198 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 



The activities resulting in available nitrates produce partial 

 breakdown of organic materials, which are further decomposed 

 by other bacteria and fungi acting upon the remaining nonprotein 

 plant materials. The partially decomposed plant remains, or hu- 

 mus, may be broken down completely in a single season if rela- 

 tively high temperatures and sufficient moisture occur most of the 

 year and permit more or less continuous functioning of the organ- 

 isms. If the organisms can operate for only a few summer months, 

 the deposition of litter usually exceeds the rate of decomposition, 

 and humus tends to accumulate. 



ANIMALS AS FACTORS 



Pollination.— Insects are by far the most important animals in- 

 volved in pollination, and bees, wasps, moths, and butterflies are 

 particularly concerned. A few birds, especially hummingbirds, 

 contribute to pollen transport, and even some small crawling ani- 

 mals may be effective at times. Most animal-pollinated flowers 

 have certain characteristics in common, such as conspicuousness 

 in size and color and the production of an odor as well as nectar. 

 It has been shown that all of these characters serve more or less to 

 attract insects. In general, the flowers are more elaborate than 

 those of wind-pollinated plants, and they have characters usually 

 interpreted as of more modern origin. 



Devices that insure insect pollination are common and often of 

 intricate design. Adaptations may occur in both insect and flower 

 limiting pollination of a particular species to a single type of insect. 

 Some adaptations are so extreme as to produce complete depen- 

 dence of plant and insect upon each other. 



Dissemination.— Plant parts, called disseminules, give rise to new 

 individuals in new places. Their food content is an attraction to 

 various animals, which, consequently, often act as agents of dis- 

 semination. Many seeds that are eaten are indigestible and retain 

 their viabilitv after they are dropped at considerable distances 

 from their sources. Others, not immediately eaten, are carried by 

 birds, rodents and even ants to places of storage or concealment, 

 where they may germinate. Of course, great numbers of seeds are 

 eaten or destroyed by animals, but dissemination from seed sources 

 is a partially compensating factor. 



