HELIOTROPISM OF SISYMBRIUM CANESCENS. l-tl 



(2) Continuity and convergence of lines of dispersal. 



( 3 ) Location of synthetic or closely related forms. 



(4) In some cases, location of dominance or g'reat abund- 

 ance of individuals. 



If to these we add : 



(5) Location of least dependence upon a restricted habi- 

 tat, we shall have the criteria most generally useful in determin- 

 ing centers of dispersal of plants. Yolxey M. Spalding. 



N^OTES OH^ HELIOTROPISM OF SISYMBRIUM! 



CANESCEXS. 



In the vicinity of Tucson, Ariz., a little cruciferous winter 

 annual, Sisymhriian ccniescois Nutt., was observed blooming 

 and fruiting in large numbers in the middle of February, 1907. 

 It exhibited a marked case of heliotropism. In open places, the 

 rather erect steins leaned uniformly to the south. In shaded 

 places no leaning was noticeable. Where the plants grew on 

 the west side of a shading object, they pointed some degrees west 

 of south, where they grew on the east side of such an object, they 

 indicated a direction a few degTees east of south. In other 

 words, they pointed in the direction of what were probably the 

 strongest rays of the sun that they received. But the stems were 

 not as straight, nor the leaning as uniform, as in case of the 

 plant growing in the open. 



Later, large numbers of this species were seen fruiting on a 

 falloAv field in tlie Santa Cruz River bottom, where the same 

 phenomenon occurred. It was noted here that the smaller plants 

 leaned southward most uniformly, and at the greatest angle, 

 while the largest plants showed only an indistinct or no tendency 

 to lean. 



On March 29, the plants being then mature, in a place where 

 the leaning was most uniform and decided, the angle of devi- 

 ation from the vertical was determined for 24 individuals. The 

 average result was 29.4 degrees. The latitude of Tucson is 

 32° 13'. From the fact that tlie midday sun at the time of the 

 vernal oijuinox in the Tropic of Cancer is 234- degrees south of 



