EXTRA CHROMOSOMES, A SOURCE OF VARIATIONS IN 

 THE JIMSON WEED 



By Alukkt F. 1{l.\kesi.ee 

 Carnegie Institution of Washinyton, Department of Qcnctics, Cold Spring 



Harbor, N. 7. 



[With 13 plates] 



Variations in plants and animals are the building stones out of 

 which the present wide range of agricultural and horticultural forms 

 have been developed. Experimental study of the origin of these 

 variations has been concerned cliiefly with changes in the ultimate 

 hereditary unit factoi-s. The present paper, however, discusses 

 variations wliich are brought about by changes in relatively large 

 groups of factors contained in chromosomes. In this summary of 

 the types that have been discovered in the Jimson weed {Datura 

 stramonium), text and illustrations from earlier publications^ have 

 been freely drawn upon, but much is presented here for the first time. 



What chromosomes are may be better understood from Figure 1, 

 which is a diagram intended to represent a plant of tlie Jimson 

 weed {Datura stramonium) with a single enlarged flower. It need 

 hardly be pointed out to most readers that all parts of the plant — 

 root, stem, leaves, and flowers — are made up of microscope units, 

 called cells, roughly analogous to the building bricks of an archi- 

 tectural structure, and that in each cell there is a definite number 

 of still smaller rod-shaped bodies called chromosomes. The name 

 chromosome means merely " colored body," and was given to these 

 protoplasmic rods because they became strongly colored when acted 

 upon by certain dyes. At the right of the diagram in Figure 1 are 

 shown three cells with chromosomes and in Plate 5 are shown 

 chromosomes as they actually appear under the microscope. The 

 student of heredity is interested in chromosomes because they are 

 the bearers of the ultimate hereditary factors wliich are transmitted 

 from parent to offspring. 



The diagram might have been improved if, instead of rods, we 

 had used narrow medifine vials and had filled each vial with a row 



' Figures 1, 2, 3, and 4 and Plates 4, 5, 7, 8, 10, and 13, Fig. 2, are taken, inoHtly with 

 modlfleatlona, from the Journal of Heredity, voIh. Iti and 20. 



Plates fl, 9, 12, and 13, Flc 1, ari' taken, with some modlllcatlons, from Annals of the 

 New York Academy of Sciences, vol. 30. 



431 



