370 THE FOOD OF PLANTS 



plants can only absorb dissolved substances, and hence the solvent action 

 of secretions is often necessary, especially in heterotrophic plants, either for 

 the extracellular digestion of organic substances, or to render possible 

 the penetration of dead materials or living plants. The death of a plant 

 or animal may be caused either by these means, or by the excretion of 

 injurious metabolic products (Bacteria, Fungi, &c.). In the economy 

 of nature, plants, and especially micro-organisms, are largely entrusted 

 with the task of disintegrating and decomposing the bodies and organic 

 remains of plants and animals, thus producing the humus necessary for 

 the growth of humus-saprophytes, and aiding in the creation of a fertile 

 soil (Sect. 51). Mutual assistance and mutual interdependence are as 

 common and as necessary in nature as is the inevitable struggle for 

 existence, and in addition to the various disjunctive, simultaneous, or 

 successive co-operations, there exists a still more intimate form of union 

 in reciprocal symbiosis. 



The importance of the root system to fixed plants can hardly be 

 over-estimated ; in a plant of Mucor, for example, the mycelial hyphae 

 may attain a total bulk very much greater than that of the sub-aerial 

 conidiophores. Organic nutriment, as well as ash constituents, may be 

 obtained in solution from the soil, and even from dust conveyed by the 

 air. Many so-called 'humus-collectors' possess various adaptions which 

 enable them to retain and utilize the dust and organic debris which falls 

 upon them, while carnivorous plants are able to capture and digest insects, 

 and thus obtain organic nitrogenous compounds. 



Freely motile organisms can seek out their food, and nutrient substances 

 commonly act as an attractive stimulus to Bacteria, Myxomycetes, and 

 Flagellatae. Similarly a localized accumulation of nutriment may attract 

 growing fungal hyphae, and induce an increased branching where the food 

 is most abundant (cf. Sect. 26). Chemotactic stimuli play an important 

 part in the penetration of parasites, and even in reciprocal symbiosis the 

 existence of interacting stimuli may frequently be made evident by the 

 changed shape of one or both consorts. 



Lichens are good examples of reciprocal symbiosis, for they are specific 

 organisms formed by the union of a fungus with one or more algae, and can 

 frequently withstand climatic conditions to which the isolated component 

 parts succumb. The fungus, especially in those lichens which grow upon 

 bare rocks, obtains organic food from the algae, while the fungal mycelium 

 supplies the latter with water and salts, or even with proteids when the 

 symbiotic algae are peptone-organisms *. 



1 Cf. p. 365. Details in the text-books, and by Lindau, Lichenolog. Unters., 1895, Bd. i; 

 Reinke, Jahrb. f. wiss. Bot., 1894 and 1895. Warming (Lehrb. d. okolog. Pflanzengeographie, 1896, 

 p. 98) terms this special case of symbiosis ' helotism.' 



