182 LABORATORY AND FIELD EXERCISES 



To what structures do these parts of the involucre of 

 the yarrow correspond in a raceme or spike ? How are 

 the flowers of the inflorescence differentiated ? Is there 

 any conceivable advantage in this differentiation ? Does 

 each separate flower spring from a separate bract? 

 Compare the parts of the inflorescence of the yarrow 

 with that of a raceme. 



2. Flowers. 



a. Central tubular flowers. 



(1) Study these flowers with a hand lens and deter- 



mine ttie relation of calyx, corolla, stamens, and 

 pistil. Is there anything corresponding to a calyx? 



(2) Are the flowers hypogynous, perigynous, or epigy- 



nous ? Are they perfect or imperfect ? complete 

 or incomplete ? regular or irregular ? 



(3) Note the bract subtending each flower. 



b. Outer ray flowers. What is the function of the ray 



flowers ? Compare these with the central tubular 

 flowers. Are the essential organs present ? How is 

 the corolla modified ? 



3. Pollination. Split the corolla of young and mature 



tubular flowers with dissecting needles so as to 

 expose the stamen tube and style. Compare the 

 young and old flowers as follows (consult the text 

 figure) : 



a. Note a stage where the upper portion of the style 



is still within the anther tube. Where are the 

 stigmatic surfaces ? Is the pollen being shed ? Can 

 it reach the stigma ? 



b. Observe a slightly older stage. Are the stigmatic 



surfaces exposed ? Can they be self-pollinated ? 

 Are the flowers of the yarrow protandrous or 

 protogynous ? 



