FLOWERS AND THEIR WORK. 233 



263- Contrivances and Conditions favouring Cross- 

 Pollination. There are in flowers many forms, conditions, 

 and mechanisms, the significance of which becomes clear 

 only on the view that cross-pollination is advantageous, and 

 that the flowers have become adapted to it. Such mechan- 

 isms, etc., either entirely prevent self-pollination, or tend to 

 do so, and are distinct adaptations for cross-pollination. 



To commence with, in many plants cross-pollination is 

 absolutely necessary if seed is to be produced, owing to the 

 flowers being unisexual. The staminate and pistillate flowers 

 are either on the same plant (the monoecious or monoicous 

 condition), e.g. Oak, or on different plants (the dioecious or 

 dioicous condition), e.g. Willow. Some plants are self -sterile, 

 i.e. the flower cannot be fertilised by its own pollen ; this 

 occurs in some species of Passion-flower, of Lobelia, of 

 Abutilon, etc. 



Dichogamy is a condition in which, though the flowers 

 are hermaphrodite, the anthers and stigmas come to maturity 

 at different times, and which, when completely developed, 

 entirely prevents self-pollination. There are two kinds of 

 dichogamy : (a) protandry, where the anthers ripen first : 

 here the pollen grains are transferred to an older flower ; (fc) 

 protogyny, where the stigma ripens first : here the pollen 

 grains are transferred to a younger flower. Protandrous 

 flowers are much more common than protogynous. Examples 

 of the former are found in Composites, Labiates, Harebells, 

 Ivy, Umbellifers, Willow-herbs, etc. ; of the latter in Plan- 

 tains, Woodrush, Figwort, Hawthorn, Christmas Eose, etc. 

 Wind-pollinated flowers are more often protogynous than 

 protandrous, but many are unisexual. 



Wind-pollinated and insect-pollinated flowers have each 

 special characters of their own, so that as a rule we can dis- 

 tinguish them at a glance. In wind-pollinated flowers the 

 pollen is produced in great abundance, as much of it must be 

 wasted ; the flowers are small and inconspicuous ; there is no 

 honey or perfume ; and frequently the stigmas are branched 

 and feathery, to catch the pollen grains. See Art. 273. 



The greatest variety, however, is shown in insect-pollinated 

 flowers ; there is no difficulty in recognising these as being 

 the most highly specialised. As a rule they have large, 

 conspicuous, brightly coloured corollas, or are arranged in 



