length of time during which production and dispersal occurs, and 

 the distance between individuals in a population. In Nebraska, 

 for example, smooth bromegrass produces more pollen than 

 crested and intermediate wheatgrasses and switchgrass, and most 

 of the pollen is dispersed within 5 to 15 rods. The average period 

 of dispersal per inflorescence varies from 8 days in crested wheat- 

 grass to 12 days in switchgrass.' '^ In eastern Colorado, Russian 

 wildrye {Elymus junceus) sheds pollen mostly between 4 and 5 P. M. 

 throughout a 9-day period at the end of May and early June. The 

 shedding is very sensitive to temperature, which varies consider- 

 ably at this time, and this may explain in part why the seed 

 production is erratic in this region. ^""^ Apparently grasses are 

 usually pollinated by nearby plants, for example, orchardgrass 

 averaged 62.4 seeds per panicle when the pollinating plants were 

 within 1.5 yd of one another, but only 27.2 seeds per panicle at 

 greater distances up to 1 1.5 yd.^^ 



Although strong winds and air currents carry many kinds of 

 pollen to great heights and distances,^!^ it is questionable how 

 much pollination is accomplished in this way. The pollen of pines 

 appears to be dispersed as much as 100 miles, but the amount is 

 limited; of cotton, up to 4200 ft; and of corn, occasionally to 400 ft. 

 The great frequency of natural hybrids in grasses is brought 

 about by the association of many individuals of different species, 

 the large production of wind-borne pollen, and the similarity in 

 reproductive organs of different species. '^^ Separation of wind- 

 pollinated plants such as pines and oaks by a few miles may not 

 prevent cross-pollination, but in insect-pollinated trees a few 

 hundred yards may be very effective. '"^^ Insect-pollination from 

 a distance, however, may not be entirely prevented, because 

 numerous insects of various species are in the air on warm days 

 to heights of several thousand feet.^'^ Plants and animals possess 

 many adaptations that facilitate cross-pollination or prevent self- 

 pollination. 



Dispersal of seeds and spores is accomplished in many well- 

 known ways but concrete data on distances they are carried are 

 scarce. Most seeds seem rarely to be dispersed beyond 300 ft, the 

 greatest distance winged seeds and fruits are carried is about 

 880 yd, but dust seeds and plumed fruits may be carried as much 



38 * Species aimd PopislaftioMis 



