. A textbook of botany for colleges and universities ... Botany. REPRODUCTION AND DISPERSAL 8S9. 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 th
. A textbook of botany for colleges and universities ... Botany. REPRODUCTION AND DISPERSAL 8S9. 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. ii8s), 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 difli- 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 uns
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