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By Jennifer Boothroyd

An attractive examine the ways in which humans have interaction with crops and animals.

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Example text

The effect is that pollen is presented during a very long period of anthesis, but only part of the total pollen production is exposed at any one time. It is a case of not putting all eggs in one basket, easily correlated with the fact that the flowers of the genera mentioned are open and their anthers unprotected. Only after the last anther is shed do the stigmas exhibit external signs of being receptive. In the dense inflorescences of many Saxífraga species, which almost form one pollination unit, there is a curious discrepancy between the very complicated mechanism preventing autogamy and the rich possibilities for geitonogamy.

Only after the last anther is shed do the stigmas exhibit external signs of being receptive. In the dense inflorescences of many Saxífraga species, which almost form one pollination unit, there is a curious discrepancy between the very complicated mechanism preventing autogamy and the rich possibilities for geitonogamy. The same applies to the dense heads of some Compositae, whereas in others second-order dichogamy counteracts geitonogamy. Also, differences between various zones of the inflorescence in the production and presentation of attractants (nectar, pollen) may serve to channel visits to flowers in a definite phase (Helianthus: Free 1970).

Conversely, since Sprengel's days it has been maintained that the wasps, said to be the principal pollinators of the protogynous Scrophularia nodosa, climb downwards. In the tropics, dichogamy is said to affect whole branch systems at the same time (Nephelium, Khan 1929, Persea, Stout 1926), producing almost a second-order dioecy. Evidently, the effectivity of the phenomenon increases the more general it is. In plants with dicHnous flowers, a static type of second-order herkogamy may be found in contrast to the above-mentioned dynamic or developmental one, viz.

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