314 THE STRUCTURE OF FLOWERS. 



Moreover, however greatly we may feel impressed with 

 the truly wonderful adaptations of flowers, a careful and 

 critical study of them reveals many features which seem 

 to counterbalance, to some degree at least, the " good " we 

 may in the first instance be inclined to assume as self- 

 evident. Indeed, the disadvantages accruing from great 

 differentiations in adaptation to insect agency are really too 

 important not to have been frequently noticed. Such are, 

 "hercogamy," or the mechanical obstruction to self-fertilisa- 

 tion, as in Orchids ; the physiological barrier, as in Linum 

 perenne ; the absence of insects required to fertilise a flower, 

 as is the case with Convolvulus sepium in England, which 

 rarely sets seed, as Sphinx Convolvuli is a rare insect ; the 

 frequent absence of bees, etc., in inclement weather, when 

 Clover sets but little seed, to the great loss of the farmer ; 

 when certain flowers are neglected for greater attractions, 

 as may be often seen when bees keep persistently to one 

 species of plant and pass over others ; the frequency with 

 which bees perforate tubular flowers without pollinating 

 them at all. Again, MuUer points out * that while honey- 

 seeking insects may legitimately cross heterostyled plants, 

 pollen-seeking insects have no need to thrust their heads or 

 proboscides down to the stigma of the short-styled forms ; 

 hence such tend to bring about illegitimate unions of the 

 long-styled forms only. This, he thinks, may be a cause of 

 the greater fertility of that kind of union. | Lastly, the 

 more highly difPerentiated a flower is, the less is its number 

 of insect visitors and the rarer may it become in nature. 

 Thus orders of plants with easy access to the honey are 

 some of the most abundant, as Ranunculacece, CompositcE,X 



* Fertilisation, etc., p. 387. f See above, p. 206. 



X The enormous number of species and wide diffusion of the Com- 

 positce are proofs of the advantages accruing to it from the peculiar 



