INSECT POLLINATION 355 



From a different point of view, that of the origin of the 

 pollen, we distinguish between autogamy, in which the 

 flower is pollinated with its own pollen, and allogamy, where 

 the pollen comes from a different flower. Allogamous 

 plants may be subdivided ; by geitonogamy is understood 

 pollination from another flower on the same plant ; by 

 xenogamy from a flower of a separate plant ; by hybridisation 

 from a flower of a different race or species. 



Finally, we may distinguish those flowers, the great 

 majority, where opening precedes pollination — chasmo- 

 gamotis — from those where autogamy takes place without 

 opening — cleistogamous. 



I. Entomophily 



Origin of Insect Pollination. — This is perhaps the most 

 important type of pollination, and not alone because the 

 majority of species are entomophilous, but because it is in 

 relation to insect visits that the angiosperm flower, the gaily- 

 coloured, scented, nectar-bearing " flower " of the popular 

 sense, has evolved. These three properties, often found 

 together in a single flower, have no conceivable use apart 

 from insects. 



The early seed plants were wind-pollinated, a condition 

 that persists in the modern gymnosperms with the exception 

 of Welwitschia, and some Cycads ; but it is likely that 

 insect visits, perhaps very irregular, may have occurred even 

 in the pteridosperm stage. The great insects of the carboni- 

 ferous strata had biting jaws, and were not nectar suckers, 

 but they may have found food by gnawing the fleshy parts 

 of the sporophylls, or by eating the microspores. It is 

 possible that the origin of attraction by brilliant colours was 

 the production of red anthocyans as by-products. The 

 female cones of fir and larch, the larch roses, are brilliant 

 red, although insects do not pollinate them. It is also 

 possible that the yellow colour of the pollen grains was the 

 beginning of this feature. In the secondary rocks, where in 

 Bennettites we have an approach to the angiosperm flower, 



