14 



GENERAL BIOLOGY 



-/.4C 



1-^ 



\ 



a 





"[ 4' ^/ V <il i> 



and the anthers are so situated that the insect rubs against 

 them at just the right time, entering or leaving : in this con- 

 sist some of 

 the most won- 

 derful exam- 

 ples of adjust- 

 ment. 



It is one of 

 the delights of 

 the student of 

 floral struc- 

 tures to trace 



Fig 9. Diagram of types of flower cluster, a, panicle; 

 h, raceme; c, spike; cf, spike contracted to heatl; 

 e, corymb; /, head; g, umbel; h, cyme. 



these organs 

 through all their modification of form, position and arrange- 

 ment, and to be able to recognize them under all their dis- 

 guises. 



The flower cluster. — Two important ends seem to be 

 served by the close grouping of flowers together in clusters, 

 i) vShowiness is secured w^ith much less expenditure of vital 

 energy in the production of sterile parts. The single flower 

 of the common elder is very small and insignificant, but the 

 big flat-topped clusters of elder flowers are borne aloft upon 

 the clumps, in such a way that they may readily be seen 

 from afar off: 2) The bringing of the flowers together in 

 compact clusters makes it possible for the insect to pass 

 from one flower to another Avithout taking flight. This 

 greatly economizes labor for the insect. The principal 

 forms of flower cluster are shown in the accompanying 

 diagram (fig. 9). It will be seen that with respect to 

 compactness and rigidity, improvements in arrangement 

 culminate in flat-topped heads. 



Study I. Flowers adapted to insect visitation. 

 Apparatus needed : A lens and a forceps. 



