23 



be noted that these specimens show not only heads of greater 

 elongation than those previously found but that on the corymbs 

 there are no heads showing intermediate stages and in all the 



Fig. I. A lengthwise section of a common form of yarrow with flat-topped 



receptacle. X 5. 

 FiG.s. 2 -AXD 3. External view and lengthwise section of a head with an 



elongated receptacle measuring 4 mm. X 5- 



Figs. 4 and 5. External view and lengthwise section of a head with an 



elongated receptacle measuring 7 mm. X 5. 



heads there is a tendency in nearby receptacles to show a similar 

 measurement. Another plant with heads similar to that shown 

 in Fig. 4 had ray flowers interspersed with the tubular flowers 

 on the elongated receptacle. These were in addition to the usual 

 number surrounding the head. 



