358 THE FLOWER 



and habits of particular insects that they are only cross- 

 pollinated when they receive visits from these insects, 

 and remain sterile, setting no seed, in their absence. 



Inconspicuous flowers are often adapted for wind- 

 poUination (grasses, sedges, most catkin-bearing trees, 

 pines and firs, etc.). Wind-pollinated flowers produce 

 pollen in large quantities, and this is smooth and dust- 

 like, not rough or sticky, like the pollen grains usually 

 conveyed by insects. A great deal is wasted because 

 its carriage to the stigmas of another flower of the 

 same species is a matter of pure chance. The stigmatic 

 surfaces are generally large, and this of course increases 

 the chance of some of the right pollen being caught. 



Darwin showed that the offspring of crossing are, 

 on the whole, and with certain exceptions, more vigorous 

 than the offspring of self-pollination, when individuals 

 arising from seed produced in the two ways are grown 

 side by side. Thus variations in the flower tending 

 to secure crossing will tend to be fixed and perpetuated 

 because the offspring of flowers with such variations 

 will tend to survive more often than those in which 

 self-poUination occurred. It seems to be only in this 

 way that we can explain the origin and flxation of the 

 various beautiful and often astonishingly accurate 

 mechanisms which bring about crossing in flowers. 



Many inconspicuous flowers (chickweed, the small 

 field speedwells, etc.) are, however, habitually self- 

 pollinated, and these perpetuate themselves indefinitely 

 with perfect success. Many garden and field crops 

 (green peas, wheat) are in the same position. There 

 is no evidence that continued " self -fertilisation " is 

 in any way harmful to the race, though a chance cross 

 in an habitually self-fertilised species will often result 

 in a distinct increase in the vigour of the offspring. 



