j.J MODES OF CHECKING SELF-FERTILISATION. 15 



The flower cannot therefore fertilise itself, and depends 

 entirely on the visits of insects for the transference of 

 the pollen. In G. pyrenaicum, where the flower is not 

 quite so large, all the stamens ripen before the stigma, 

 but the interval is shorter, and the stigma is mature before 

 all the anthers have shed their pollen. It is therefore 

 not absolutely dependent on insects. In G. molle, which 

 has a still smaller flower, five of the stamens come to ma- 

 turity before the stigma, but the last five ripen simul- 

 taneously with it. Lastly, in G. pusillum, which is least 

 of all, the stigma ripens even before the stamens. Thus, 

 then, we have a series more or less dependent on insects, 

 from G. pratense to which they are necessary, to G. pusil- 

 lum, which is quite independent of them ; while the size 

 of the corolla increases with the dependence on insects. 



In those species in which self- fertilisation is prevented 

 by the circumstance that the stamens and pistil do not 

 come to maturity at the same time, the stamens 

 generally ripen first. 



The advantage of this is probably connected with the 

 visits of bees. In those flowers which grow in bunches 

 the lower ones generally open first. Consequently in 

 any given spike the flowers are at first all male ; subse- 

 quently the lower ones, being the older, have arrived at 

 the female stage, while the upper ones are still male. 

 Now it is the habit of bees to begin with the lower 

 flowers of a spike and work upwards. A bee, therefore, 

 which has already dusted herself with pollen from 

 another flower, first comes in contact with the female 

 flowers, and dusts them with pollen, after which she 

 receives a fresh supply from the upper male flowers, 

 with which she flies to another plant. 



