SAPROPHYTISM AND SYMBIOSIS 761 



transpiration. Probably saprophytes culminate in mesophytic woods, 

 where soil bacteria are abundant, and where numerous saprophytic 

 fungi permeate the soil and the rotting trunks. There is also a char- 

 acteristic saprophytic flora in the humus underlying ponds and swamps, 

 and even in the dark abysses of the ocean. Both in the forest and in 

 the ocean depths the saprophytes play a part of great import to all 

 organic life; as the organisms of decay and as agents of nitrification 

 they make much material available for other organisms, and their sym- 

 biotic relations with green plants also are most important. 



Probably saprophytic fungi display their greatest luxuriance in old 

 mines, where the decaying timbers furnish a rich supply of food. The 

 mycelia spread abundantly over the surface of the wood as well as within, 

 and even far out on the adjoining rocks, the hyphal strands remaining 

 connected with the source of food. Undoubtedly the uniformly high 

 humidity is the chief factor in determining this astonishing luxuriance; 

 that this factor is not darkness is shown by the fact that similar rich 

 cultures may be obtained in the light on bread or on cheese enclosed 

 in moist chambers. In caves, where there are no timbers, saprophytes 

 often luxuriate, the necessary organic matter being supplied by streams 

 entering from the outer world, by the excrements of cave animals (as 

 the blindfish), and of cave-frequenting animals (such as bats), and even 

 by tallow dropped from the candles of human visitors. Were it not for 

 the saprophytes, ocean depths and caves would be essentially devoid of 

 plant life. 



2. PARASITISM 



General considerations. Definitions. Parasites are plants or 

 animals that derive foods or food materials from other plants or animals 

 to which they are attached. A holoparasite is one that is entirely de- 

 pendent for its food upon the organism to which it is attached, com- 

 mon illustrations being parasitic fungi, broom rapes, and parasitic ani- 

 mals. A partial parasite is one that is only partially dependent upon 

 the organism to which it is attached, being capable of manufacturing a 

 part or even all of its food; in the former case it derives food from its 

 host, in the latter case only food materials. Representative partial 

 parasites are the mistletoe and many of the Scrophulariaceae. 



Inter gradations. All possible gradations connect partial parasites 

 with holoparasites; the former also grade similarly into autophytes and 

 partial saprophytes, and the latter into holosaprophytes. The mistle- 



