REPRODUCTION AND DISPERSAL 



859 



many plants the chief secretion of nectar occurs before and in others after anthesis; 

 rarely, if ever, is there any exact correlation with this period, as in the case of floral 

 nectaries. The theory that these nectaries have no r61e of importance is more ten- 

 able than the theory of protection from ants. This view of the case is supported by 

 the fact that extrafloral nectaries occur in flowerless plants, as in Pteris and in various 

 other ferns. 



Flower structure. Many flowers are so constructed that certain flying insects, as 

 well as ants, are unable to disturb the pollen or nectar ; this is most obvious in flowers 

 with long corolla tubes and in zygomorphic flowers. 

 In flowers with long corolla tubes (such as the petunia, 

 fig. 1185), or with long spurs, only such insects as 

 various Lepidoptera, which have corresponding elon- 

 gated mouth parts, can reach the nectar; in some 

 cases the corolla tubes are lined with bristly hairs 

 which still further tend to keep out small insects, 

 though they offer practically no obstruction to long 

 probosces. In a number of zygomorphic flowers (as 

 in the snapdragon and in various legumes) it is diffi- 

 cult for small and weak insects to force their way to 

 the nectar or pollen. Among the features which tend 

 to exclude undesirable insects, floral zygomorphy, long 

 corolla tubes, and spurs are much the most impor- 

 tant. Since insects with long mouth parts can get 

 freely exposed nectar, the chief value of zygomorphy 

 and tubular corollas would seem to be the exclusion 

 of undesirable insects. While such structures may 

 have some evolutionary connection with insect visita- 

 tion, the connection is too complex to be understood 

 at present. It would seem much better for a flower to be pollinated in any manner 

 than to run the chance of no pollination if the proper insect were not present. The 

 significance of flower structure, here as elsewhere, is an unsolved enigma. 



Some instances of specialized cross pollination. General remarks. The con- 

 sideration of cross pollination cannot be concluded without a short account of some 

 of the more striking instances of extreme specialization. In some of the cases to be 

 mentioned the dependence of the flower upon the insect (and often of the insect 

 upon the flower) is absolute, and therefore to be regarded as illustrating obligate 

 symbiosis. When such specialized forms are taken to other countries for cultiva- 

 tion, they may not produce seed, unless the insects also are transported. 1 



Silene and the orchids. Some of the night-blooming catchflies (Silene) are 

 visited by nocturnal Lepidoptera, especially Dianthoecia, whose movements in 

 getting nectar incidentally effect pollination; later the moth deposits eggs in the 

 ovary with its long ovipositor, and the developing larvae feed upon the ovules. In 



1 An excellent illustration of this is afforded by the orchid, Vanilla, whose fruits fur- 

 nish commercial vanilla; the absence of the proper pollinating insect in certain regions 

 makes artificial pollination a necessity for profitable cultivation. The day lily (Hemero- 

 callisfulva) never fruits in Europe, probably because of the absence of the proper insect. 



FIG. 1185. A flower of 



Petunia ; note the long tube 

 (0 of the sympetalous corolla 

 (Oi well-fitted for pollina- 

 tion by moths with long 

 mouth parts; c, calyx of 

 five sepals. 



