98 CARNEGIE INSTITUTION OF WASHINGTON. 



generation, grown under glass, produced normal foliage early in the 

 season, while later leaves show the abnormal characters. Examination 

 of the affected areas of the leaf shows that within them the secondary 

 physiological responses of the palisade and sub-palisade tissues have 

 been suppressed and the structure remains therefore in an embryonic 

 condition. This would seem to indicate that we are dealing with a 

 ''physiological" disease. 



Negative results have also followed from the study of the second 

 (Fi) generation from seeds produced by abnormal green fruits of the 

 Tucson plant No. 1. All the progeny appear normal at present. 



This year a departure from the normal, consisting in the production 

 of very narrow lanceolate leaves, was synchronously entered upon by a 

 small number of plants (in lot No. 131) at Tucson and Carmel. While 

 this may be an expression of the general leaf-distortion above and pre- 

 viously noted, the only feature noticeable is the change in shape due 

 to the reduction of the transverse measurements. 



The Genetic Analysis of Guayule (Parthenium argentatum) under Cultivation, 



by W. B. MacCalluni. 



When a wild plant is domesticated and transferred from a natural 

 state to the condition of intensive cultivation, new forms make their 

 appearance sooner or later. The cause of this occurrence of new vari- 

 eties is not altogether clear. They may be forms already existing 

 unobserved in nature, or they may be variations that would have 

 occurred any way, and not necessarily^ caused by the environment of 

 cultivation. To what extent the direct influence of the conditions 

 of domestication may induce permanent variations is not apparent; 

 indeed, many exclude this altogether as a factor in the origin of new 

 varieties. The absence of accurate record, however, and the obscur- 

 ity of time have left a very inadequate knowledge of the behavior of 

 plants in this respect when first brought into cultivation. In Par- 

 thenium argentatum we have the unique case of a plant, not only brought 

 suddenly into domestication from a wild state, but from extremely 

 arid and desert conditions to an environment of intensive cultivation 

 in a region of exceptionally favorable chmatic conditions, grown in 

 very large numbers, and every step a matter of most accurate record. 



In the dry and rather stunted condition in which the desert plants 

 usually exist it was difficult to determine with certainty more than two 

 or three unquestioned varietal forms, although many differences, espe- 

 cially in regard to size and habit, were discernible that did not seem to 

 be altogether environmental. Grown under culture, however, with uni- 

 formity of soil and other conditions, the differences of environment are 

 largely eliminated and the recognition of strictly varietal differences 

 becomes easier. Over 100 different forms, quite distinct from one 

 another, have been isolated. Between some the differences are slight, 



