AN INTERESTING MODIFICATION IN XANTHIUM 
Charles A. Shull 
Two years ago I received through the kindness of Mr. F. F. Creve- 
coeur, of Onaga, Kansas, some burs of Xanthium which show a very- 
unique and interesting modification. The ordinary burs of Xanthium 
are too common and famihar to need description. Normally they 
enclose but two ovaries, and possess only two beaks which arise con- 
jointly from the outer end of the bur. Through these beaks the styles 
protrude at the time of pollination. 
These modified burs, however, enclose a considerable number of 
ovaries, usually between twenty and thirty. The beaks are corre- 
spondingly increased in number, and are arranged in two or three 
concentric rows about a central depression which occupies the central 
part of the distal half of the bur. ' Figure i shows the burs about 
natural size. The form of the bur is probably determined by the 
cessation of growth by the centripetal portion of the receptacle, while 
the. centrifugal zone continues to develop, and imbeds a number of 
flowers which are apparently arranged in more or less concentric rows. 
Figure 2 shows one of the burs with the outer wall of the receptacle 
removed so as to show the outer row of seeds, each of which is enclosed 
in its black ovarial wall. 
The exact structure of the bur is most easily understood from an 
examination of the cross section of the bur taken slightly above the 
equator, just beyond the bottom of the depression previously men- 
tioned. Such a section is shown in Figure 3. Not all of the burs 
had the same number of ovaries, but the general structure of all was 
the same. In this particular bur there were twenty-six ovarial cavities 
in the receptacle, twenty-three of which contained the remains of 
ovaries. The position of the cavities which contained ovaries are 
indicated by small circles. The other three cavities contained no 
trace of ovaries, but their position indicates clearly enough that they 
correspond to a third row of florets. 
There is a very strong tendency to sterility, apparently, for many 
of the ovaries were empty. Of the twenty-three ovaries found in the 
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