THE MICROBIOLOGY OF THE ATMOSPHERE 



The characteristics of wind-borne pollen become clear when contrasted 

 with insect-borne pollen (Table IV). There are many exceptions to the 

 generalizations in this table and, in particular, some plants make the best 

 of both methods. Both anemophilous and entomophilous plants often 

 protect their pollen from the rain, and many store it within the flower for 

 some time after shedding from the anthers. Anemophilous pollen is not 

 generally shed into very calm or very damp air. 



TABLE IV 



TYPICAL CHARACTERISTICS OF ANEMOPHILOUS AND ENTOMOPHILOUS 



PLANTS 



Wind-pollinated 



Flowers Lack conspicuous and attractive 



petals, scent, and nectar. 



Flower Projecting into air: hanging from 



position bare branches before leaves open 



(catkins); on erect stalks (grasses, 

 etc.); or at ends of branches (coni- 

 fers). 



Prevention Male and female organs often in sep- 

 of self- arate flowers or inflorescences, or on 



fertilization separate plants. If flowers herma- 

 phrodite, one sex commonly matures 

 before the other, or, if sexes are in 

 separate inflorescences, the female 

 is often above the male. 



Pollen Often shed into the air in vast quan- 



tities. 



Shape rounded, often nearly spheri- 

 cal or ellipsoidal. 



Size-range narrower than entomo- 

 philous pollen and seldom less than 



15 M- 



Surface typically smooth as seen 

 under the microscope, non-sticky, 

 easily separating into single grains in 



Insect-pollinated 



Often with bright colours, 

 scent; nectar attractive to in- 

 sects. 



Tend to be exposed to view, but 

 not exposing anthers to wind. 

 Flowers usually maturing when 

 plant in full growth and insects 

 abundant. 



Flowers usually hermaphrodite, 

 with structural or genetic bar- 

 riers to selfing. 



Usually restricted pollen pro- 

 duction with little shedding. 

 Shape very variable. 



Size very variable, 3 to 250 /n, 

 but often less than 15/11. 



Surface typically rough, spiny 

 or war ted, often oily or sticky, 

 tending to adhere in clumps. 



GYMNOSPERMS 



Conifer pollen, instead of being formed in stalked anthers as is that of 

 Angiosperms, is produced in two or more pollen sacs on the lower side of 

 the male cone-scales. The pollen grains are large and often bear two 

 conspicuous air-filled bladders which decrease the density of the particle 

 and so retard its fall under the influence of gravity. 



In PmuSy cone-scales of the erect male cone separate as they mature, 

 and pollen shed from the paired sacs falls into small hollows on the upper 

 surface of the cone-scale below. From these hollows the pollen is blown 

 away when the wind reaches sufficient velocity. Some other conifers have 

 hygroscopic mechanisms protecting their pollen from rain and allowing 



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