762 ECOLOGY 



toe, like the autophytes, probably makes its own carbohydrates, differ- 

 ing from them chiefly in taking its water and salts from a host plant 

 instead of from the soil; such a plant is termed a water parasite. Color- 

 less parasites take organic foods from their host plants; even among 

 these there probably are degrees of parasitism, some manufacturing 

 proteins, while others absorb them along with carbohydrates. Grada- 

 tions between autophytes and partial parasites are best illustrated by 

 the Scrophulariaceae, and are described below. There are many fungi 

 and bacteria that can exist either as holosaprophytes or as holoparasites; 

 such forms are called facultative saprophytes or facultative parasites, in 

 contrast with obligate saprophytes (as most molds) and obligate para- 

 sites (as the more familiar phases of rusts and smuts). 



Various bacteria that ordinarily are saprophytic become parasitic when several 

 successive generations are grown on carrots or turnips. The fungus, Empusa, 

 infects living flies, causing their death, but it grows with undiminished vigor after 

 the death of the host. Similarly, a species of Saprolegnia causes the death of fishes, 

 continuing to live afterward as a saprophyte. Various parasitic bacteria are ca- 

 pable of saprophytic existence, and a number of the fungi causing plant cankers, 

 stem rots, and leaf spot diseases also may live as saprophytes ; some of the latter are 

 regularly saprophytic in their later stages. The common saprophytic mold, Mucor 

 Mucedo, causes a destructive rot in sweet potatoes. Similarly, no sharp lines of 

 demarcation separate saprophytic and parasitic timber fungi. Most smuts and 

 rusts have alternating stages of parasitism and saprophytism. Even among the 

 seed plants there are a few forms which usually are parasitic, but which seem ca- 

 pable of partial saprophytism, notably Lathraea and Melampyrum, the latter also 

 having a capacity for autophytism. In many cases parasites, while attached to 

 living plants, do not come into contact with living cells, secretions from the invad- 

 ing fungus killing the cells before any of the contents are absorbed. In water 

 parasitism (e.g in the mistletoe) the dead hadrome of the parasite is in contact with 

 the dead hadrome of the host. Yet the latter instances properly belong to the 

 phenomena of parasitism, since the host plants are injured and are deprived of 

 materials which otherwise they might have used. 



Parasitic fungi and bacteria. General characteristics. The best- 

 known parasitic bacteria are the pathogenic forms, which occasion many 

 diseases in man and in other animals, and also in plants (for example, 

 the organisms causing tuberculosis, cholera, and typhoid fever; figs. 

 13, 18, 19). These forms generally are less differentiated than are the 

 saprophytic bacteria, perhaps giving an illustration of reduction from 

 a more specialized ancestry. Some bacteria that apparently are para- 

 sitic are not pathogenic, such as Leptothrix and Sarcina, which live 

 respectively in the mouth and the stomach. Bacillus radicicola, the 



