REPRODUCTION AND DISPERSAL 



843 



in such a way that it is Ukely to come into contact with the stigmas.' 

 Commonly pollen-gathering insects are relatively non-specialized (ex- 

 cept in the case of bees), corresponding in general to the lack of speciali- 

 zation in the flowers, iii which the pollen is so exposed that it may be 

 taken readily by any insect that visits it. 



Nectaries and nectar. — Nectar-secreting flowers commonly are more 

 specialized than are nectarless flowers, and 

 the nectar-gathering insects are the most 

 specialized of pollinating insects. How- 

 ever, there are many simple actinomor- 

 phic flowers with exposed nectar (notably 

 among the umbellifers) or with nectar but 

 sKghtly concealed (as in the crucifers) , which 

 are frequented by flies and by other insects 

 with short probosces. From these simple 

 nectar-producing flowers there are grada- 

 tions in the degree of concealment of nectar 

 to the highly specialized and often zygomor- 

 phic forms in which it is concealed at a 

 considerable depth at the base of long co- 

 rolla tubes or in elongated spurs (fig. 1171), 

 where as a rule only the most specialized 

 insects with long mouth parts can obtain 

 it. In most cases it is difiicult or even 



impossible for insects getting the nectar nous and zygomorphic- From 



i^ . . . Barnes (Part II). 



to avoid rubbing agamst anthers and 



stigmas, thus facilitating pollination. As a rule, the pollen in nectar- 

 bearing flowers is not abundant, and in long-tubed, and zygomorphic 

 flowers it commonly is concealed. 



Most arctic and alpine flowers and also most vernal flowers of temperate climates 

 are comparatively simple in structure and have their nectar supply relatively ex- 

 posed. On the other hand, many tropical flowers and a large number of estival 

 flowers of temperate climates have more specialized structures, their nectar supply 

 being hidden in spurs or at the base of long corolla tubes. With the former there 

 may be associated the general prevalence of insects with short probosces, charac- 

 terizing climates or seasons of low temperature, and with the latter there may be 



^ The high degree of specialization here present is shown by the fact that the pollen 

 which is used for food does not readily germinate on account of the absence of the proper 

 enzym ; when this is supplied artificially, it germinates as readily as does the pollen from 

 the other stamens. 



Fig. 1 171. — A longitudinal 

 section through a nasturtium 

 flower {Tropaeolum majus), 

 showing the spurs {s) with nec- 

 tar in) collected in its lower por- 

 tion; this flower is hypogy- 



