medium is necessary for reproductive 

 activities. 



MUTATING STEM STOCKS. 



The two chief series, C.H.15.7 and C.H. 156.8, continue to breed 

 true to type and behavior, as noted in previous reports. 



The main effort of the year with these experiments has been the 

 study of the composition and nature of the different mutants. As 

 previously noted, these are either (1) recombinations, or (2) recombina- 

 tions with alterations. 



During the past year genetic data regarding the nature and behavior 

 of six of the "recombinations" has been completed, i. e.: 



