WSAT IS A FLOWEBf CONCLUDED 



137 



150. It so happens that in the greater number 

 of plants the pistils and stamens in any flower 

 mature at different times. That is, the pollen may 

 all be discharged before the stigma is receptive, 

 or the stigma may shrivel and die before the 

 anthers open. In other words, there is frequently 

 only a small chance of a flower fertilizing itself. 

 There must be some means, then, of assuring the 

 transfer of pollen. The commonest means are 

 wind and insects. The flower does not need to 

 attract the wind, but it must have some means 

 of letting the insects know where it is. The 

 showy petals are perhaps the sign -boards. At all 

 events, insects may 

 not visit some 

 flowers when the 

 petals are re- 

 moved, although 

 they are attracted 

 by them when the 

 petals are undis- 

 turbed (see 273&). 



150a. This non-con- 

 currence in maturity of 

 the essential organs is 

 known as dichogamy. 



Fig. 137. 

 Stamlnate flowers of willow. 



151. If the pollen may be carried from flower 

 to flower, it is not essential that every flower 



