136 COACTION: THE INTERRELATIONS OF ORGANISMS 



as truly carnivorous. The free-living bloodsuckers are chiefly Diptera, 

 especially in the tundra and swampy areas of the coniferous forest. 

 Ectoparasites are numerous among insects and arachnids, and an in- 

 significant number of bats are also bloodsuckers. 



PLANTS AS ACTIVE AGENTS (COACTORS) 

 Plants as Passive Members (Coactees) 



Like animals, this group is best divided with respect to the type 

 of coactee, whether plant or animal. Apart from the host of para- 

 sites represented by the fungi and bacteria, the number of genera is 

 small, the mosses and ferns having practically none and the flowering 

 plants a few among scattered families of the dicotyledons. To this 

 group are also to be assigned the many saprophytes on dead or decay- 

 ing matter, a few of w'hich occur among flowering plants. 



Flowering Plants. In this group belong those species that manu- 

 facture no food or but a part of what they require. Like other depen- 

 dent plants, they belong to the great physiological category of hyster- 

 ophytes, and may be more or less definitely divided into partial 

 parasites, parasites proper, and saprophytes. They exhibit practi- 

 cally all possible stages in the evolution of this special habit, from 

 rooted green plants such as Castilleia to chlorophyll-free genera re- 

 duced to flower and haustorium, as in the tropical Rafflesia. Many 

 of them' are more singular than important in the community, but some, 

 like dodder and mistletoe, may exhibit a destructive coaction of signifi- 

 cant effect. 



Hysterophytes occur in a small number of families, i.e., twelve, 

 some of which contain no holophytic or autonomous genera, though 

 this may be a consequence of basing the family on the food habit. 

 Several of these are tropical, containing such unique forms as Rafflesia, 

 which is reduced to a single flower sometimes three feet across. In 

 temperate regions, the most important families are Loranthaceae, 

 Monotropaceae, and Orobanchaceae, together with scattered genera 

 such as Corallorhiza among orchids and Cuscuta among bindweeds. 

 A large majority of these are root parasites and do not often cause 

 serious injury to the host, but certain of the mistletoes and many of 

 the dodders may exert fatal effects. Cuscuta is especially destructive 

 in California, often producing bare areas of considerable extent, on 

 which succession is initiated. 



Fungi and Bacteria. The destructive coactions of many of the 

 flowerless hysterophytes are too well known to require comment apart 

 from the role they take in modifying community processes such as 



